Indian Journal of Chern is try Vol. 38A, June \999, pp. 547 - 552

Mechanism of oxidation of ethanolamines by sodium N-chloro-p­toluenesulphonamide in alkaline buffer medium: A kinetic study

S M Mayanna*, Puttaswamy & S Madhumathy Department of Post-Graduate Studies in Chemistry, Central College, Bangalore Uni versity, Bangalore 560 00 I, India

Received 25 July 1997: revised 6 January 1999

Kineti cs of ox idation of monoethanolamine (MEA), diethanolamine (DEA) and trieth anolamine (TEA) by sodium N­chloro-p -to luenesulphonamide (chloramine-T, CAT) in alkaline buffer medium has been studi ed at 3 13K. The reacti ons followed identical kinetics be ing first order in [CAT] and fractional order each in [substrate] and [OW]. Under comparable experimental conditions the rate of ox idation increases in the order: DEA > TEA > MEA. Addition of the reacti on product p-toluenesulphonamide retards the reacti on rate. Added chloride ions and change in ioni c strength of the mediulll do nut have any effect on the rate of the reaction . Change in di electric constant of the medium affects the reaction rate. Acti vation parameters for ihe rate determining step are eva lu ated and isokinetie relati on is observed with P=390 K ind icati ng th at the reactions are enthalpy-cont ro ll ed. Fonnation and decomposition constants of substrate - CAT complexes are eva lu ated . Suitable mechani sms and rate equat ions are proposed based on the f.l bserved kinetic data.

Ethanolamines' find extensive applications in the synthesis of surfactants, pharmaceutica ls and as addition agents in metal fini shing industries ' . Kinetic results have been reported in I iterature for the oxidation of ethanolamines by va ri ous ox idi sin g agents2

-6

. Much informati on is not ava ilable on the kinetics of ox idati on of ethanolamines by N­haloamines. The di verse nature of the chemistry of aromatic sulphonyl haloamines is due to their abili ty to act as sources of halonium cations, hypohalite species and N-ani ons which act both as bases and nucleophiles. As a result these reagents react with a wide-range of functi onal groups7 We report herein the results of ox idati on of ethanolamines by chloromine-T (p-CH3C6H4S02NCIN a. 3H20 or CAT) in alkaline buffe r of pH 9.69 at 3 13 K.

Experimental

Chloramine-T (E Merck) was purified by the method of Morri s et al8 An aqueous so lut ion of CA T was prepared., standardi sed iodometrically and preserved in amber co loured bottl es to prevent photochemi cal deterioration. Ethanolamines were of accepted grades of purity and were used without further purification. Standard buffer system (borax+NaO H) were employed. A ~nstant ioni c strength of the reacti on medium was maintained at 0.50 mol dm-' by adding required amount of concentrated NaC I0 4 solution. All other chemicals were of analytical grades. Doubly di stilled water was used for preparing aqueous solutions.

Kinetic measurements Mixtures containing requi site amounts of substrate ,

NaCI04 and buffer were taken in stoppered pyrex glass tubes whose outer surface was coated wi th bl ack paint. The tube was thermally equilibrated in a water bath (3 13±0.1 K). A measured amount of CAT so lution pre-equilibrated at the same temperature, was added and the reaction mi xture was shaken. The progress of the reaction was monitored iodollletrica lly by withdrawing aliquots (5 ml each) of the reacti on mixture at regular time interva ls. The course of tbe reaction was studied for two half li ves. The pseudo­first order rate constants, k, ca lculated were reproduc ible within ± 3%.

Stoichiometry and product analysis Varying ratios of CAT to ethanolamines

([CAT]»[ethanolamine]) were equilibrated at 3 13K in the presence of alkaline buffer for 48h. Estimat ion of unreacted ox idant lead to stoic hi ometric reactions ( I, 2 and 3).

