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3 Ozonolysis of alkenes in liquid phase

3.1. Olefins

The reactions of ozone with alkenes are the best known of all reactions of ozone. These reactions take place with high rates, low activation energy and are a basic source for ozonide preparation, i.e., cyclic 1,2,4-trioxalanes [1-7]. The interaction of ozone with the C=C bonds is a powerful method for structural studies of polydienes, functionalising of complex organic compounds at low temperatures and synthesis of valuable oxygen-containing compounds [8-22]. This reaction plays an important role in the atmosphere [23-30] during degradation of rubber materials [31, 32]. All this makes the study of the theoretical and practical aspects of these reactions attractive and relevant for many researchers [1-13]. The ozonation of alkene provoke a great interest both for elucidating the mechanism and reactivity of alkenes and for ozone application in organic synthesis, protection of rubber products against ozone degradation and creation of novel efficient and economical technologies [1, 2, 8, 13, 15, 25-34]. We have carried out detailed and systematic studies on the ozonolysis of olefins and their polymeric analogues [35-51].

3.1.1 Mechanisms

The reactions of ozone with alkenes are a subject of interest and study since the discovery of ozone in 1840 [52-84]. The first experimental research and conclusions concerning these reactions dated from the beginning of the 20th century [52-55]. Harries observed that certain olefinic compounds reacted with ozone to give peroxide oils to which he firstly assigned structure (1) and later he changed the structural assignment to structure (2) as shown in Scheme 3.1 [56-63]:

C C

O OO O

OO

CC

1 2

180

Ozonation of Organic and Polymer Compounds

2H2O

C=O + H2O2 + O=C

2 CO

O

H+ O=C

H

O

OC C-OH

O

Scheme 3.1

Harries proposed that the compound with structure (2) decomposed to hydrogen peroxide and carbonyl compounds. However, the possibility for self-decomposition of (2) to 1,2-dioxalane rearranging further to a carboxylic acid and a carbonyl compound could not be excluded.

Staudinger [64] suggested a more complicated mechanism of olefin ozonolysis (Scheme 3.2) than Harries:

R 2C=CR 2+ O 3R 2C CR 2

O O OR 2C CR 2

O O

O

R 2C CR 2

O O

O

R 2C=O + O=O=CR 2O

OO

CR 2R 2C

O

R-C-OH

O

R=H

;+OOCR2

Polymer

Scheme 3.2

He assumed that ozone reacts with olefin producing an intermediate adduct with a four-member ring, which undergoes rearrangement to 1,2,3-trioxalane, diperoxide,

181

Ozonolysis of alkenes in liquid phase

carbonyl compound or an acid or polymerises. Later it was established that this scheme failed to explain a number of experimental observations [66].

In 1949-1951 Criegee proposed another mechanism of ozonolysis which can be considered as classical [66-71]. This mechanism is outlined in Scheme 3.3.

R2C=CR2

O3R2C CR2

O OO

CR2

O O

O

R2C CR2

O O O

R2C=O + O-O-CR2

O

OO

CR2R2C

O

R2C

+ _

_ ++ polymeric ozonides

zwitterion

primary ozonide

ozonide

diperoxide

polymericperoxides+ R2C

OOH

G

HG

;

G=OH, OR, RCOO_

; rearrangementproducts

Scheme 3.3

According to Scheme 3.3, ozone reacts with C=C bonds forming a primary ozonide (PO) - 1,2,3-trioxalane. The reaction is exothermic producing more than 50 kcal/mol. PO is very unstable and rapidly decomposes to a zwitterion and a carbonyl compound. At least four ways have been suggested by which the zwitterion can stabilise itself: (1) reaction with its own ketone or aldehyde to give ozonide (1,2,4-trioxalane), (2) reaction with the wall of the solvent cage, composed mainly of solvent molecules thus producing hydroperoxides, (3) reaction with other zwitterions outside the cage yielding diperoxide - 1,2,4,5-tetraoxalane and polymeric peroxides, -(-O-O-C-O-O-)

n-, reactions with carbonyl compounds resulting from the decomposition of another PO and formation of ‘cross’ ozonides, reactions with preliminary added aldehydes, ketones, alcohols, water, and so on leading to the formation of ‘foreign’ ozonides or peroxide compounds, and (4) undergoes monomolecular rearrangement giving 1,2-dioxalane which isomerises further to acid.

The proposed mechanism has been confirmed by much experimental data from various researchers [1, 2]. The low temperature reduction of olefin ozonolysis products,

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obtained at low temperatures, gives 1,2-alcohol with 1,2,3-trioxalane as a precursor [87-89]. The formation of a zwitterion is demonstrated by the formation of: (1) ‘foreign’ ozonide [84], (2) methoxy hydroperoxides in methanol solution [91-94], (3) ‘cross’ ozonides [95-98], and (4) ozonide during the photochemical decomposition of diazo compound solutions in the presence of oxygen and an aldehyde [99]:

(C6H5)2CN2 (C6H5)2C: (C6H5)2C-O-O

(C6H5)2C-O-O

hν O2

radical reactions

(C6H5)2C=O+-O-

(C6H5)2C=O+-O-

RCHO C6H5

C6H5

O O

O

R

H

Rieche [73, 74] confirmed the ozonide structure through a counter synthesis, and later it was established by different spectral and theoretical methods [74-83].

The Criegee mechanism is very close to the contemporary concepts for the mechanism of olefin ozonolysis. The additional studies carried out by Criegee accounting for the effect of the number, size, spatial arrangement, electronic properties of the substituents, conformational state and electronic structure of the zwitterion [84-86] on the alkene ozonolysis throw light on the mechanism of the ‘anomalous’ ozonolysis of some olefins with more particular structure and substituents [87-101].

The ozonide stereochemistry can be predicted if the configuration of the zwitterion - anti or syn is known, which is strongly dependent on the nature and size of the substituents and the experimental conditions [100-102]:

R HC

OO

+

_

anti

_

+O

O

CH R

syn

The studies on olefin ozonolysis up to 1958 were summarised by Bailey [3]. Many other authors have carried out research on separate problems regarding these reactions [4-16]. These works stimulated a number of new studies on the mechanism and stereochemistry of the reaction, on the ozonolysis of new types of olefins, the

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preparation of new products and the application of novel methods of analysis [54-57, 71-76].

Concurrently with the accumulation of the convincing evidence favouring the Criegee mechanism, other evidence began to appear which could not be explained by the classical Criegee mechanism. For example, upon ozonolysis of cis- and trans-1-arylpropenes, the ratios of ozonide to polymer peroxides and/or free aldehydes are quite different for the two isomers [77-79]. It was also reported that the cis:trans ratios of cross ozonides [80-88] obtained from cis and trans unsymmetrical olefins, often differ whereas according to the simple Criegee mechanism cis-olefin should yield cis-ozonide and trans-olefin trans-ozonide.

For this reason, Loan and co-workers [87] proposed a scheme which explains the formation of trans- and cis-ozonide from cis- and trans-olefin, respectively (Scheme 3.4).

+ O3o

o

oo

oo o

o o

oo

;

Scheme 3.4

The mechanism indicated above includes the formation of a seven-member ring intermediate with conformation responsible for the final ozonide conformation. Upon ozonolysis of trans-olefins the cis:trans-ozonide ratio should be close to 1, and higher than 1 upon cis-olefin ozonolysis. In case of hindered olefins, the formation of a σ-complex rearranging further to ozonide, is considered as most probable.

This mechanism lost its importance very quickly as many new experimental results emerged which could not be completely explained by it. Thus the ozonolysis of trans-di-tert-butylethylene yields trans-ozonide [102] while according to the Story mechanism it should give cis-ozonides. Moreover, the Story mechanism could not explain the formation of epoxides which are the major products in ozonolysis of highly hindered olefins. It was found that the foreign aldehyde with deuterated 18O atom is incorporated into the zwitterion in preference to the epoxy orientation of the ozonide cycle [103-112]. This observation is in accord with the classical concept of the Criegee mechanism whereas the Story mechanism supposes the preference of the peroxy-bridge orientation [100, 101].

