Indian Journal of Chemistry Vol. 40A, November 2001 , pp.1182-1186

Kinetics and mechanism of the oxidation of some unsaturated acids by 2,2'-bipyridinium chlorochromate

Shashi Vyas, Pradeep K Sharma & Kalyan K Baner/

Department of Chemistry, J.N. V. Uni versity, Jodhpur 342 005, India

Received J 5 January 2001 .. revised 15 June 200J

The ox idation of maleic, fumaric, crotonic and c innamic acids by 2,2'-bipyridinium chlorochromate (SPCC) in dimethyl sulphox ide leads to the fo rmation of corresponding epoxide. The reaction is of first order each in BPCC and the reductant. The reaction is catalysed by hydrogen ions. The hydrogen-ion dependence has the form: kobs = c + d [H+]. The ox idation of these ac ids has been studied in nineteen organic solvents. The analys is of the sol vent effect by multiparametri c equati ons indicates the importance of the cation-solvating power of the solvent. A mechani sm involving a th ree-centre transition state has been postul ated.

Halochromates have been used in synthetic organic chemistry as mild and selective ox idizing reagents 1.4.

We have been interested in the kinetic and m chani stic aspects of the oxidation by halochromates and several reports have emanated from our laboratori·8

. Though the oxidation of alkenes by chromyl chloride and chromic acid has received much attention9

.1O

, there seems to be no report on the ox idation of alkenes by halochromates. We have, therefore, undertaken an investigation of the oxidation of fumaric (FA), malejc (MA), crotonic (CrA) and cinnam ic(CiA) acids by 2,2' -bipyridinium chlorochrom~te (SPCC) in dimethyl sulphox ide (DMSO) as a solvent. Mechani sti c aspects are discussed.

Materials and Methods The unsaturated acids were co mmercial products

and were used as supplied. SPCC was prepared by the reported method4 and its purity was ascertained by an iodometri c method. Solvents were puri fied by the usual methods II . Due to non-aqueous nature of the medium, to luene-p-sulphonic ac id (TsO H) was used as a source of hydrogen ions.

Product analysis Product analys is was carried out under kinetic

conditions i.e. with an excess of the reductant over S PCc. In a typical experiment, the unsaturated ac id (0.2 mol) and SPCC (5.86 g, 0.02 mol) were di ssolved in 100 ml of DMSO and was allowed to stand fo r ca. J 2 h to ensure completion of the reaction. The soluti on was then treated with an excess

(200 cm3) of a saturated soluti on of

2,4-din itrophenylhydrazine in 2 mol dm·3 HCI and kept overnight in a refri gerator. The precipitated 2,4-dinitrophenylhydrazone (DNP) was filtered off, dried, weighed, recrystallized from ethanol , and weighed again . The yields of DNP before and after recrystallization were 2.46 g (86%) and 2.20 g (77%) respecti vely. The DNP was found identi cal (m.p. and mi xed m.p. ) with the DNP of acetophenone, acetone and pyruvic acid in the oxidation of cinnami c, crotonic and male ic/fumaric acids respec ti vely. The oxidation state of chromium in completely reduced reaction mi xtures, determined by an iodometri c method, was 3.97±0.08 .

Kinetic m easurements

The pseudo-first o rder conditions were attained by keeping an excess (x 10 or greater) of the reductant over SPCc. The solvent was DMSO, unless specified otherwise. The reactions were fo llowed at constant temperature (±0.1 K). The reactions were fo llowed by monitoring the decrease in the concentrati on of SPCC spectrophotometrically at 356 nm fo r at least three half-li ves. The pseudo-first order rate constant , k obs,

was evaluated from the linear (r2 > 0 .995) plots of log [BPCC] against time. Dupli cate kinetic runs indicated that the rate constants are reproducible to within ±3%. The second order rate constant, k2' was evaluated from the relation: k2 = k obs / lreductant]. All experi ments, other than those for study ing the effect of hydrogen ions, were carried out ~n the absence of TsOH.

+

-.

VY AS et al. : KINETICS OXIDATION OF UNSATURATED ACIDS 1183

Results The rate and other experimental data were obtained

for all the unsaturated acids studied. Since the results are similar, only representative data are reproduced here.

The final product, after the workout, in the oxidation of crotonic, cinnamic, maleic/fumaric acids IS acetone, acetophenone and pyruvic acid respectively . These must have arisen from the corresponding epoxides by rearrangement and decarboxylation [Eq. (I)].

R - Cl( - 5H - COOH -7 R - h - CH2COOH -7

o 0 R - ff -CH3 + CO2

o (R = Ph, Me or COOH)

... (1 )

Epoxides are known to rearrange to ketones 12. B­Ketoacids readily decarboxylate in acidic solutions l3

.

Therefore, the overall oxidation process may be written as follows.