H2N (C H 2C1 1 20 H )+2 RNC INa+2H20~HCHO+

HCOO H+NH1+2RNH2+2NaC I ... (I) HN(CH2CH 20Hh+4RNC INa+4 H 20~2HCHO+

2HCOOH+NH1+4RNH2+4NaC I ... (2) N(CH2CH20H )3+4 RNC IN a+4 H 20~CH3CHO+2 HC

HO+2 HCOO H+N H3+4RNH 2+4NaC I ... (3) where R=p-C H,C(, H.1S02-

p-To luenesulphonam ide was identi fi ed'l among the reaction products by paper chromatography using benzy l alcohol saturated with water as the sol vent ancl

548 INDIAN J CHEM, SEC. A, JUNE 1999

Table I-Effect of reactants on the rate of reaction pH=9.69; 11=0.5 mol dm-3

; temp=313 K

104[CAT]o 103[EA]o k xI05(s- l)

(mol dm-3) (mol dm-3

) MEA DEA TEA

8.0 10.0 0.89 65.1 7.70

9.0 10.0 0.87 67.1 7.85

10.0 10.0 0.89 65.1 7.65

11.0 10.0 0.88 66.2 7.85

12.0 10.0 0.88 65.5 7.65

13.0 10.0 0.86 65.0 7.50

14.0 10.0 0.89 67.0 7.70

10.0 5.0 0.64 51.0 5.05

10.0 10.0 0.89 65 .1 7.65

10.0 20.0 1.28 85.0 11.1

10.0 40.0 1.71 III 18.2

10.0 60.0 2.22 131 23 .1

10.0 80.0 2.59 156 28.2

10.0 100 2.90 171 32.5

0.5% vani llin in 1% HCI solution in ether as the spray reagent (RFO.905). Formate ion was detected by the chromatropic acid procedure'o and formaldehyde by schiffs reagent test". Acetaldehyde in the case of TEA was identified by iodoform and nitroprusside tests". Ammonia was detected by Nessler's reagent test according to the method of Vogel'2 and quantitatively estimated by the microkjeldahl procedure. The experiments were also carried out separately witli I x 10-3 mol dm - 3 oxidant and I x I 0-2

mol dm-3 acetaldehyde, formaldehyde and formic acid and rate constants were evaluated . These values are much lower «5%) when compared to the rate constant values of ethanolamines under comparable experimental condition.

Results and discussion Oxidation of ethanolamines by CAT was carried

out at 313K in various buffer media. The reaction was found to be facile in the pH range 8.5-10.5. Hence a detailed investigation was made on the kinetics of oxidation of ethanolamines in alkaline buffer solution of pH 9.69.

Dependence of rate on [CAT} and [EA}

Kinetics of oxidation of ethanolamines by CAT in alkaline buffer solution of pH 9.69 was investigated at several initial [CAT]. Plots of log [CAT] versus time werc linear indicating a first order dependence of rate on [CAT]. Similar results were noticed with other buffer solutions. The pseudo-first order rate constants,

Table 2-Thermodynamic parameters for the oxidation of ethanolamines by CAT in alkaline buffer solution

[CAT]o=IO.OxlO-"'mol dm-3; [EA]o= IO.Ox 10-lmol dm

3; pH=9.69; 11=0.5 mol dm-3

Thermodynamic parameter

MEA DEA TEA

Ea (kJ mol- I)

Mr(kJ mor ' )

/IS (JK- I mor ' )

tlG~ (kJ mor ' )

110

107

- 1.50

107

65.6

63.0

90.1

87.5

- 104 -45.8

95 .8 102

k, are independent. of [CAT] and these values are given in Table I. An increase in [ethanolamine] increases the rate of reaction (Table I) and a plot of log k versus log [EA] was linear. The order in each of the ethanolamines was found to be fractional (0.50 for MEA, 0.40 for OEA and 0.75 for TEA) . Under • comparable experimental conditions the reaction rates were in the order: OEA > TEA > MEA.

Dependence of rate on [OH} The reaction rate increases with increase in pH of

the reaction medium. Plots of log k versus pH were linear with fractional slopes (0.30 for MEA, 0.46 for OEA and 0.32 for TEA), giving positive fractional orders in [OH-].

Effect of ionic strength and p-toluenesulphonamide on the rate

A constant ionic ~trength (0.5 mol dm-3, NaCI04)

was maintained during oxidation . However, the effect of change in ionic strength of the medium (0.1-0.8 mol dm-3

) on the reaction rate was found to be negligible. Addition of the reaction product, p­toluenesulphonamide to the reaction mechanism retarded the rate. Further, plots of log k versus log [RNH2] were linear with negative fractional slopes (- ' 0.20 for MEA, -0.18 for OEA and -0 .2 1 for TEA).

