Indian Journal of ChemistryVol. 22A, February 1983, pp. 173-174

Kinetics of Reaction of Bromo methylPheny 1 Sulphoxide with trans-

Cinnamate Ions

R S RAGHAVAN*, K SINGANAN & N MATHIVANAN

Department of Chemistry, Pachaiyappa's College,Madras 600030

Received 7 April 1982 ; revised 12 July 1982 and accepted18 August 1982

The reaction of bromomethyl phenyl sulphoxide withsubstituted trans-cinnamate ions in 90 per cent (v/v) acetone-water mixture follows second order kinetics. The reaction isfacilitated by electron-releasing groups and retarded byelectron-withdrawing groups in the cinnamate ion. TheHammett equation is found to be applicable giving a p-valueof -0.316. The attenuation ratio (It) is found to be 0.386.From a Bronsted-type correlation, 01; is found to be 0.73 at25°C. The reaction rate decreases with a rise in [cinnamate-ion], probably due to the existence of ion pairs in the mediumemployed.

The kinetics of reaction of phenacyl bromide withsubstituted anilines', benzoate ions", naphthoateions" and trans-cinnamate ions' have been reported.However, very few studies" have been reported onsimilar reactions of bromomethyl phenyl sulphoxidewith various nucIeophiles. The title investigationis an extension of our earlier work on the kinetics ofthe reaction of this bromosulphoxide with benzoateions in 90 per cent (v/v) acetone-water mixture".

Bromomethly phenyl sulphoxide was preparedfollowing the literature procedure? and purified bydistillation under reduced pressure. The substitutedtrans-cinnamic acids were prepared as described in theliterature8-12 and purified by recrystallisation fromsuitable solvents. The procedure for kinetic runs

was similar to that reported by us earlier",Equal volumes of equimolar solutions of reactants

in 90 per cent (v/v) acetone-water mixture weremixed and the reaction was allowed to go to comple-tion. Evaporation of solvent yielded a solidproduct, m.p. 118°. IR data [1500, 1454 (C=C,aromatic) ; 1240, 1140, 1064 (vas CoO) ; and 1070,1020 crrr+ (v S -+0)] of the product and its positiveresponse to unsaturation tests pointed out to structure(J) for the product.

o 0t "CeH5-S -CH2-O-C-CH=CH-C6H5

IThe reaction follows total second order kinetics,

first order in each reactant. The bromosulphoxidereacts with cinnamate ions according to Eq. (1)

C6HGSOCH2Br+XC6H4CH=CH-COO----+C6H5S0CH20COCH=CHC6H4X +Br .. (1)

The second order rate constants and the activationparameters for the reaction of the bromosulphoxidewith cinnamate and several substituted cinnamateions are given in Table 1. The kinetic data revealthat the reaction is facilitated by electron-releasingsubstituents and retarded by electron-withdrawingsubstituents indicating that the overall reaction ratedepends on the electron density on the oxygen atomof the carboxylate anion. The reactivities of differentcinnamate ions follow the order:p-NMe2>p-OH>p-OMe>p-Me>-H>p-CI>o-Cl>

m-F>m-CI>m-N02>o-N02>p-N02

Analogous reactivity orders have been recordedfor the reaction of phenacyl bromide with benzoateions" and also with cinnamate ions-. On the basis ofthe inductive effects of fluoro and chloro groups thegreater nucIeophilicity of the m-Buoro isomer relativeto the m-chloro isomer is rather surprising. Thistrend is, however, in conformity with the a valuesfor these two substituents C1m-F= +0.34; am_Ol =+0.37).

Table I-Second Order Rate Constants and Activation Parameters for Reaction of trans-Cinnamate Ions with BromomethylPhenyl Sulphoxide in 90% (vjv) Acetone-Water Mixture

[Cinnamate ion] = 0.02 mol dm-s, [Bromomethyl phenyl sulphoxide] = 0.02 mol dm"?103k.