R - CH = CH - COOH + 0 2CrCIObpyH--7 R - C~ - JH - COOH + OCrCIObpyH . .. (2)

o SPCC undergoes a two-electron change. Thi s is in

accord with our earlier observations with BPCC and other halochromates5

-8

.

The reactions are first order with respect to BPCC. Individual kinetic runs were strictly first order in BPCC. Further, the pseudo-first order rate constants do not depend on the initial [BPCC]. The reaction rate increases linearly with an increase in the concentration of the acid. For example, with [BPCC] constant at 0.001 mol dm-3 when [cinnamic acid] was increased from 0.05 to 0.50 mol dm·3, 104

k obs,

increased from 7.0 to 70.5 S-I at 298 K. The oxidation of acids in an atmosphere of nitrogen

failed to induce the polymerization of acrylonitrile. Further, the addition of acrylonitrile had no effect on the rate.

The reaction is catalysed by hydrogen ions. With the [SPCC] = 0.001 and [cinnamic acid] = 0.10 mol dm-3, the values of rate constant, 104 kabn are 19 A, 24.6,35.1,47.0,56.5 and 67A S- I at [TsOH] = 0.10, 0.20, OAO, 0.60, 0.80 and 1.00 mol dm·3 respecti vely at 298 K. The hydrogen ion dependence has the following form [Eq. (3)]. The values of c and d, for the oxidation of cinnamic acid are 14.0 x 10-4 S- I and 53.5 x 10-4 dm3 mol- I S- I (r2 = 0.9994, sd = 0.50) kobs =c+d[H+] ... (3)

The rates of the oxidation of the cinnamic ac id were obtained in nineteen different organic solvents.

Table I - Effect of solvents on the ox idation of cinnamic acid by BPCC at 298 K

Solvents 104 k2 Solvents 104 k2

(dm3 mol- ' s- ') (dm3 mol- ' s- ')

Chloroform 56.5 Acetic ac id 16.3

1,2-Dichloroethane 64.8 Cyclohexane 5.75

Dichloromethane 63.3 Toluene 25.2

DMSO 141 Acetophenone 75.7

Acetone 59.6 THF 37.7

N,N-Dimethylfonnamide 88. 1 t-Butyl alcohol 29.0

Butanone 48.9 lA-D ioxane 38.4

Nitrobenzene 72.3 1,2- Di methox yethane 24. 1

Benzene 28 .9 Carbon di sulphide 15.2

Ethy l acetate 3 1.5

Table 2 - Rate constants and ac ti va tion parameters of the oxidati on of unsaturated acids by BPCC

Subst 104 k2/ (dm3 mol ·' s·,) f..H· f..S· f..C ·

288 K 298 K 308 K 3 18K (kJ mol- ') (1 maliK- I) (k1 mol- I)

FA 4.53 7.45 12.5 20.4 35.8±0.5 - 185± 2 90.8±0.4

CrA 14.0 22.4 36.0 57.5 33.3±0.5 - 184± 2 88. t±0.6

MA 17.7 27.9 45.0 71.0 32.8±0.7 -1 84± 2 87.5±0 6

CiA 94.5 141 220 328 29.3±0.6 -183± 3 83.5±0.5

1184 INDlAN J. CHEM., SEC A, NOVEMBER 200 1

The solubility of reagents and reaction of BPCC with primary and secondary alcohols limited the choice of solvents. There was no reaction with the chosen solvents. Kinetics was similar in all the solvents. The values of k2 are recorded in Table 1.

The rates of oxidation of the four unsaturated acids were determined at different temperatures and the activation parameters were calculated (Table 2) .

Discussion The rate constants, k2, in eighteen solvents (CS2

was not considered, as the complete range of solvent parameters was not available) were correlated in terms of the linear solvation energy relationship [Eq.(4)] of Kamlet et al. 14

•

log k2 = Ao + pn * + b13 + aex . . . (4)

[n this equation, n * represents the solvent polarity, 13 the hydrogen bond acceptor basicities and ex is the hydrogen bond donor acidity . Ao is the intercept term. It may be mentioned here that out of the 18 solvents, 12 have a value of zero for ex. The results of correlation analyses in terms of Eq. (4), a bi­parametric equation involving n* and 13, and separately with n* and 13 are given below [Eqs (5) -(8)].

log k2 = - 3.16 + 1.44 (±0.14) n* + 0.13 (±0.12) 13-0.13 C±O. II ) ex ... (5)

R2 = 0.8624; sd = 0.13 ; n = 18; \jI = 0.29

log k2 = - 3.20+ 1.I9(±0.14)n* +0.08 (±0. 11)13 ... (6)

R2 = 0.8482; sd = 0.13 ; n = 18; \jI = 0.30

log k2 = - 3.18 + 1.22 (±0.13) n* ... (7)

r2 := 0.8424; sd = 0.14; n = 18; \jI = 0.32

log k2 = - 2.50 + 0.30 (±0.26) 13 ... (8)

r2 == 0.0753 ; sd = 0.31; n = 18; \jI = 0.88

Here n is the number of data points and \jI is the Exner's statistical parameter l5

.

