Oxidation of Some Aliphatic Alcohols by Pyridinium

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2009, 6(1), 237-246

Oxidation of Some Aliphatic Alcohols by Pyridinium

Chlorochromate -Kinetics and Mechanism

SAPANA JAIN*, B. L. HIRAN and C.V.BHATT

Chemical Kinetics and Polymer Research Laboratory,

Department of Chemistry,

Mohan Lal Sukhadia. University, Udaipur (Raj)-313 001, India.

[email protected]

Received 26 June 2008; Accepted 20 August 2008

Abstract: Kinetics of oxidation of some aliphatic primary and secondary

alcohols viz. ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and 2-

methyl butanol by pyridinium chlorochromate (PCC) have been studied in water-

perchloric acid medium. The reaction shows first order dependence with respect

to pyridinium chlorochromate [PCC] and hydrogen ion [H+]. The rate of

oxidation decreases with increase in dielectric constant of solvent suggests ion-

dipole interaction. Activation parameters have been evaluated. Products are

carbonyl compounds and free radical absence was proved. A tentative

mechanism has been proposed.

Keywords: Kinetics, Oxidation, Aliphatic alcohols, Pyridinium chlorochromate, Perchloric acid-water

Introduction

In 1975, Correy and Suggs1 reported PCC, C5H5NHCrO3Cl as a readily available stable

reagent, oxidizes a wide varity of alcohols to carbonyl compounds. It is used as an oxidant

for of alcohols2-4

, amino acids5-6

, aldehydes7-10

, L-cystine11

and aniline12

etc. Oxidation of

alcohols, deuteriated alcohols, cycloalkanols13

, vicinal and non-vicinal diols14-17

and

homobenzylic alcohols18

etc. has been reported. We described here comparative kinetics of

oxidation of some aliphatic alcohols and also the appropriate reaction mechanism.

Experimental

All chemicals were used of ‘Anala R’ grade. Double distilled water was used as a medium.

All the alcohols were used after their distillation by proper method and purity checked by

their boiling point. The solution of perchloric acid was prepared by diluting known volume

of acid in water and standardized by sodium hydroxide using phenolphthalein as an

indicator. PCC is prepared by improved method of Correy and Suggs described by Agrawal19

.

238 SAPANA JAIN et al.

The orange solid, which is collected on a sintered glass funnel dried for 1 h in vacuum and

m.p. (148-150ºC) checked. The solid was not hygroscopic and highly soluble in water,

acetonitrile, DMF etc. PCC solution was prepared by dissolving the known amount of this

reagent in water and standardized by iodometrically using starch indicator.

Kinetic measurements

The solution of oxidant in acid-water medium obeys Lambert Beer’s law i.e. absorbance

versus [oxidant] is a straight line therefore reactions were followed by monitoring the

decrease in oxidant concentration.

The reaction have been arranged to study under pseudo first order conditions by keeping

the excess of substrate upon oxidant i.e. [Substrate]>> [Oxidant] ratio not less than 8:1 in any

reaction set. The reactions were carried out in a glass stoppered cell at constant temperature

±0.1ºC. The reaction mixture was prepared by mixing the requisite amount of substrate,

perchloric acid and water and allowed to stand in a thermostatic bath for a sufficient length of

time. Adding the solution of the oxidant started the reaction and mixed well. The optical

density of the reaction mixture was followed spectrophotometrically at 354 nm by using

Simadzu U.V/Visible spectrophotometer model Jasco 7800 with recording facilities.

Product analysis and stoichiometry

Product analysis was carried out under kinetic conditions. In a typical experiment, large

volume containing [alcohol] is in excess over [PCC] and kept for reaction completion.

Moles of PCC consumed were determined by difference in absorbance before and after

completion of reaction. The whole reaction mixture (after completion of reaction) was

treated with 2,4-dinitrophenylhydrazine. A yellow-orange precipitate obtained which was

filtered, washed, dried and weighed. From the weight of the precipitate, moles of carbonyl

compound (product) were determined and hence stoichiometry was confirmed.