Bauld, Bailey and co-workers [102] suggested refinements for the Criegee mechanism in order to account for the strong and varied experimental facts on the ozone

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reaction with olefins. For this purpose, they accepted that: the PO exists in C-C half-chair conformation; the zwitterion (carbonyl oxide, CO) exists in syn and anti configurations depending on the geometry of the reagent and the experimental conditions; the preferred ozonide conformation is C-O half-chair; the addition of ozone to the C=C bond is 1,3-cis-stereoselective yielding a five-member ring - 1,2,3-trioxolane; the concerted cleavage of PO gives zwitterion and carbonyl compounds, which via 1,3-bipolar cycloaddition results in ozonide formation in the cage, and in the solution volume, the products suggested previously by Criegee; and the reaction of the zwitterion and the carbonyl compound occurs stereoselectively depending on the zwitterion configuration and the preferred conformation of the end ozonide. Thus, Bauld and Bailey formulated the following stereochemical rules: (1) equatorial (trans) substituents in PO are preferentially converted into anti, and axial (cis) substituents into syn carbonyl oxide, (2) the equatorial substituent in the PO is incorporated into a carbonyl oxide zwitterion in preference to an axial substituent, (3) aldehydes interact preferentially with anti-carbonyl oxides so as to orient bulky substituents into a,e-conformation, i.e., cis-ozonide formation, and with syn-carbonyl oxides so as to orient bulky substituents into a,a or e,e-conformation (trans). Thus Bailey succeeded in explaining the cis:trans ratios of normal and cross ozonides, particularly those obtained from olefins with bulky substituents, and trans-olefins yield trans-ozonides and cis-olefins yield cis-olefins, predominantly.

These suggestions, however, are true only for olefins with bulky substituents such as, iso-C3H7 or tert-C4H9, but not for olefins with smaller substituents, i.e., CH3, C2H5 [113-116]. On the basis of microwave spectra Lattimer and co-workers [112] showed that ethylene, propylene and trans-2-buteneozonide have an oxygen-oxygen half-chair rather than carbon-oxygen chair conformation as suggested by the Bailey mechanism. This means that the e,e- and a,s-substituents are located in the trans-position and the a,e-substituents in the cis-position [117-123]. The studies on the conformation of PO, ozonide and zwitterions reveal that Bailey’s rule 2 is not always valid and should be refined. It was shown that during the decomposition of cis-2-butene PO the anticarbonyl oxide yields cis-ozonide and the syn-carbonyl oxide trans-ozonide, regardless of the ozonide conformation. The small substituents - CH3, C2H5 and C3H7

in the PO of 1,2-disubstituted ethylenes are oriented in the e,e-position and the bulky ones, iso-C3H7 and tert-C4H9, in the a,a-position [124-129].

According to reference [112] the ozonolysis of olefins takes place as a supra-supra-cycloaddition of 4n-2n-systems via a five-member ring formation. The cycloreversible decomposition of PO, depending on the configuration, results in: (1) syn-carbonyl oxide with a-orientation of the R substituent, and (2) anti-carbonyl oxide in the case of e-orientation of the substituent. The stereochemistry of the olefins ozonolysis is shown in Table 3.1.

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Table 3.1 Stereochemical course of ozonide formation and decomposition of PO after the mechanism of Bauld-Bailey (before the slash) and Kuczkowski

(after the slash)

Olefin PO, C-C half chair/

O-envelope

COX OZ, O-O-half

chair/-

OZ

1-Alkene e/e anti/anti e,a/- cis/cis3

Trans1 a,a/e syn/syn a,a/- trans/trans

Cis2 e,a/e anti/anti e,a/- cis/cis

Cis3 -/a -/syn - -/trans

Note: 1 - bulky substituents; 2- bulky substituents; 3 - small substituents.

Later, Criegee summarised the available data and modified the mechanism of olefin ozonolysis taking account of the different conformations of the carbonyl oxide [125-127].

Murray, Hagen and Bailey established that the cis:trans-ozonide ratios vary at: (1) addition of various complex-forming reagents such as toluene, o-xylene, isodurene, 1-mesithyl-1-phenylmethane, hexamethylbenzene, and (2) change of the heating rate during PO decomposition (Scheme 3.5) [128-130]:

HC

O O

+

_

CH

CH3CH3

R

HCOO

+_ _

+

OO C

H

R

anti-complex syn-complexH - bond

Scheme 3.5

The more stable transition state of PO would give predominantly anticarbonyl oxide, being dependent on the size of the substituents. Thus, trans-1,2-di-isopropylethylene in its reaction with ozone in the presence of complexing agents, gives a cis:trans-ozonide ratio >1. It has been found that this ratio remains the same in the absence of a complexation agent and at slow heating from (–155 oC) which, however, is against the stereochemical predictions. This fact could be explained by the occurrence of an equilibrium between the syn- and anti-isomers of CO, which takes place at high rate even at –155 oC passing through a carbonyl oxide formation (Scheme 3.6).

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syn

RHC

OO

+

_

anti

_

+O

O

CHR

_

+

OO

CHR

Scheme 3.6

This hinders the complexation between the CO and the complexing agent. These results have been observed during the study on the low-temperature ozonolysis of cis-olefins.

On the basis of conformational considerations, the anti-isomer of CO appears to be more stable than the syn-isomer although in some cases the latter form can be stabilised on account of H-bond formation (Scheme 3.5). On the other hand, it can be assumed that CO preferentially leads to anti-CO-complex formation. Its stability will increase with the increase of the substituent size and then the equilibrium syn-anti will be shifted to the latter. This will result in rise of the cis-ozonide yield as predicted by the third rule of Bailey [129].

On the basis of experimental and theoretical data [130-134], and applying the rule of least motion whereby the PO decomposition occurs via a minimum change of atom coordinates, Bailey [130] demonstrated that the CO conformation depends rather on the rate of PO decomposition than on the thermodynamic stability of the stereoisomers. On this basis he proposed some additional refinements to the mechanism of olefins ozonolysis. The cis-trans-ozonide ratio depends on: (1) the decomposition of the thermodynamically preferable PO conformer, (2) the decomposition of the kinetically favourable PO conformer, (3) the competition in regard to the second and probably the third rule of Bailey for olefins with small substituents, and (4) the equilibrium of syn-/anti-CO conformers prior to their recombination with the carbonyl moiety. The slow heating of the e,e-conformer, which most likely has a lower activation energy of decomposition than the a,a-conformer, would produce preferentially anti-CO and the cis-ozonide in a smaller amount. In the case of rapid heating the decomposition would occur so quickly that it would not affect the equilibrium of the two conformers as it is shown below:

anti-COX ← e,e-PO ↔ a,a-PO → syn-COX

In references [129, 130] it is reported that rule 3 should be altered so as to conform to the O-O half-chair conformation of the end ozonide [111, 112, 134]:

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Thus the revised rule 3 should be: upon the interaction of the aldehydes with anti-CO the bulky substituents are oriented equatorially-axially (e,a) producing cis-ozonide, while aldehydes react with syn-CO to orient them diaxially (a,a) producing trans-ozonide. Bulky substituents in PO are oriented diaxially (a,a) resulting in trans-ozonide formation.

The mechanism of ozone reaction with olefins has been studied by means of thermochemical and quantum chemical methods in a series of contributions [135-145]. Harding and Goddard [137] have found that the exothermicity of the reaction between ozone and ethylene amounts to 92-96 kcal/mol, while it is 101.7 kcal/mol according to reference [11]. Goddard states that the ozonolysis of cis-trans olefins takes place through a biradical pathway (Scheme 3.7):

R

R+ O3

R

HC O

O.

.+ RCHO

(I)

.O

OCH

RO

CHR .

C C

O O

OR

R

RR

O

OO

CC

. HRC

O

R

HC O

O.

(II)

+ RCHO.

.O

OCH

R+ O3

R R

anti

trans1,5-biradical

1,5-biradical cis

(II)

bulky R

trans-ozonide

syn

1,5-biradical

.H RC

OR

HC O

O.

(I)

A B

(II)

1,5-biradical

.O

OCH

R

O

CH R.

C D

Csmall R

Scheme 3.7

In the transition states A, B, C and D, the orientation of the carbonyl oxygen adjacent to the carbon atom of the peroxymethylene allows the stabilisation of the lone electron pair on the oxygen p-orbital through exchange with the C-O bond. The arrows around the biradical C-O bond denote the most favourable orientations for rotation leading

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to cyclisation (the bulky substituents should be kept away from their neighbours in their movement). The orbital phase consideration predicts supra-surface addition whereas the rotation around the biradical bond should be clockwise in the transition states B and D, and counter-clockwise in A and C. The pathway of ozonide formation from olefins with bulky substituents (tert-butyl) is denoted by arrows. In the case of olefins with small substituents, i.e., CH3, the transition state C might be assumed to follow the orbital permissible suprasurface addition even when the other transition states (A, B and D) follow the steric directions. In the light of the biradical mechanism the authors interpret the Baileys rules in the following manner: (1) an equilibrium between the syn- and anti-peroxymethylenes (not carbonyloxides) in the absence of a complexing agent, does not occur or take place to a very small extent, because of the high conversion barrier of 29 kcal/mol, (2) the complexing agents accelerate the rate of equilibration which leads to an increase of the anti-peroxymethylene concentration of the cis-ozonide, which is in contrast with the data from reference [131], (3) at higher temperature the cyclisation reaction is less stereoselective which would favour the formation of the more stable trans product, a fact which is experimentally confirmed, and (4) the increase of the solvent polarity increases the lifetime of the 1,5-biradical species and thus reduces the stereoselectivity of the reaction.