(mol-l dm" S-I)Substituent E8

30° 35° 40°C (kJ mol=')

H 1.14 1.85 2.96 75.86poOH 1.53 2.40 3.73 70.21p-Me 1.26 2.01 3.17 73.08p-Cl 0.943 1.52 2.43 75.08o-Cl 0.872 1.42 2.27 75.86m-F 0.809 1.32 2.13 76.15m-Cl 0.773 1.26 2.04 76.58p-NMe2 2.050 3.28 5.15 73.00p-OMe 1.36 2.13 3.41 73.33m-N02 0.701 1.15 1.87 77.71o-NO. 0.648 1.07 1.74 78.48p-NOo 0.573 1.01 1.65 85.47

6Ht(kJ mol=')

73.3067.6470.5572.4973.3073.5874.0170.4770.7975.0875.9582.92

59.5075.6567.7863.7161.6762.1061.5563.9666.2857.7555.5633.27

91.6290.9791.4392.1292.3092.7292.9890.1691.2292.8393.0793.19

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INDIAN J. CHEM., VOL. 22A FEBRUARY 1983

The applicability of the Hammett equation wasrevealed by the linear plot of log ka of the m- and P:substituted cinnamate ions against the correspondinga values at different temperatures. The linear plotgives a p-value of -0.316 (at 35°) (corr. coeff.,r=0.993, s=0.0199); -0.332 (at 30°) (r=0.992,s=0.0231); and -0.305 (at 40°) (r=0.993, s=0.0197).The negative p-values imply that the reaction isfacilitated by electron-release from the benezene ringtowards the reaction centre. The decrease in p-valuewith increase in temperature is expected in view ofthe temperature independence of a (ref 12, 13).

The p-value for the present reaction (-0.316 at 35°)is slightly higher than the value of -0.208 (at 35°)reported by Arumugam et al," for the reaction ofphenacyl bromide with cinnamate ions; indicating asomewhat higher susceptibility of the reaction tothe polar effects of ring subtituents in the cinnamateion. The p-value is significantly lower than thatobserved for the reaction of benzoate ions (-0.305 at40° for cinnamate ions as against - 0.79 at 40° for

benzoate ions"). The attenuation ratio (n = -f. ) isPo

0.386. This compares excellently with the value of0.39 reported by Chapman et al.l5 for the reaction ofdiphenyldiazomethane with cinnamic acids and thevalue of 0.30 reported by Arumugam et al." . for thereaction of phenacyl bromide with cinnamate ions.This points to a consistently attenuated effectivenessof n-electronic transmission in cinnamic relative tophenyl system'".

A linear correlation is observed for the presentreaction when the value of log k2 are plotted againstp K; of the conjugate acids corresponding to the m-and p- substituted trans-cinnamate ions. The valueof the Bronsted coefficient (0() is 0.73 (r=0.979). The

. positive value of 0( signifies that the rate of thereaction increases with increasing basicity of thenucleophile. The value of IX. is close to the value of+0.75 reported for the reaction of the bromosulpho-xide with benzoate ions", indicating that there ismore or less the same degree of bond formationbetween the nucleophile and the reaction centre inboth these cases. The «-value is much higher thanthe value of 0.404 reported by Arumugam et al.4 forthe reaction of cinnamate ions with phenacylbromide, suggesting that with the replacement of

o 0II t

- C-by -S-, the extent of bond formation betweenthe nucleophile and the reaction centre increases.The isokinetic relationship", !:;Ht = !:;Ht +~!:;stis found to hold good in the present reaction series.A plot of !:;Ht versus L,st is linear with a slope~ (isokinetic temperature) = 4 13K (r=0.98)_

174

Table 2--Effect of [Sodium Cinnamate] on Second OrderRate Constants

Solvent: 90 per cent (vjv) acetone-water; temp 40°C

[Bromo- [Sodium 10' k,sulphoxide cinnamate] (mol-1 dm" S-I)(mol dm") (mol drrr=)

0.Q2 0.01 3.550.02 0.02 2.960.02 0.03 2.330.02 0.04 2.04

It is observed that ka decreases with increase in[sodium cinnamate] (Table 2) and a plot of log k'l.versus [cinnamate ion] is linear. This can be rationa-lised by considering that the salt is not completelydissociated since acetone is a poor solvating medium.The cinnamate ion and the sodium ion may be heldtogether by electrostatic attraction to give ion-pairssince 90 per cent acetone lacks adequate solvatingpower to free the ions from each other. At high[salt], the incidence of this phenomenon is morepronounced and hence the rate decreases.

We wish to express our grateful thanks to Prof SpShanmuganathan for encouragement and facilitiesand Dr B R Pai, Amrutanjan Ltd, Madras forassistance and helpful suggestions. One of us (K S)is thankful to the Government of Tamil Nadu forsponsorship under the State F I P Scheme. Partialfinancial support of the U G C, New Delhi is alsogratefully acknowledged.

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