Conformation of carbonyl (aldehyde/ketone) compound was done by melting point, IR and

nitrogen percentage analysis of precipitate obtained. Cr(III) was confirmed by visible spectra

of the reaction solution after completion of reaction. The stoichiometric equation is:

33RR'CH(OH) + 2Cr(VI) + H 2O 3RR ' C O 2 Cr (II I ) H O 7H+ +→ = + + +

There was no change in rate or absorbance on addition of stabilizer free acrylonitrile in

nitrogen atmosphere. This confirms absence of free radical in these oxidations.

Results and Discussion

Effect of oxidant concentration

The reactions are of first order with respect to PCC i.e. log absorbance versus time is straight

line for more than 80% reaction. Further the value of kobs is independent of the initial

concentrations of PCC. The rate can be expressed as:

-d [PCC] / dt = k′ [PCC]

Effect of substrate concentration

The rate of oxidation increased on increasing the concentration of alcohols (Table 1). Plot of

log kobs versus log [substrate] is a straight line in all the cases with slope 1.0, 1.01, 0.89,

0.98, 1.02, 0.88 respectively for ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol

and isoamyl alcohol i.e. first order with respect to substrate. Plot of 1/kobs versus

1/[Substrate] gave linear line passing through origin or very small intercepts nearly zero

suggest that the rate does not obey Michalis Mentane type kinetics.

Oxidation of Some Aliphatic Alcohols by Pyridinium Chlorochromate 239

Figure 1. Variation of rate with substrate concentration.

Table 1. Variation of rate with substrate concentration.

[PCC]=2x103 M; [HClO4] = 5x10

1 M; Temperature = 298 K

kobs x 104, sec

1

Substrate x 102

mol dm3 Ethanol Propan-1-ol Propan-2-ol Bunan-1-ol Butan-2-ol

2-Methyl

butanol

1.0 2.43 2.82 1.54 2.14 2.87 4.99

1.5 3.41 3.99 2.30 3.09 4.27 6.98

2.0 4.45 5.22 3.31 4.41 5.70 8.77

2.5 5.79 7.07 3.58 5.24 7.39 11.01

3.0 7.67 8.06 4.34 6.18 8.12 13.25

4.0 9.97 11.51 5.05 8.33 12.06 16.37

5.0 12.56 13.56 6.14 10.55 14.50 19.95

Effect of pyridine and picolinic acid concentration

The addition of pyridine and picolinic acid does not affect the reaction rate. This suggests

that PCC is quite stable in perchloric acid water medium in the concentration range studied

and dose not dissociate to chromic acid

Effect of ionic strength

There was no effect of SO42-

and CH3COO¯ observed on the reaction rate in the debye Hukle

limit. It proves that interaction in rate determining steps is not ion-ion type and one of the

reactant molecules is neutral.

2 + log [substrate]

4 +

log

k o

bs

R2 = 0.9926

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Prop an-2-ol

Ethanol

Prop an-1-ol

Butan-1-ol

Butan-2-ol

2-methy l butanol


Effect of perchloric acid concetntration

The effect of hydrogen ion concentration on the rate of the oxidation was studied by varying

[H+] while keeping the concentration of other reactants constant. Since there is no effect of

ionic strength on reaction rate therefore ionic strength was not kept constant. A steady

increase in oxidation rate with increase in the acidity of the medium suggests the formation

of protonated PCC in the rate determining step20

. The plot of log kobs against log [H+] is

linear with slopes 2.3, 2.2, 1.9, 2.14, 2.10, 2.0 respectively for ethanol, propan-1-ol, propan-

2-ol, butan-1-ol, butan-2-ol and isoamyl alcohol i.e. second order, suggesting that two

protons may involve in the rate determining step21

(Table 2).

Table 2. Variation of rate with perchloric acid concentration

[PCC]=2x103 M; Temperature = 298 K; [Alcohol] = 2x10

2 M.

kobs x 104, sec

1 [HClO4] x 10

1

mol dm3 Ethanol Propan-1-ol Propan-2-ol Bunan-1-ol Butan-2-ol 2-methyl butanol

2.5 - 1.16 - 1.15 1.60 2.61

3.0 - 1.62 1.70 1.75 2.19 3.56

4.0 3.21 3.63 3.18 3.20 4.13 6.73

5.0 5.47 5.22 3.81 4.50 5.70 9.06

6.0 9.34 8.04 7.32 8.04 10.41 14.91

7.0 12.82 12.92 9.40 10.36 14.52 19.62

7.5 15.35 16.90 10.8 12.60 17.10 22.64

9.0 20.34 25.51 13.2 20.10 29.60 35.11

10.0 32.24 41.26 13.7 27.70 40.30 48.90

12.5 64.06 - 30.06 39.92 - -

Figure 2. Variation of rate with prechloric acid concentration.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

+

Ethanol

Propan-1-ol

Propan-2-ol

Butan-1-ol

Butan-2-ol

2-methyl butanol

1+ log [H+]

4 +

lo

g k

ob

s


Figure 3. Variation of rate with solvent composition.