On the basis of quantum chemical calculations Cremer [126-129] proposed a revised rule for determination of the stereoselective pathways of olefins ozonolysis (Table 3.2).

Table 3.2 Stereochemical courses of ozonide formation depending on the olefin conformation at ozone 1,3-cycloaddition and 1,3-cycloconversion of

PO to CO at early (x<0.5) and later (x>0.5) transition state

Olefin Size of R1 & R2

COX x<0.5

COX x>0.5

Ozone x<0.5

Ozone x>0.5

cis- small anti- syn- cis- trans-

trans small anti- syn- cis- trans-

cis bulk anti- anti- cis- cis-

cis bulk syn- syn- trans- trans-

It should be noted that the studies discussed above and the conclusions drawn in them are based mainly on the analysis of the products obtained and on the activation energy calculations of the intermediate and stable products. However, the kinetic methods could also provide essential evidence for the elucidation of the mechanism of this reaction.

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3.1.2 Kinetics

The reaction of ozone with olefins usually proceeds with high rate and low activation energy at temperatures ranging from –100 oC to 300 oC [95, 96, 143, 146].

3.1.2.1 Gas phase

Cadle and Schadt were the first to determine the values of k for the ozonolysis of ethylene and hexene-2 in the gas phase [147]. Later, the ozonolysis kinetics of various olefins has been studied by many investigators [146-161]. The kinetics have had second-order with k values (20 oC) in the range of 0.8-450 × 103 M-1.s-1 (Table 3.3).

Table 3.3 Arrhenius parameters for the ozone reaction with olefins in the gas phase

No. Olefin k × 10-3 (20 oC) (M-1.s-1)

log A Ea (kcal/mol)

1. Ethene 1.6 ± 0.2 1.6; 1.8;

0.8; 1

2.2; 6.2; 6.73; 5.5

4.2 ± 0.4; 4.2; 2.6; 4.9

2. Propene 7.1 5.46 3.9

3. Butene-1 6.2; 3.9 6.2 1.7

4. trans-Butene-2 29; 260; 13 5.55; 5.41; 6.93

0.2 ± 0.3; 2.3; 1.1

5. cis-Butene-2 200; 17 6.28 0.96

6. iso-Butene 14 ± 2 6.15; 6.28 2.8 ± 0.4; 1.7

7. 2,3-Dimethylbutene - 6.23 0.82

8. Pentene-1 3.9; 3.2 - -

9. Hexene-1 4.5 - -

10. 2-Methylbutene 450 - -

11 Trimethylethene 12 - -

12. Cyclopentene 6.1; 4.6 - -

13. 1,1-Dichloroethene 2.2 - -

14. 1,2-Difluoroethene 0.24 - -

15. Tetrafluoroethene 81 - -

16. Hexafluoropropene 13 - -

17. Octafluorobutene 1.1 - -

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The various investigators have reported similar values for k for the reaction of ozone with ethene, but the values of A differ more than four-fold [147, 148].

It appears that the values of A should be similar if one takes into account the structure of trans-butene-2 and iso-butene with a lower Ea for iso-butene. In fact, the values of A and Ea differ essentially and the magnitude of Ea for iso-butene is higher (Table 3.3).

It has been found that the yield of ozonides, the major products in the liquid phase, is very low. The reaction mixture contains both lower and higher molecular compounds than the initial olefin, such as hydrocarbons, acids and aldehydes, etc. These products are obtained from the consequent reactions of the C=C bond cleavage products [153, 154]. Compounds with mass numbers up to 5 times order of magnitude higher than the initial compounds have been identified by means of mass spectrometry (for butenes with a molecular mass >200) [154]. It has been reported that epoxides are the major products of halogenated olefin ozonolysis [155].

The ozonolysis in the gas phase is accompanied by chemiluminescence, a fact which has found practical application for the production of analytical equipment for ozone analysis in the atmosphere. The light emission is due to the electronic transfer of various excited species such as: H2COX (1A′) and OH (X2πi; v ′ ≤9; A2Σ+).

In the reaction schemes outlined below the composition of the various products and the formation of the intermediate excited species are associated with the occurrence of radical steps, due to the breakdown of the O-O bond of the primary ozone [153] (Scheme 3.8) and further isomerisation of the bipolar ion (Scheme 3.9).

Scheme 3.8

C C

OO

OR

H

R

H H

R

H

RO

OO

CC

.

R-C-CHR

O OOH *

R

HC OO

_+ R CO

OH

* R + CO2 + H

RH + CO2

RCOOH

. .

Scheme 3.9

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The stoichiometry of the olefin-ozone reaction varies in the range from 1:1.4 to 1:2 depending on the reaction conditions. The stoichiometry is exactly 1:1 in the absence of side reactions.

The kinetics of the ozonolysis in the gas phase is strongly dependent on the absence or presence of molecular oxygen [153, 156]. Many workers underline that the ozonolysis kinetics is much more complex in its absence, i.e., in an inert atmosphere. Thus the kinetics of the ozone reaction with tetrafluoroethylene is described by the following relationship (Equations 3.1-3.6):

d[O3]/dt = k1[O3][C2F4] + k2 [O3]2.[C2F4], (3.1)

logk1 = (8.2 (0.5) – [(9.5 ± 0.7)/2.3.RT] (3.2)

logk2 = (14.6 (0.5) – [(10.1 ± 0.6)/2.3.RT (3.3)

and the reaction of ozone with allene [150]:

d[O3]/dt = k1[O3][C3H4] + k2 [C3H4].([O3]2/[O3]0) (3.4)

logk1 = (6.0 (0.7) – [(5.5 ± 1)/2.3.RT] (3.5)

logk2 = (6.9 (0.7) – [(6.2 ± 0.8)/2.3.RT] (3.6)

The foregoing discussion shows that the analysis of the kinetic data in the gas phase requires particular attention. Thus one should bear in mind that probably some of the reported constants for the gas phase ozonolysis are actually products, summations or partial quotients of parallel reaction constants.

3.1.2.2 Liquid phase

The study of the kinetics of olefin ozonolysis in the liquid phase appears to be a more complicated task than in the gas phase because of the high reaction rate and more complex analysing equipment [145, 146]. The first studies in this respect were devoted to determination only of the relative rates in regard to a standard olefin-cyclohexene [162]. The first values of k were reported in 1969-1970. It has been found that olefin ozonolysis in the liquid phase follows also second-order kinetics [163-165].

We have established that the outlet ozone concentration is proportional to the change of the olefin concentration during acrylic acid ozonolysis (Figure 3.1).

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0 100 200 300 400 500 6000.0

0.4

0.8

1.2

1.6

[O3]g

CH3COOH

[O3]0

Time, s

Figure 3.1 Kinetics of ozone concentration at reactor outlet upon ozonolysis of 0.02 M acrylic acid in acetic acid medium at 200 oC. The arrows denote the points

of stoichiometry and rate constant estimation

The outlet ozone concentration drops abruptly after the start of the reaction and it remains low, close to zero, almost up to the complete olefin depletion (up to 4 minutes). Then it begins to rise rapidly and approaches the ozone concentration at the reactor inlet. The pattern of the kinetic curve depends on k - it becomes rectangular at high values of k and at low values of k the ozone concentration in the stationary region rises, the time necessary for reaching the inlet concentration becomes longer and the curve form is changed dramatically. A 1:1 stoichiometry has been found. After the addition of 1 mol ozone to 1 mol olefin the reaction rate is reduced by a factor of about 6-8. This fact suggests that the bipolar ion and the carbonyl compounds hardly interact with ozone. The major part of the products (>95%) are generated via one single reaction pathway at low and moderate temperatures. The values of A are found to amount to 106 M-1.s-1 which are typical for simple liquid phase reactions. The methods for kinetic studies are based on determination of the consumption rate of one of the reagents. In the absence of intermediate equilibrium steps, the rate of the first reaction step can be directly found as follows:

The absence of a reverse reaction is testified by the following two facts: (1) the PO formation is an exothermic process –38 kcal/mol [143], and (2) the accumulation

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of significant amounts of PO at temperatures 110-120 oC. If an inert gas is then blown through the reaction mixture only the dissolved ozone is liberated. If a reverse reaction occurs:

R-CH=CH-R + O3C C

OO

OH

R

R

H

at blowing through, the equilibrium will be shifted to the starting compounds and the ozone concentration will be equal to the whole amount absorbed by the system. In this case an olefin rather than PO should be identified. Upon repeated ozonation the olefin will absorb again an equimolar amount of ozone. The experiments, however, show that only PO is identified in the reaction mixture, practically no olefin is found and ozone is not consumed which demonstrates the absence of a reverse reaction.