Effect of dielectric constant, temperature and so/vent isotope on the rate

The effect of solvent dielectric constant on the kinetics of oxidation of ethanolamines was a lso studied by adding methanol to the reacting system . A decrease in the rate constant was noticed with decrease in the dielectric constant of the reaction . Plots of log k versus 110 were I inear with negative slopes for all the amines. The reaction rates were studied at different temperatures (303- }23 K). From the linear Arrhenius plots of log k versus I IT. activation parameters (~u, ~) and &r) for the overall reactions were evaluated. These values are presented in Table 2. The reaction between CAT

MA Y ANNA et al. : OXIDATION OF ETHANOLAMINES BY CHLORAMINE-T 549

Table 3--Values of K" formation constant K2 and decomposition constant k3 (from double reciprocal plots)

Ethanolamine 104 KI K2 k3 (dm3mor l

) (S- I)

MEA 7.77 47.3 2.87x 10-5

DEA 5.69 61.0 1.82x 10-3

TEA 10.0 28.8 3.44x 10-4

(l.Ox 10-3 mol. dm - 3) and OEA (l.Ox 10-2 mol dm -3)

was studied in 0 20 medium containing buffer (PH 9 .69) at 313K . . The solvent isotope effect, k(H20)/k(020), was found to be 0.94.

Oxidation potential of CAT -PTS system decreases with increase in pH of the medium' 3. In alkaline solution of CAT, RNCI 2 or HOCI does not exist. In the present alkaline solution one could expect the presence of RNCr anion and RNHC I. The latter, however, is formed in base retarding reactions'4 .

~onoethanolamine

H H

HO~O + H

Br

-HBr ~V H HCHO-4--H~O~

Table 4---Values of the decomposition constant (k3) of ethanolamine-CAT complex at different temperature

[CAT]o=1 O.Ox lO-4mol dm··3; [EAlo=IO.0x 1O-3m~1 dm-3; pH=9.69; f..l = 0.5 mol dm-3

Temperature 105xk3(s- ')

(K) MEA DEA TEA

308 1.04 103 25.0

313 2.87 182 34.4

318 4.54 333 100

323 8.00 500 333

RNCr +H20 ( ) RNHCl+OH ... (4)

Hardy and Johnston' 5 have indicated that there is considerable concentration of RNHCI even in alkaline CAT solution . Further with increase in [NaOH] , RNHCI gives NaOCI. First order dependence of rate on [CAT], fractional order dependence each on [EA]

e -H

~ H

HO~N/

H Br

+ .4---- .\.. b O HO~ (H

H20

Scheme-2a

550 IND IAN J CHEM, SEC. A, JUNE 1999

Di~thanolamine

r<0 HO H +

~ 1 mole

+ Br_Oe*

• HCHO+ HCOO

*As shown in scheme 2a

Tr iethanolamine

HO, /"... H ~ '~j

HO~ "Br

( X)

HO~ NH2

~ 2 moles

+. Br_Oe * t

NH3+ HCHO+HCOO-

Scheme -2b

e - 3moles Br-O *

NH3+2HCHO + 2HCOO ~ ~ ..-HO~

*As shown in scheme 2b

N - Br + HO~/

( Xl )

Schem e -2 c

~OH 1 ~

CHf HO

Tabl e 5--K in ctic and thermodynami c parameters fo r the rate and [OW ] and the observed reta rdati on o f rate by the reaction prod uct RNH2 can be explained by Scheme I :

li mi ting step

Ethano lamin e Ea Mt t:S ~G" (kJ mor l

) (kJ mor l) (JK- I mor l

) (kJ 11101- 1)

MEA 12 1 11 9 43 .1 105

DEA 82 .9 80.3 -4 1.2 93.3

;.: RNHC l+O H ~ RNH 2+OC I .. . ( i) fast

T EA 11 2 109 40.1 96.4 f·: J

oel +EA ~X .. . ( i i) fa st

MAY ANNA el af.: OX IDATION OF ETHANOLAMI ES BY CHLORAMINE-T 551

X~X' X '+n ocr ~ Products

... (iii) slow and rls

... (i v) fast

Scheme 1

Here n= 1 for MEA, n=3 for Dr .r\ In ·A.

From Scheme I , assum ing [OCr]+(X]

Ilt-= [RNHCI]+

Rate law (5) can be dC:1 i ed.