Figure 4. Variation of rate with temperature.

4 +

lo

g k

ob

s

R2 = 0.998

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0.0030 0.0031 0.0032 0.0033 0.0034 0.0035

(1 / T)

4 + lo

g k

obs

Propan-2-olEthanolPropan-1-olButan-1-olButan-2-ol2-methyl butanol

4 +

lo

g k

ob

s

4 +

lo

g k

ob

s

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60

1/D x 10-2

Ethanol

propan-1-ol

propan-2-ol

Butan-1-ol

Butan-2-ol

1/T


Figure 5. Zucker hammett plots in perchloric acid media.

Figure 6. Bunnett plots in perchloric acid media.

H0

H0

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

4 +

log

kob

s

Ethanol

Propan-1-ol

Propan-2-ol

Butan-1-ol

Butan-2-ol

4 +

lo

g k

ob

s

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.022 -0.017 -0.012 -0.007 -0.002

a

3 + lo

g k

obs-l

og[

H]

Ethanol

Propan-1-ol

Propan-2-ol

Butan-1-ol

Butan-2-ol

3 +

log

ko

bs+

log

[H+]

log aH2O


.

Figure 7. Bunnett plots in perchloric acid media.

Effect of solvent composition

At fixed ionic strength and [H+] the rate of oxidation of alcohols with PCC increases with

decrease in polarity of solvent. In other words a decrease in rate with increase in dielectric

constant of solvent (1,4-dioxane) is observed. This is due to polar character of the transition

state as compared to the reactants. According to Scatchard22

, the logarithm of the rate

constant of a reaction between ions should vary linearly with the reciprocal of the dielectric

constant if reaction involves ion-dipole type of interaction (Table 3).

Table 3. Variation of rate with solvent composition

[Alcohol] = 2x102

M [HClO4] = 5x101 M [PCC] = 2x10

3 M Temp. = 298 K.

kobs × 104, sec

1 Solvent composition

1,4-dioxane % v/v Ethanol Propan-1-ol Propan-2-ol Bunan-1-ol Butan-2-ol

0 5.47 5.22 3.18 4.50 5.67

10 7.28 8.21 3.59 5.65 12.61

20 10.06 10.31 4.58 10.36 17.27

30 14.20 13.40 5.68 12.22 23.67

40 18.23 21.11 7.91 19.95 33.00

50 41.50 35.50 9.97 44.83 57.70

Effect of temperature

The rates of oxidation of alcohols were determined at different temperature (Table 4) and the

reactions obey Arrhenius equation. Energy of activation was calculated by slopes of straight

line obtained plotting log k versus 1/T. The activation parameters for ethanol, propan-1-ol,

propan-2-ol, butan-1-ol, butan-2-ol and 2-methyl butanol were calculated (Table 5).

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-0.020 -0.016 -0.012 -0.008 -0.004 0.000

Ethanol

Prop an-1-ol

Prop an-2-ol

Butan-1-ol

Butan-2-ol

Isoamy l alcohol

4 +

log

ko

bs+

H0

log aH2O


Table 4. Variation of rate with temperature

[Alcohol] = 2×10–2

M; [HClO4] = 5×10–1

M; [PCC] = 2×10–3

M.