The rate constants were measured: (1) through ozonation of a mixture composed of unknown olefin and a reference, for example cylcohexene [163, 164] as discussed above, (2) through ozone bubbling through the olefin solution in an inert solvent (in regard to ozone) and registration of the change of outlet ozone concentration [165], and (3) through fast mixing of ozone and olefin solutions by means of stopped-flow techniques [166].

All these methods provide similar results for the k values but the bubbling methods appears to be more convenient and experimentally available. We have successfully applied this method throughout our studies in all cases when it is possible.

The rate constants of ozonolysis of some olefins in liquid phase are presented in Table 3.4.

The comparison of the data in Tables 3.3 and 3.4 reveals that, mostly, the rate constants measured in the liquid phase are an order of magnitude several times higher than those found in the gas phase which is in accordance with the collision theory [166].

The study of the rate dependence on the concentration of the reagents shows that in most cases it is first order with respect to ozone and slightly greater than one in regard to olefin. However, the kinetics approximate first order with respect to olefin at low or sufficiently low olefin concentrations. The increase of the concentration results in an increase of the reaction rate more than that predicted by the second-order kinetics. This could be explained by the fact that the PO formation is preceded by the formation of an ozone-olefin complex (Scheme 3.10).

194


Table 3.4 Rate constants (k × 10-5 M-1.s-1) of olefin ozonolysis in CCl4 at 20 oC

No. Olefin [153]* [155] [156]

1. Ethene - - 0.4

2. Butene-1 - - 1.3

3. trans-Butene-2 0.4-1.6 - -

4. cis-Butene-2 0.3-1.0 - -

5. Isobutene 0.2 0.97 -

6. Pentene-1 2.4 - 5.0

7. 2-Methylbutene-2

8. Cyclopentene 4.5 2.0 4.0

9. Hexene-1 0.27 0.76 1.4

10. Hexene-2 - 1.48 5.0

11. Tert-methylethene - 2.0 -

12. Styrene 0.30 1.0 3.0

13. p-Methylstyrene 0.31 - -

14. m-Methylstyrene

Note: *measured in CHCl3 with respect to cyclohexane.

R-CH=CH-R + O3 C C

OO

OH

R

R

HR-CH CH-R

O3

Olefin

1 2

3

Scheme 3.10

This complex is thermodynamically stable and at collision with a new olefin molecule is broken down to the initial products. Then the dependence of the observed rate constant (kobs) on the olefin will be described as shown in Equations 3.7 and 3.8:

kobs = k1.k2/(k2 + k3 .[Olefin]) (3.7)

1/kobs = 1/k1 + k3/(k1 .k2.[Olefin]) (3.8)

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Upon ozonolysis of acrylic acid the values of k1 and k3/k2 amount to 4.2 × 103 M-1.s-1 and 8 × 106 M-1.s-1, respectively. It should be pointed out that these values are higher than those reported in reference [163, 164] as the olefin concentrations are lower at the points of rate constants determination (denoted by arrows). In this case the rate constant was calculated by applying the real olefin concentration which is equal to the difference between the initial and the consumed concentration up to the moment of calculation. Generally, this difference is within the limits of the experimental error and thus the reported constants could be used for kinetic characteristics of the process. However, when possible we have used the values of k1.

The temperature dependencies of the rate of hexene-1 and maleic anhydride ozonolysis, obtained by us, are presented in Table 3.5.

Table 3.5 Dependence on temperature of the rate constant k1 of the ozone reaction with some olefins

Hexene -1 17 oC 0 oC –20 oC –40 oC –60 oC –78 oC E (kcal)

k1 (M-1.s-1) 1.4 × 105

1.5 × 105

1.2 × 105 0.7 × 105 1.0 × 105 0.6 × 105 0.5 ± 0.5

Maleic anhydride

43 oC 35 oC 26 oC 17 oC 9 oC E (kcal)

k1 (M-1.s-1) 68.3 61.5 36.1 25.8 18.4 6.1 ± 0.5

As seen, the rate constant of hexene-1 does not change with temperature while the activation energy of maleic anhydride reaches the value of 6.1 kcal/mol. The dependence of the rate constant on the temperature for maleic anhydride, according to our data [32, 33], is described by the following expression (Equation 3.9):

k1 = (1.1 ± 0.2).106.exp(-6100 ± 500/RT) (3.9)

The approximate value of A for hexene-2 is 106-107 M-1.s-1.

Generally the olefin ozonolysis is characterised by very low values of Ea = 1-10 kcal/mol. However the inaccurate determination of the rate constants often does not allow the precise definition of the Arrhenius parameters which in turn could lead to misleading interpretation of the experimental results. The values of Ea are commensurable with a number of other energy factors, influencing the reactivity, which impedes the evaluation of their contribution [167]. Among them are the reorientation of the surrounding solvent, the thermal energy of the molecules, the temperature

196


dependence of the diffusion coefficients, etc. Consequently, the dependence of the rate constant on temperature cannot be outlined as according to definition (Equation 3.10):

Ea = E0 + 0.5 RT (3.10)

where Eo is the height of the potential surface at -273 oC.

The rate constants for olefins with various structures are given in Table 3.6.

The rate constants of the reaction of ozone with olefins with terminal double bonds (hexene-1, octene-1 and decene-1) are high and similar. The isomeric olefins with more internal double bonds (hexene-2 and methyloleate) react much more readily that those mentioned above. The rate of the ozone attack increases as the number of alkyl substituents on the double bond increases (hexene-1 and 2-methylpentene-2, acrylic acid and methacrylic acid, polybutadiene and polyisoprene). Cycloolefins C5-C12 react with relatively similar rate constants but more slowly than those with an open chain (cyclohexene k1 = 4 × 105 and k1 for methyloleate = 1 × 106 M-1.s-1). The presence of phenyl groups results in acceleration of the reaction (styrene) while the introduction of second, third and fourth groups (stilbene, 1,1,4,4-tetraphenylbutadiene) has no further influence on the reaction rate. The olefin ozonolysis can be regarded as an electrophilic 1.3-addition and similarly to the reactions of Prilezhaev and Diels-Alder which have close electronic multiplicity, one can expect that electron-accepting substituents in the molecule will decrease the reaction rate. Actually, Br, Cl, -COOH and NO2-substituted olefins react much more slowly with ozone [32-35, 168-170].

The comparison of the rate constants allows disclosure of the effects depending on the structure of the olefins under study. Thus, the values of k for high molecular compounds (polybutadiene, polyisoprene and squalene) are lower than those found for their low molecular analogues (methyloleate and tri-trans-cyclododecatriene and 2-methylpentene-2) [171, 172]. It has been also found that the reactivity of cis- and trans-isomers (maleic and fumaric acids, cylcohexene and tri-trans-cyclododecatriene-1,5,9, gutta-percha and natural rubber) varies as the trans-isomers are more active in their reaction with ozone [141, 168, 169]. The rate constant of cis-diphenylethene is 8.9 × 104 M-1.s-1 while for trans-diphenylethene it is 1.8 × 105 M-1.s-1. Usually these facts are attributed to the occurrence of the compensation effect. Huisgen [173, 174], however, explained this fact by the repulsion of the cis-substituents in the transition state. The hybridisation of the carbon atoms of the double bond changes from sp2 to sp3 in the activated complex which results in overlapping of the van der Waals radii of the cis-substituents. Thus the rate of the ozone attack is retarded whereas overlapping is impossible in the case of trans-isomers and thus the reaction is faster. Similar differences in the reactivity of cis- and trans-isomers have been also observed in Prilezhaev reactions.

197


Table 3.6 Rate constants of olefin ozonolysis in CCl4 at 20 oCNo. Olefin k1 × 10-5 (M-1.s-1)

1. Ethene 0.4

2. Butene-1 1.3

3. cis-Butene-2 1.6

4. Isobutene 1.0

5. 1,3-Butadiene 0.74

6. Pentene-1 1.4

7. 2-Me-Butene 5.0

8. Hexene-1 1.4

9. Hexene-2 5.0

10. 2-Me-Pentene-1 5.0

11. Octene-1 1.3

12. Octadecene-1 1.8

13. Cyclopentene 4.0

14. Cyclohexene 4.0

15. Cyclodecene 4.0

16. Tri-trans-cyclo-dodecatriene-1,5,9 3.5

17. Norbornene 5.0

18. Styrene 3.0

19. Stilbene 1.8

20. 1,4-Diphenybutadiene 0.9

21. 1,1,4,4,-Tetraphenyl butadiene 0.8

22. Acenaphthylene 2.2

23. Acrylic acid 0.042

24. Methacrylic acid 0.078

25 Methylmetacrylate 0.078

26. Maleic acid 1.42.10-3

27. Fumaric acid 0.084

28. Maleic anhydride 3.6.10-4

29. 2,6-Dimethylhept-2-en-2-carbonic acid 0.026

30. Oleic acid 10.0

31. Methyloleate 10.0

32. Vinylchloride* 0.022

33. Allylchloride* 0.085

34. cis-1,2-Dichloethane* 3.57 × 10-4

198


35. Vinylidene chloride* 2.11 × 10-4

36. Trichloroethene* Tetrachloroethene*

3.6 × 10-5

1.0 × 10-5

37. 2-Bromopropene 0.028

38. Acrylonitrile 1.0 × 10-3

39. Tetracyanoethene 0.4 × 10-5

40. 1,1,2-Triphenyl-2,2-dinitroethene 3.6 × 10-5

41. 1,1-Diphenyl-2,2-dinitroethene 0.7 × 10-5

42. Polybutadiene 0.6

43. Squalene 7.5

44. Natural rubber 4.4

45. Gutta percha 1.7

46. Polychloroprene 0.042

Note: *the values are taken from [154, 158].

