Indian Journal of ChemistryVol. 22A. May 1983, pp. 415-416

Kinetics & Mechanism of Cerium(IV)Oxidation of Cinnamic & Substituted

Cinnamic Acidst

S VENIMADHAVAN & T R BALASUBRAMANIAN*

Department of Chemistry, Ramakrishna Mission VivekanandaCollege, Madras 600004

Received 13 September 1982; revised and accepted 25 Norember 1982

The kinetics of oxidation of cinnamic and substituted cinnamicacids by eerie ammonium nitrate has been studied in acetonitrile-water (80:20, % v/v). The reactions follow Michaelis-Menton type ofkinetics suggesting the formation of complex between Ce(IV) andcinnamic acids. The equilibrium constant (K) is found to besusceptible to electronic effects. The reaction occurs by a free radicalmechanism as revealed by the formation of polystyrene and carbondioxide.

Though the literature on the kinetics and mechanismof oxidative decarboxylation of carboxylic acids by avariety of metal oxidants like Co(III), TlOII), Ag(1I)and Ce(IV) is quite exhaustive", similar studies usingCe(IV) as an oxidant are limited to aliphatic acids" andphenylacetic acids:'. The present work is an attempt tostudy the kinetics and mechanism of oxidation of CI., {3-unsaturated acids, viz. cinnamic acid, m-nitro-, p-chloro-, m-chloro-, and p-methyl-cinnamic acids byeerie ammonium nitrate (CAN).

The reactions were carried out in acetonitrile-water(80:20, % v/v) and the progress of the reaction wasmonitored by following the disappearance of Ce(IV)with time (i) titrimetrically and (ii) spectrophoto-metrically at 405 nm using Baush and LombSpectronic-20 spectrophotometer.

The reactions exhibit a first order dependence in[Ce(IV)] as evidenced by the linearity of the plots oflog [Ce(IV)] versus time. However, at constant[Ce(IV)], the order with respect to the substrates is notsimple and the slopes of the plots of log kobsversus log[substrate] are not strictly unity. This means that thereactions follow Michaelis-Menton type of kineticssuggesting the formation of a complex between CANand cinnamic acids. The plots of l/kobs versusI/[substrate] are also linear with definite intercepts onthe ordinate axis for all the substituted cinnamic acidsstudied suggesting a rate law of the form (1)

tThis paper was presented at the Annual Convention of Chemists,1980 held at Indian Institute of Technology. Bombay duringDecember 1980.

Table 1- Effects of Substituents on k obs

[CAN = 2 x 10 -3 mol dm -3; solvent: 80/~ CH3CN-20,/o H20(v/v); [HN03] = 0.8 mol dm -3; [NaN03] = 0.2 mol dm -3; temp

=27cc[Substrate] kOb' x 104 (s -I) for R =mol dm >'

H* m-CI m-N02 p-C1 p-Me

0.01 1.06 2.73 2.94 2.02 1.81

0.015 1.46 2.94 3.35 2.67 2.470.02 1.90 3.45 3.85 2.86 2.990.025 2.30 3.63 4.03 2.99 3.39

"Spectrophotometric value at i. = 405 nm at 2TC

Table 2-Effect of Substituents on K and k,[CAN] =2 x 10 -3 mol dm -3; solvent: 80,!;, CH3CN-20% H20 (v/v);

[HN03] =0.8 mol dm -3; temp 27'C [NaN03] =0.2 mol dm >'Cinnamic acid k, x 104(s -I) K(dm3 mol "")

Parent 3.14 26.1p-Methyl 8.69 26.6m-Chloro 4.67 143p-Chloro 3.64 143m-Nitro 5.56 131

- d[Ce(IV)] = k [C (IV)]dt obs e

kIK[substrate] [Ce(IV)]I + K[substrate]

.. .(1)

The values of K, the equilibrium constant or theformation constant of Ce(IV)-cinnamic acid complexand k., the disproportionation constant of the complexhave been calculated from the plots of l/kobs versusL'[substrate]".

It is noticed that the formation constant K issusceptible to electronic effects (Tables 1 and 2). It isinteresting to note that the magnitude of the formationconstant falls into tW9 groups. This could be due to theoperation of mesomeric effect of the substituents likenitro and chloro which erihances the coordinatingability of oxygen.

When the disproportionation constants (kJ of theparent and substituted cinnamic acids are cast into aHammett plot, it is found that the parent along withsubstituted cinnamic acids having electron-withdrawing substituents falls on a line whileconsiderable deviation is observed in the case ofsubstituted cinnamic acids having electron releasingsubstituents like p-methyl. The kinetics of P:nitrocinnamic acid could not be followed because of

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

solubility problems and the reactions of p-methoxycinnamic acid was found to be too fast to bemeasured under the experimental conditions.

The intermediacy of the free radicals is implied bythe qualitative tests for vinyl polymerisation, rateretardation in the presence of oxygen atmosphere anda low Hammett rho value of + 0.46. Product analysisshows that when eerie ammonium nitrate in aqueousacetonitrile containing nitric acid is treated withcinnamic acid, carbon dioxide, polystyrene andanother product (in < 10% yield) are obtained. Theisolation of polystyrene indicates that it can be derivedfrom the initially formed styryl radical. The styryl

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radical, once formed can give styrene which undergoesready polymerisation to give polystyrene. The IRanalysis of the minor product (m.p. 80CC) shows that ithas all the characteristic features of cinnamic acid.However, the melting point was quite different fromthat of cinnamic acid (m.p. 132CC).

ReferencesI Bamford C H & Tipper C F H, Comprehensive chemical kinetics,

Vol. 7, (Elsevier Publishing Company, Amsterdam) 1972.2 Sheldon R A & Kochi J K, J Am chem Soc, 90 (1968) 6688.3 Trahanovsky W S, Cramer J & Brixius D W. J Am chem Soc. 96

(1974) 1077.4 Balasubramanian T R & Venkatasubrarnanian N, Indian J Chern,

9 (1971) 1243.