kobs × 104, sec

–1 Temp,

K Ethanol Propan-1-ol Propan-2-ol Butan-1-ol Butan-2-ol 2-Methyl butanol

293 - - - - 5.64 -

298 4.22 5.22 3.41 4.50 7.62 7.14

303 4.98 6.02 4.82 6.42 10.14 9.21

308 6.49 8.60 6.93 9.59 13.84 11.64

313 8.81 11.40 9.16 12.61 18.73 13.92

318 11.21 14.71 12.91 17.05 26.84 18.21

323 14.12 20.10 8.81 21.87 - 23.2

Table 5. Thermodynamic parameters for various substrates

[Alcohol] = 2x102

M [HClO4] = 5x101 M [PCC] = 2x10

3 M Temp. = 303 K.

kobs x 104, sec

1

Thermodynamic

parameter Ethanol Propan-1-ol Propan-2-ol Bunan-1-ol Butan-2-ol 2-Methyl

butanol

Energy of activation

∆Ea# ,

kJ mol1

38.29 48.50 53.61 50.1 46.57 36.39

Entropy of activation

∆S#, J mol

1 K

1

-108.44 -66.63 -58.81 -62.37 -70.10 -104.58

Free energy of

activation ∆F#,

kJ mol

1

68.13 65.88 68.65 66.25 64.95 65.04

Discussion

Kinetics of oxidation of aliphatic alcohols by PCC was investigated at several initial

concentrations of the reactants. At low concentrations of PCC and when substrates are in

large excess, the reaction is found to be first order in PCC. The plot of log (a-x) ie log

absorbance against time is found to be linear with a correlation coefficient 0.9982, showing

first order dependence in PCC. A plot of log k1 vs log [Substrate] gave a straight-line with

slope ≈ 1 showed first order dependence over substrate. The thermodynamic parameters are

mentioned in Table 5. The entropy of activation is negative as expected for a bimolecular

reaction. The negative value also suggests the formation of a cyclic intermediate from non-

cyclic reactants in the rate determining step23

. The large negative value of ⊗S suggests that

the transition state is less disorderly than the reactants24

.

A study increase in the oxidation rate with an increase in the acidity of the medium

suggests the formation of protonated PCC in the rate-determining step. The plot of log k1

against log [H+] is linear with a slope of nearly two suggesting that two protons may involve in

the rate-determining step. Different possibilities are one for PCC and other of substrate or both

the protons are taken by oxidant. Since protonation of alcohol is less probable it is more likely

that both protons are used by the oxidant. Many workers have suggested protonated PCC25-27

.

The protonated PCC and alcohol combine to give intermediate, which was also indicated by

decrease in rate with increase in dielectric constant of reaction medium due to more polar

character of the transition state as compared to the reactants which is further attacked by proton.


Although the intermediate is already positively charged hence second proton attack will be

difficult and hence very slow. Energy of activation suggests C-H bond breaking in rate

determining step and negative entropy of activation indicates formation of cyclic from

non-cyclic or more polar than reactants structure formation. From H+ effect and applying

various hypothesis like Zucker-Hammett28

, Bunnett29

and Bunnett-Olsen30

concludes

water molecule is acting as proton abstracting agent or say solvent helps to remove proton.

According to Zucker-Hammett hypothesis plot of log kobs against H0 is straight line in range

between slope ≈1.Bunnett has proposed that plots of log kobs + H0 against log aH2O (activity of

water) is linear or approximately so. The slope in such plots constitutes a parameter called

Bunnett function and designated as ω. If the ω value > +3.3 suggest water to act as proton

abstracting agent in rate-determining step. In case of Bunnett-oleson plots log k1 – log

[H+] versus log aH2O is linear with slope ω

value > -2.0 means water act as a proton-

abstracting agent in rate determining step. The slopes in such plots are in good agreement

with the criterian given by Zucker Hammett and Bunnett-oleson. This proves that water

act as a proton-abstracting agent in oxidation reaction. B.L. Hiran et al31

observed the

same results in the oxidation of C3 alcohols viz allyl alcohol, 1-propanol and 2-propanol

by 3-methyl pyridinium bromochromate in acid medium. ∆E# versus ∆S

# is almost straight

line indicates similar mechanism is operating in all the compounds which have been taken

for study.

According to the above results and data following mechanism has been proposed.

Mechanism

.

Over all reaction

33RR'CH(OH) + 2Cr(VI) + H 2O 3RR 'C O 2 Cr (III) H O 7H+ +→ = + + +

The rate law can be given as follows:

Rate = k' [Oxidant] [Substrate] [H+]

2

Rate = kobs [Oxidant]

kobs = k' / [Substrate] [H+]

2

This rate law and suggested mechanism explains all the observed facts.


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Oxidation of Some Aliphatic Alcohols by Pyridinium