The small sizes of the ozone molecule predetermine the insignificant role of steric effects in the reaction. Only one of the examples in Table 3.6 shows an anomaly in the rate which could be ascribed to steric influence (k of acrylic, methacrylic and 2,6-dimethyl-hept-2-ene-2-carboxylic acids are 2.3 × 103, 7.8 × 103 and 2.8 × 103 M-1.s-1, respectively). If one takes into account only the inductive effects of the substituents, then the rate constant of 2,6-dimethyl-hept-2-2-carboxylic acid would be 7.8 × 103 M-1.s-1. In fact the latter is 3 times greater than the experimental value which demonstrates the effect of steric factors on the reactivity.

The rate of olefin ozonolysis depends significantly on the olefin structure (k for oleic acid is six-fold higher than that for tetracyanoethylene) (Table 3.6). The quantitative relationship between the electronic and geometric structure of olefins and their reactivity is a very important stage in studying ozone chemistry. The first attempts in this respect were the studies of Cvetanovic and co-workers [163, 164] who found a good linear relationship between log of the rate constant and the ionisation potential of chlorinated ethenes.

It has been found that the linear dependence of the reactivity on the free energy change in the system is valid for many organic reactions. The Hammett relationship is well known and widely applied [170] (Equation 3.11):

lgki /k0 = ρ.σ (3.11)

199


where kj are the rate constants of the compounds studied; k0 is the rate constant of the reference; ρ is coefficient accounting for the reaction character; σ is inductive constant of the substituents.

Some other relationships, like that of Taft, have also been used for evaluation of the contributions of inductive, steric and mesomeric effects of various substituents on the rate constant.

A dependence of Hammett type has been found upon decomposition of PO and ozonides [171, 172, 175]. It is quite natural that a similar relationship might be observed during ozonolysis of olefins.

Table 3.7 Logarithms of the relative values of the rate constants and the Hammett inductive constants (σpara) of the substituents on the double bonds

No. Olefin Substituent Σσpara log ki /k0

1. Methyloleate 2CnH2n+2 –0.302 1.40

2. 2-Me-pentene-2 2CH3, C2H5 –0.491 1.097

3. Hexene-2 CH3, C3H7 –0.321 1.097

4. Styrene Ph –0.01 0.875

5. Styibene 2Ph –0.02 0.65

6. Hexene-1 C4H9 –0.151 0.544

7. Octadecene-1 C16H33 –0.151 0.65

8. Ethene H 0 0

9. Methylmethacrylate CH3, COOCH3 0.22 –0.7

10. Methacrylate acid CH3, COOH 0.28 –0.71

11. 3-Chloropropene-1 CH2Cl 0.18 –0.67

12. 2-Bromopropene CH3, Br 0.062 –1.15

13. Acrylic acid COOH 0.45 –0.98

14. Acrylonitryle CN 0.66 –2.6

15. Maleic acid 2COOH 0.9 –2.45

16. Fumaric acid 2COOH 0.9 –1.68

17. 1,1,2-Triphenyl-2- nitroethene

3Ph, NO2 0.748 –4.04

200


The rate constant of the reaction of ozone with ethene, equal to 4 × 104 M-1.s-1, is accepted as a standard in studying the kinetics of ozonolysis of various olefins. It is calculated on the basis of the k value in the gas phase multiplied by a coefficient of 25 which accounts for the heat of dissolution, 1.9 kcal/mol.

The Hammett relationship in Figure 3.2 is plotted on the basis of the data of Table 3.7. As seen the data fall on a straight line (Figure 3.2) thus indicating a linear dependence.

-0.8 -0.4 0.0 0.4 0.8

-4

-3

-2

-1

0

1

2

3

R = 0.89883ρ = -3.2

8

17

16

1514

1312

1110

9

7

6543

21

σpara

Figure 3.2 Hammett relationship of ozone reaction with olefins

The negative value of ρ = –3.2 indicates the electrophilic nature of the ozone reaction with olefins. The low value of the correlation coefficient R = 0.89883 could attain much higher values if the steric effects of the substituents and the electrostatic interactions are considered. Regardless of this the dependence in Figure 3.2 is quite acceptable for evaluating the rate constants of reactions with new olefins.

Another very important question related to the reactivity of olefins, which has not been sufficiently studied, is the relationship between the reactivity of C=C bonds and the deformation and steric factors responsible for the olefin conformation. These factors depend on the inter, intra, supra, micro and macro effects. Thus the intramolecular strains arising in the cyclic olefins are due to the deformation of valent, dihedral angles, interatomic distances and transanular interactions [175-183]. The values of the different deformational and steric components responsible for cyclohexene conformation, as calculated by us, are presented in Figure 3.3.

201


2 4 6 8 10 12

0

10

20

30

40

50

60

BND

TOR

MMXE

SE

Energy, kcal

Cycloolefins

Figure 3.3 The values of C2-C12 cylcoolefin energies calculated according to the molecular mechanics theory. MMXE - the Alinger energy; SE - the strain energy;

TOR - the torsion energy and BND - the bond energy

It is seen that the dependencies have specific character with a minimum in case of cyclohexene. Among the olefins, given in Table 3.6, there are a number of olefins with highly strained molecules, such as cyclododecene, cyclododecatriene, norbornene, ethylidenenorbornene, acenaphthylene and 2,6-dimethylhept-2-2-carbonic acid. It has been found that these olefins react with diazomethane in contrast to those with strain-free molecules. The reduction of strain in the sequence of C4, C5 and C6 cycloolefins correlates with the reduction of the corresponding yield of pyrazolines [183].

The reactivity and size relations of cyclo-olefins are demonstrated in Figure 3.4. It is seen that cyclohexene, the olefin with the lowest strain energy of the initial molecule, exerts the lowest reactivity towards ozone. Simultaneously, its activated complex (AC) has the highest conformation energy. Thus Ea - the difference between the energy of the starting olefin and that of the AC - would be the highest which accounts for the lower reactivity of cyclohexene with respect to the other cycloolefins [1].

Olefins may contain several double bonds separated by three simple C-C bonds. In these cases the C=C bonds react as independent kinetic units. The ozone addition to one of them hardly affects the neighbouring ones. The substituent inductive effect is reduced by a factor of about 2.7 through each C-C bond and thus its influence on the other carbon atoms is significantly reduced [183, 184].

202


4 6 8 10 12

0

2

4

6

8

10

SEn/SE6

kn/k6Rela

t. va

lues

Cycloole�n

Figure 3.4 Dependency of the relative rate constants and strain energies of cyclo-olefins on the cycle size

Upon ozonolysis of compounds with conjugated bonds (1,3-diene, p-divinylbenzenes, etc.), the different reactivity is determined by the structure of the starting diene, the nature of the olefin after the first ozone addition and the effect of the ozonide inductive factor on the second C=C bond. This is illustrated by the ozonolysis of divinylbenzene (Scheme 3.11).

Scheme 3.11

The proceeding of the reaction in two steps results in the appearance of two stationary regions on the curve [O3]g = f(τ) thus allowing the determination of the two rate constants k and k′ (Table 3.8).

203


Table 3.8 Rate constants of ozone reaction with dienes and trienes in CCl4 at 20 oC

Compounds k × 10-5 (M-1.s-1) k′ × 10-5 (M-1.s-1)

Divinylbenzene 2.0 0.2

Ethylidenenorbornene 3.6 2.0

Vinylnorbornene 3.3 0.8

Vinylcyclohexene 1.4 0.8

Tetrahydroindene 1.4 1.4

Cyclododecatriene-1,5,9* 3.5 3.5 (3.5)

Note: *The third double bond reacts with the same rate as the second.