_ - d[CAT] k1 K , /\ ~ r CATl IF HOW l .' te- = - -- -- - -df [R NH .1 ~ K.fOW l fI I ~~ [EA]}

... (5 )

Rate law (5 ) is ill accorda l1L'c \\ "1 111(' observed ki ne tic bta and call be Ira nsl'urilled ! :0 I lj s. (6) and (7). Since rate=k[CATj I where [(,\'1'1 , represent::. the total [CATI I rRNH , l I - -= ------- - + -- -. k ~3 K I K2 rEA][() I-I' ] k,Kcl E: 1 l,

I -- ... (6)

... (7)

From the double reci procal plol'> of I 'k versus I / [EA] and I lk versus [RNI-I 2] , values l)f K1, form ation constant K2 and decomposit ion constants k, were calculated Crable 3). K I val ues arc fairly constant supporting the proposed mec ha ni sm. The values are small indicating the smal l equi librium because of the acid - base character of the reaction. The formation constants K2 are in the order DEA > MEA > TEA ; on the other hand decomposi tion constants (kJ) which indicate the stability of X are in the order DEA < TEA < MEA. The decomposition con tant k, fo r the rate­limi~ing step was al so determi ned by varying [EA]o at different temperatures (308-323K) and the values are given in Table 4. Using these va lues activation parameters for the decompos ition step of the ethanolamine -CAT complex [step (iii) of Scheme I] were determined by an Arrhen ius plofof log k3 versus lIT, these are shown in able 5. A detailed mechanism of oxidation of ethanolamines by CAT is shown in SchelJle 2.

The non-bonding electron pai r is important in the chemistry of amines, since it is responsible for the typical basic and nucleophilic properties of these compounds. In the series MEA, DEA and TEA since ethanoyl (-CH2CH20H) group has a+1 effect, the larger the number of these groups on nitrogen atom

the greater will be the electron density on it. The increasing negative charge on the central nitrogen atom favours the attack of electron deticient oxidising species at thi s centre. It is anticipated that TEA should form complex with electrophilic species read ily. However. in the present study the increasing order of .the reaction rate is found to be DEA > TEA > MEA, vhich is anomaloLi s compared to the expected order

i.e., TEA > DEA > MEA. This is attributed tl) an inc reasing steric effect, which is operating in the case of TC , where side cha ins may be hindering the ;1 proach of the reagen t towards nitroge n atom. In the ca:;(' of DCA hoth po ar and steric factors arc fOll nd to favour the reaction It is noticed that TEA reacti on is slow in itially. ;\s SOtln as a molecule or CH1CHO is elf1ninated, the reaction is fOllnd to be raster as shO\\n in Scheme 2C. ~ It is seen from Tab le 2 that the acti vation energy is

highest for the slowest reaction, ind icat ing thar the react ion is enthalpy-controlled. r urther, the va lu es of L\ft and .1\...')'" can be corre lated linearly result ing in an isokinetic relati onship . rrom the slope, the val ue of isoki netic tem perature f3 is found to be 390K. wh ich is much hi gher than the experi mental tem perature indicating that the reaction is enthalpy-controlled . The reaction is characteri sed by a moderate energy of act ivati on and the near constancy of !t.G" values indicates that a similar mechanism is operati ve in the oxidation of ethanolamines by CAT. Values of !t.S" are negative and are in the order DEA > TEA > MEA. Thus the transition state appears to be more ordered and rigid for the diethanolamine while in the case of MEA the transition state more or less resembles the reactants.

For the limiting case of zero angle of approach, between two dipoles or an ion-dipole system, Amis l 6

has shown that a plot of log k versus liD is linear with a negative slope for a reaction between a negative ion and a dipole or between two dipoles, while a pos itive slope results for a positive ion-dipole interaction . The negative dielectric effect observed in the present studies clearly supports the ion-dipole interaction as given in Scheme I. Solvent isotope studies show that k (H 20)/k(D10) < I. This is generally correlated with the fact that OD- is a stronger base 17 than 01-1 and hence in base catalysed reaction enhancement of rate in 0 20 medium can be expected .

Acknowledgement The authors are grateful to Dr. D.S. Mahadevappa,

Emeritus Professor, Department of Chemi stry,

552 INDIAN J CHEM. SEC. A, JUN E 1999

University of Mysore, Mysore and Dr. M. A. Pasha, Department of Chemistry, Bangalore University, Bangalore, for their helpful discussions.

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