In the example of divinylbenzene the change in the rate constants after ozone addition could be followed. Its correlation with the general principles of the theory can be also evaluated. The rate constant of the ozone reaction with the first C=C bond should be slightly smaller than that of styrene because of the higher conjugation in divinylbenzene. After the ozone attack on the first C=C bond the conjugation degree is reduced and ozone interaction with the second C=C bond should take place with a rate similar to that of styrene (3 × 105 M-1.s-1). However, taking into account the electron-acceptor effect of the ozonide cycle, the rate of the second step should be smaller than that of the first step, which has been also confirmed by experimental data.

The comparison of k for ethylidene- and vinylnorbornene, allows the assumption that ozone attacks firstly the double bond in the side chain, as the changes in its structure affect the value of k while the values of k′ are equal for the two isomers. The C=C bonds in cyclododecatriene-1,5,9 are equivalent due to the long distance between them and the equal configuration of the neighbouring environment. This is demonstrated by the equal rate constants for the three bonds and by the value of the hydrogenation rate of cyclododecatriene-1,5,9 on palladium [185].

3.1.2.2.1 Effect of solvent

It is known that the rate of many reactions depends strongly on the solvent polarity [181, 182]. There are a number of examples whereby the reaction mechanism changes with solvent variation and the ozonolysis of olefins is among them. It has been found that alkoxy- and aryloxy-hydroperoxides [186, 187] instead of ozonides, are the major ozonolysis products of olefin ozonolysis in solvents such as alcohols, acids and other compounds.

204


In order to assess the influence of the solvent on the rate constant we have followed the kinetics of acrylic acid ozonolysis in various solvents. The choice of acrylic acid was mainly for two reasons: (1) its good solubility in the solvents used, and (2) its relatively low rate of interaction with ozone which simplifies the registration of the reaction progress. The following solvents which were used for this study - CCl4, decane, acetic acid and water - differ essentially in regard to their dielectric constants (ε) and their ability to dissolve ozone. Decane and CCl4 are inert towards the intermediate and end products but acetic acid and water react with the dipolar ion. We have found that the reaction obeys first-order kinetics with respect to each reagent.

Table 3.9 Rate constants of acrylic acid ozonolysis in various solvents at 20 oC

Solvent Dielectric constant (ε)

Henry’s constant (α)

k × 10-3 (M-1.s-1)

CCl4 2.24 1.8 2.9

n-Decane 2.05 1.5 2.8

Acetic acid/H2O (%)

99 6.2 1.70 2.6

84 18.2 1.26 2.6

67 30.9 0.95 2.6

50 43.6 0.77 2.6

16 69.0 0.65 2.8

100 80.2 0.42 2.7

The data in Table 3.9 show that the value of k does not depend significantly on the solvent. Thus two basic conclusions emerge: (1) the solvent does not affect the first step of the reaction or, if participates, it is in fact during some of the next reaction steps, as:

RHC+OO- + R′COXOH (RH(HOO)C-O(O)CR′

RHC+OC(OO-)CHR + R′C(O)OH (RHC[O(O)CR′]-O-C(OOH)HR

and (2) formation of polar compounds or polar transition states [188-192] is not observed:

205


C

C

O

O

O

C

C

OO

O

C

C

OO

O+

_

+ _

_

+

The polar intermediate compounds should solvate or react with ‘the active solvents’. This should change the k value with the increase of the solvent polarity, and lead to the formation of reaction products with C-C bonds instead of C=C bonds. However, similar compounds have not been experimentally identified.

The constancy of k values in various solvents testifies that the reaction occurs without formation of kinetically active polar compounds due to the high rate of electron delocalisation in the AC. The latter is much slower than the relaxation time of the solvent shell (∼10-13 s).

3.1.2.2.2 Effect of configuration

We have studied the ozonolysis of structural derivatives of one and the same chemical compound: butane dicarbonic acid and its anhydride, maleic anhydride (I) cyclic form, maleic acid (II) cis-form and fumaric acid (III) trans-form:

HC CH

O=C C=OO

I

C C

COOHHOOC

H H

II III

H

H

HOOC

COOH

CC

These compounds, similarly to acrylic acid, react relatively slowly with ozone which makes them suitable objects for investigating the reaction kinetics [32, 33]. The concentrations of the reagents vary from 6 × 10-5 to 0.9 M. Water, glacial acetic acid and chloroform were used as solvent media. The products of the ozone reaction with the double bond in the ethers of maleic and fumaric acids have been identified using chemical methods and infrared (IR) spectroscopy. The low values of the rate constants suggest that at definite conditions ozone may also attack the C-H bond (1b) in addition to the basic interaction with the C=C bond (1a):

H

H

HOOC

HOOC

C

C+ O3

C

C

HOOC

HOOC

H

H

O3 èëè

C

C

HOOC

HOOC

H

H O3

1a 1b

206


In this connection we have determined the reaction stoichiometry and analysed the IR spectra of the solutions during the reaction course. The stoichiometry we found to be 1:1. The iodometrically estimated content of active oxygen in the samples corresponds to the amount of absorbed ozone. The progress of the reaction was accompanied by an increase in the intensity of the C-O bands in the range of 110-1200 cm-1 and narrowness of the C=O bands in the range of 1710-1850 cm-1.

However, at high conversion degrees the C=O bands are moved from their initial range due to the inductive effect of the ozonide cycle. With the increase of ozonation duration the bands of the C=C valent vibrations in the 1610-1680 cm-1 range decrease their intensity and an abrupt decrease of the rate of ozone uptake is observed. At complete consumption of C=C bonds the ozone concentration at the reactor outlet becomes equal to that at the reactor inlet. This fact suggests that ozone interacts mainly with the C=C bonds, while its consumption in parallel and consecutive steps is very low. This occurs regardless of the aldehyde formed in ozonolysis, which may also react with ozone. On the other hand the band intensity of the C-H and H-O valent vibrations at 3020 cm-1 and 3550 cm-1, respectively, do not change significantly in the course of the ozonation. These experimental observations confirm the assumption that ozone does not attack the C-H bonds at the C=C bonds. This is also shown by the low k value for butandicarbonic acid (1 × 10-3 M-1.s-1) at room temperature. Briener and co-workers [193-195] also confirmed the absence of ozone interaction with the hydrogen of the OH group. Upon ozonation of the aforementioned acids and their esters they obtained a similar product composition. Consequently ozone reacts predominantly with the C=C bonds of I, II and III according to pathway 1a.

It has been found that the rate of the reaction and the product compositions do not vary substantially with the solvents used, i.e., water (= 80.1), acetic acid (= 6.2) or chloroform (= 10). However, it should be noted that because of the lower solubility of fumaric acid in chloroform most of the comparative kinetic studies were conducted in acetic acid medium (Table 3.10).

Table 3.10 Kinetic parameters of the ozone reaction with maleic anhydride, maleic and fumaric acids in acetic acid at 2-6 × 10-6 M ozone concentration

Substrate k1 (M-1.s-1) k3/k2 (M-1) log A1 E1 ± 0.5 (kcal/mol)

I 26 (17.30); 36 (25.80C); 51 (35.00C); 64 (42.50C)

63 (17.30C) 6.30 6.5

II 114 (17.00C); 142 (23.00C); 162 (35.00C); 232 (42.00C)

4560 (17.00C) 5.50 4.6

III 765 (17.00C); 840 (22.00C); 919 (28.50C);1050 (42.50C)

1000 (22.00C) 5.13 3.0

207


We established that at relatively low reagent concentrations the rate of ozone absorption is not proportional to the reagent concentrations although the amount of absorbed ozone is equal to that of the olefin consumer. The reaction exhibits first order with respect to ozone and the dependence on the olefin concentration is more complicated. The rate constant decreases with the increase of olefin concentration. The reciprocal magnitude of the observed rate constant depends linearly on the olefin concentration (Figure 3.5).

0 1 2 3 4 50.00

0.01

0.02

0.03

0.04

0.05

0.06

42.50C

25.80C

17.30C

[I].103, M

Figure 3.5 Dependence of the reciprocal value of the rate constant on the concentration of maleic anhydride at various temperatures in acetic acid and [O3]

= 5 × 10-6 M

These results indicate the occurrence of a side non-efficient reaction, in which the olefin is consumed via a reaction with an intermediate product formed between the ozone and olefin (Scheme 3.12):

Olefin+O3 → P1 (1)

P1 → Products (2)

P1+Olefin→2Olefin+O3 (3)

Scheme 3.12

208


On the basis of the kinetic analysis of Scheme 3.12 an expression for the reciprocal value of the observed rate constant, similar to Equation 3.8, is derived:

1/kobs = 1/k1 + (k1.k3/k2) [Olefin] (3.12)

Actually Figure 3.5 illustrates graphically Equation 3.12. The values of the rate constants as determined on the basis of Scheme 3.12 are given in Table 3.10.

The analysis of the data in Table 3.10 reveals two significant facts: (1) the high values of k3/k2 ratios, and (2) the increase in the rate constant k1 in the sequence: maleic anhydride (cyclic form)<maleic acid (cis-isomer)<fumaric acid (trans-isomer). The first observation discloses the high efficiency of reaction (3) and the second accounts for the anomalous reactivity of the geometrical isomers. Thus it was expected that the trans-isomer would react faster than the cis-isomer. The high value of k2 may be attributed to the specific character of P product which is probably an unstable π-complex and its decomposition to the initial products is facilitated by the olefin molecules.

We determined the kinetic parameters of the ozonolysis of maleic anhydride, maleic and fumaric acids at very low olefin concentrations whereby reaction (3) can be ignored. In these experiments the rate of the ozone reaction with I, II and II depends linearly on the concentration of the two reagents and the Arrhenius parameters differ from those determined at high olefin concentrations (Table 3.10). The reaction stoichiometry is 1:1 and the rate of ozonide formation is proportional to the rate of ozone uptake. The kinetics of ozonide formation was followed both iodometrically and spectrally by monitoring the IR band intensity at 1125 cm-1.

The scheme of the reaction accounting for the observed experimental data is shown in Scheme 3.13:

>C=C< + O3 → Products

Scheme 3.13

However the literature provides data that the olefins ozonolysis probably takes place via the formation of π-complex according to Scheme 3.14:

>C=C< + O3 → π-Complex → Products

Scheme 3.14

209


• ApplicationofERM

In order to propose the most probable mechanism of the ozone reaction with I, II and II we have applied the method based on comparison of the calculated values of the pre-exponential A according to the activated complex method with different forms of the activated complex in the transition state with those obtained experimentally using two forms of TS:

OO

O

CR2R2C

OOO

CR2R2C

LC CC

The calculated and experimental values for A are presented in Table 3.11.

It is seen that the values of A calculated with CC of the activated complex are lower than those with LC. The values of A calculated on the assumption that the energy of free rotation is zero are given in column 4 and those taking account of this energy are placed in column 5. Compared to Aexp, the values of A-CC are lower by about 1-2 orders. The conclusion based on these facts is that the transition state cannot have a cyclic form as these values of A are the highest possible predicted by the theory with this form of the activated complex. The values of A calculated with the linear form of the activated complex are similar to those obtained experimentally. Therefore, it can be assumed that the reaction proceeds via the formation of a π-complex according to Equation 3.13:

kobs = k1.k2/(k-1 + k2) (3.13)

at k2 >>k-1 and kobs = k1

The assumption that k2>>k-1 is supported by the good fit between the calculated value for A of the right reaction with that obtained experimentally for I, II and III ozonolysis. This is also confirmed by the absence of any additional bands in the IR spectra of the ozonised solutions of I, II and II in methylene chloride registered for 48 h at –80 oC.

3.1.2.2.3 Effect of structure

For detailed analysis of the effect of the chemical structure of olefins on their reactivity, we have synthesised a number of substituted cyanostilbenes:

210


R 2R 1C

H

N

CC

Earlier, these compounds were investigated as phosphorescent and colouring agents [196, 197]. Their interaction with ozone, however, has not received considerable attention although their specific electronic structure provides a unique possibility for assessing the effect of such an important chemical factor on the reaction ability.

Some characteristics of the synthesised and investigated cyanostilbenes are presented in Table 3.12 [35, 36].

Table 3.11 Arrhenius parameters for the ozonolysis of I, II and III

Compound kobs (20 oC)

(M-1.s-1)

log ACC log ALC at

Efr= 0

log ALC, at real Efr

log Aexp

Ea

(kcal/mol)

Maleic anhydride

29.1 4.48 7.48 6.30 (1.60) 6.30 6.50

Maleic acid 128 4.42 7.42 5.50 (2.60) 5.50 4.60

Fumaric acid 810 4.33 7.33 5.13 (3.00) 5.13 3.00

Numbers in brackets are the estimated energies of free rotation in kcal/mol.

Table 3.12 Kinetic and physical constants of synthesised and studied cyanostilbenes in CCl4 at 20 oC

No. Cyanostilbene k × 10-5

(M-1.s-1)

mp

(oC)

log k Σσ

1. 2-Phenyl-3-phenyl 1.3 5.114 0

2. 2-Tolyl-3-(4-chlorophenyl) 1.3 5.114 0.687

3. 2-(4-Chlorophenyl)-3-tolyl 1.2 5.079 0.687

4. 2-(3,4-Dimethoxyphenyl)-3-nitrophenyl

- - 0.625

5. 2-(4-Nitrophenyl)-3-(4-chlorophenyl)

0.6 4.778 1.005

6. 2-(4-Nitrophenyl)-3-(4-nitrophenyl)

- - 1.556

211


7. 2-(4-Nitrophenyl)-3-(4-dimethylaminophenyl)

1.6 5.204 –0.052

8. 2-(4-Aminophenyl)-3-(4-methoxyphenyl)

3.0 5.477 –0.928

9. 2-(4-Nitrophenyl)-3-(4-methoxyphenyl)

1.1 5.041 0.510

10. 2-(4-Methoxyphenyl)-3-(4-methoxyphenyl)

2.6 5.415 –0.536

11. 2-(4-Aminophenyl)-3-(4-methoxyphenyl)

0.5 4.699 0.608

12. 2-(4-Nitrophenyl)-3-(3-methylphenyl)

2.4 5.380 –0.091

13. 2-(3,4-Dimethoxyphenyl)-3-(3-fluorphenyl)

- - 0.074

14. 2-(3,4-Dimethoxyphenyl)-3-(4-chlorophenyl)

1.7 5.230 –0.170

15. 2-(4-Cyanophenyl)-3-(4-dimethylaminophenyl)

5.9 5.771 –0.200

16. 2-Tolyl-3-(4-aminophenyl) 1.4 184 5.146 0.470

17. 2-(3,4-Dimethoxyphenyl)-3-(3,4-dibromphenyl)

- 107 - –0.163

18. 2-(3,4-Dimethoxyphenyl)-3-(1-naphthyl)

- - - 0.768

19. 2-Phenyl-3-(4-dimethylaminophenyl)

- - - –0.840

20. 2-(3,4-Dimethoxyphenyl)-3-piperonyl

- 145 - -

21. 2-(3,4-Dimethoxyphenyl)-3-(3,4-dimethoxyphenyl)

- - - –0.306

22. 2-(3,4-Dimethoxyphenyl)-3-aminophenyl

- 85 - –0.813

23. 2-(4-Nitrophenyl)-3-phenyl - - - 0.768

24. 2-(3,4-Dimethoxyphenyl)-3-(2-methoxy-5- bromophenyl)

- 175 - –0.156

25. 2-(3,4-Dimethoxyphenyl)-3-(3-fluorophenyl)

- - - 0.184

Note: The constants are measured only for tetrachloromethane-soluble compounds.

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The values of the melting points are given only for the newly synthesised compounds. The concentration of cyanostilbenes in the kinetic experiments amounts to 1.33 mM.

For determining the rate constants upon a high reaction rate we have applied the absorption theory which is enhanced by the proceeding of a first-order chemical reaction (Equation 3.14) [34]:

R0 = [O3]*(k1.D)1/2 (3.14)

where R0 is the absorption rate at bubbling liquid in stationary conditions, mol/(cm3); [O3]* = α.[O3]g is the equilibrium ozone concentration on the liquid-gas surface (mol/cm3), where α = 2 is Henry’s coefficient at 20 oC; D = 7.8 × 10-8(κMB)1/2.T, (µB.VA

0.6)-1 = 3.06 ×10-5 cm2/s - is the coefficient of ozone diffusion (A) in CCl4 (B), calculated by the Wilke and Chang equation [198]; κ = 1 is the coefficient of association; MB is the molecular weight of CCl4, g; µB = 1.2 x 10-3 Pa-s which is the viscosity of B; VA = 30 cm3/g, mol the molecular volume and k1 is the rate constant.

Equation 3.14 is true at high accelerations of the physical absorption resulting from the chemical reaction at (D × k1)1/2/kL>>1. At known values for D = 3.06 × 10-5 cm2/s, kL ≈ 10-2 cm/s and k1 = 106 × 10-3 = 103 s-1, the condition (D × k1)1/2/kL>>1 is fulfilled since the left side of the inequality has a value of 15.

The ozone reaction with the cyanostilbenes is a bimolecular process as the rate of the reaction increases linearly with increasing concentrations of both reagents. Consequently ozonolysis is first order with respect to each of the reagents. The quantitative analysis of the band at 1152 cm-1, which is responsible for the ozonide formation, reveals that its accumulation rate is almost equal to the rate of cyanostilbene consumption.

It has been found that 4,4′-dinitro-cyanostilbene ozonolysis yields 4-nitro-benzenecyanate and 4-nitro-benzaldehyde as the major products, while the ozonide amount is very low. The scheme of the reaction is given in Scheme 3.15.

C C

H

CN CNH

CC+O3

OO

O

_

+O O

OC C

H CN

+

NO2O2N NO2O2N

NO2O2N

O2N CHO

O2N CNCO

Scheme 3.15

213


In view of the concepts on the PO decomposition mechanism, with electron-donating substituents, a zwitterion at the carbon atom with a CN group could not be formed due to its destabilising effect. The conjugation between the carbonyl compound formed at this carbon atom and the CN and Ph groups prevents ozonide formation via a zwitterion. For this reason the zwitterion leaves the cage and then it interacts with another zwitterion in the solution volume liberating oxygen. This reaction results from the strong electron-accepting action of the nitro groups in the para-position and provides an example of anomalous ozonolysis.

Generally, the cyanostilbenes studied obey the Hammett’s relationship and give a straight line with ρ = –0.4 (Figure 3.6). This value, although substantially lower than that found for substituents directly attached to the double bond, shows clearly the occurrence of conjugation leading to re-distribution of the substituents inductive effects.

-1.0 -0.5 0.0 0.5 1.04.6

4.8

5.0

5.2

5.4

5.6

5.8

ρ = -0.4

1614

12

11

10

97

5

8

σ

Figure 3.6 Dependence of the rate constants on the Hammett inductive constants

The small difference in the rates depending on the inductive constants can be attributed to the low A values, most probably, because of the low activation entropy.

214


The cyanostilbenes studied have substituents in the both benzene rings (α and β). This could be regarded as the reason for the neutralisation of the substituents effect, which contributes to the low value of ρ. The electronic structure of cyanostilbene is shown as:

The values of the charges are determined by quantum-chemical calculations with AM1 Hamiltonian (MOPAC6).

Aiming at more accurate assessment of the substituents effect on the reaction ability, we have synthesised 13 new compounds substituted only in the α- and 14 in the β-benzene ring. The rate constants of the ozone reaction with all these compounds have been evaluated and are outlined in Table 3.13.

The correlation of these constants with the Hammett constants (Figure 3.7a, b) gives the following values of ρ = –0.266 and –0.286, for substitution in the α- and β-benzene ring, respectively. The correlation coefficients are not higher than 0.8.

It is evident that the two coefficients (are almost equal since the difference of about 7% is within the limits of the experimental error. Thus the reactivity actually does not depend on the substitution position. The similar slopes of the kinetic curves suggest that the role of the CN group in the re-distribution of the electron density and its effect on the reaction ability of these compounds cannot be differentiated in this reaction.

215


Table 3.13 Rate constants of cyanostilbene ozonolysis in CCl4 at 20 oC

No. α-Substituents/σ k × 10-5 (M-1.s-1)

1. p-NH2/-0.66 4.13

2. m,p-CH3O/–0.153 4.13

3. m-CH3O/0.115 2.82

4. m-NO2/0.710 4.36

5. p-F/0.062 2.08

6. p-CH3/–0.170 3.06

7. p-Cl/0.227 2.08

8. p-CN/0.660 2.08

9. Naphthyl/–0.01 1.49

10. Pyrene/–0.01 0.56

11. Biphenyl/–0.01 2.22

12. Stilbene 1.80

13. Cyanostilbene 1.30

14. p-Br/0.232 1.90

15. p-Cl/0.227 2.09

16. p-NHCOXCH3/0.00 2.14

17. p-NO2/0.778 1.39

18. m-NO2/0.710 1.50

19. m-Cl/0.373 1.80

20. m-CH3O/0.115 1.56

21. m-Br/0.391 1.56

22. p-CH3O/–0.268 2.12

23. p-CH3/–0170 2.60

24. Pyridine 2.01

25. Naphthyl 3.05

26. m,p-CH3O/–0.153 3.13

27. m-OC2H5,p-CH3O/–0.258 3.08

216


-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.85.2

5.4

5.6

α

ρ = - 0.266

13

7 8

6

5

3

21

lgk

σ

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

5.2

5.4

5.6β

ρ = - 0.286

27 26 25

24

23

22

2120

19

1817

16 15

14

lgk

σ

Figure 3.7 Dependence of the rate constants on the Hammett inductive constants

3.2 Polydienes

The interest in the ozone reaction with polydienes is closely related to the problem of ozone degradation of rubber products. The reaction kinetics is found to be strongly dependent on the polymeric state, a question which has attracted a special interest in recent years [199].

Flory [200] showed that the reactivity of the functional groups in the polymer molecule does not depend on its length. It is also known that some reactions of the polymers proceed much more slowly compared with their low molecular analogues (catalytic hydrogenation). At the same time many fermentative reactions with polymers take place rapidly while similar reactions with low molecular compounds do not occur

217


at all [201]. The folded or unfolded form of the macromolecules provides various conditions for contact of the reagents with the reacting groups [202]. By using the modified version of this principle [203] it was possible to explain the proceeding of reactions without specific interactions between the adjacent C=C bonds and the absence of diffusion limitations. The study of the mass-molecular distribution (MMD) is in fact a very sensitive method for establishing the correlation between molecular weight and reactivity. The theory predicts that the properties of the system, polymer-solvent can be described by the parameter of globe swelling (γ) which defines the free energy (F) of the system and thus the rate constant of the reaction. For a reversible reaction, i.e., polymerisation-depolymerisation, the dependence of the rate constant of the chain length increase on the molecular weight is expressed by Equation 3.15:

ln kpj/kp∞= –const.(5γ - 3/γ).(dγ/dM).M0 (3.15)

where M0 is the molecular weight of the studied sample and kp∞ is the rate constant for infinitely long macromolecules.

A good correlation between the theoretical and experimental data for polystyrene solutions in benzene was found in reference [204].

The study of polymer degradation is complicated by the structural peculiarities at the molecular and supramolecular level and by diffusion reasons. It is difficult to find simple model reactions for clarification of the particular properties and for the express examination of the proposed assumptions. An exception in this respect is the ozone reaction with C=C bonds whose mechanism has been intensively studied and could be successfully applied upon ozonolysis of polymeric materials [178]. This question will be discussed in detail at the end of this Chapter and in Chapter 4.

Table 3.14 summarises the rate constants of the ozone reaction with some conventional elastomers and polymers and their low molecular analogues, synthesised by us.

It is seen that the reactivity of elastomers and polymers and their corresponding low molecular analogues, as demonstrated by their rate constants, are quite similar, thus suggesting similar mechanisms of their reaction with ozone. This statement is also confirmed by: (1) the dependence of k on the inductive properties of substituents such as k of polychloroprene is higher than that of vinylchloride due to the presence of two donor substituents, and (2) the dependence of k on the configuration of the C=C bond in trans-isomer (gutta percha) and cis-isomer (natural rubber).

It has been found that the effects related to the change of the macromolecule length or the folding degree do not affect the ozonolysis in solution. Probably this is due to the fact that the reaction is carried out with thermodynamic elastomer solutions in which the macromolecules can perform free intramolecular movements and do

218


not react with adjacent macromolecules. Moreover, the rate of macromolecule reorganisation is probably higher than the rate of their reaction with ozone as the experiment does not provide evidence for the effects of the change of the parameters mentioned above [205-207].

Table 3.14 Rate constants of ozone with polymers and low molecular analogues in CCl4, 20 oC

Compound Mw k × 10-4 (M-1.s-1)

Polyvinylchloride 8 × 105 0.42 ± 0.1

Vinylchloride 62.45 0.18

2-Bromopropene 121 0.28 ± 0.05

Polybutadiene 3.3 × 105 6.0 ± 1

Cyclododecatriene-1,5,9 162 35 ± 10

Polybutadiene-styrene 8 × 104 6 ± 1

Gutta percha 3 × 104 27 ± 5

Natural rubber 1 × 106 44 ± 10

2-Me-pentene-2 85 35 ± 10

Squalene 410 74 ± 15

Polystyrene 5 × 105 0.3 × 10-4

Cumene 120 0.6 × 10-4

Polyisobutylene 1.7 × 105 0.02 × 10-4

Cyclohexane 84 0.01 × 10-4

However, it should be noted that k of the elastomers are about 2-6 times lower that those of the low molecular analogues. The accuracy of Ea determination does not allow estimation as to which of the two parameters A or Ea one should relate the decrease in k. If we assume that the mechanism of ozone reaction with monomers and elastomers is similar, i.e., the reactions are isokinetic, then Amon = Apol. At kmon/kpol = 2 - 6 the difference in Ea at 20 oC will be 0.5-1.0 kcal/mol. At the low experimental

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