Kamlet's 14 triparametric equation explains ca. 86% of the effect of solvent on the oxidation . However, by Exner's criterion l5 the correlation is not even satisfactory (ef Eq. 5). The major contribution is of solvent polarity. It alone accounted for ca. 84% of the data. Both 13 and ex play relatively minor roles .

The data on the solvent effect were analysed in terms of Swain's equation l6 of cation- and anion-solvating concept of the solvents also [Eq. (9)].

log k2 = aA + bB + C ... (9)

Here A represents the anion-solvating power of the solvent and B the cation-solvating power. C is the intercept term. (A + B) is postulated to represent the solvent polarity. The rates in different solvents were analysed in terms of Eq. (9), separately with A and B and with (A + B).

log k2 = 0.40 (±0.0l) A+1.24 (±0.01)B-3.32 ... (10)

R2 = 0.9998; sd = 0.01; n = 19; \jI = 0 .0 1

log k2 = 0.22 (±0.41) A - 2.47 ... (II)

r2 = 0.0174; sd = 0.33; n = 19; \jI = 0.96

log k2 = 1.21 (±0.07) B - 3.19 .. . (12)

r2 = 0.9448; sd = 0.08; n = 19; \jI = 0.17

log k2 = 0.96 ± 0.11 (A + B) -- 3.30 ... (13)

r2 = 0.8253; sd = 0.14; n = 19; \jI = 0.3 1

The rates of oxidation of cinnamic acid in different solvents showed an excellent correlation in Swain's equation [ef Eq.(9)] with the cation-solvating power playing the major role. In fact, the cation-solvation alone accounts for ca. 94% of the data. The correlation with the anion-solvating power was very poor. The solvent polarity, represented by (A + B), also accounted for ca. 83% of the data. In view of the fact that solvent polarity is able to account for ca. 83% of the data, an attempt was made to correlate rate with the relative permittivity of the solvent. However, a plot of log k2 against the inverse of the relative permittivity is not linear (r2 = 0.5102; sd = 0.23 ; \jI = 0.55).

Mechanism The observed acid-dependence of the reaction rate

may be explained on the basis of a protonation of BPCC in a pre-equilibrium [Eq. (14)] , with both the protonated and deprotonated forms being active oxidizing species .

0 2CrCIObpyH + H+ ~ (HOCrOClObpyH) . . . (14)

The reactions of alkenes with various Cr(V I) derivatives have been widely studied. The most extensively studied derivative is chromyl chloride. The results of the present study are comparable with those obtained with chromyl chloride.

The low values of the enthalpies of activation indicate that bond-cleavage is not extensive in the formation of the activated complex. The reaction is

.~

-

......

yy AS et al.: KINETICS OXIDATION OF UNSATURATED ACIDS 1185

isoentropic i.e. the change in the rates is governed by the enthalpies of activation. The formation of a rigid cyclic activated complex, from two acyclic reactants, is indicated by the large negative values of the entropies of activation. The probable structures of the activated complex are I, II, III and IV. III is akin to the activated complexes of concerted cis-l,3-cycloaddition reactions, which are characterized by values of reaction constants close to zero 17

• Therefore, an activated complex of type III is unlikely in view of the fact that the reactions of substituted styrenes IS and alkenes l9 exhibit moderately large negative reaction constants (-l.99 and - 2.63 respectively).

In reactions involving fairly large degree of carbocationic character in the activated complex, reaction constant values of -3 to -5 have been observed2o

.2 1

. In the oxidation of trans­monosubstituted cinnamic acid by acid bromate, Reddy and Sundaram22 observed that electron­withdrawing groups have little effect on the rate of oxidation while electron-donating groups have substantial effect on the reactivity. They obtained a reaction constant of -3.7 for the electron-donating groups. The activated complex is proposed to be a benzylic carbocation in character. Therefore, the reported value of ca.-2 in the oxidation of styrenes IS

by chromyl chloride mitigates against an activated complex with a fully developed positive charge (IV). In the present reaction also, replacement of a methyl group by a phenyl group results in a modest rate enhancement (ca. 7 times).

A perusal of the relative rates of the oxidation (cf Table 2) showed that maleic acid is oxidized at a rate ca. 3 times that of fumaric acid. This militates against the formation of the activated complex II. The formation of a four-member cyclic activated complex is likely to be more vulnerable to steric factors. Sterically the attack of BPCC on the open face of maleic acid (having two hydrogens) is more facile than on fumaric acid . Had the activated complex been a four-membered cyclic structure, the difference in the rates of maleic and fumaric acids would have been much sharper. Formation of even the three-member cyclic activated complex (I) is less hindered in the oxidation of the cis-acid as compared to that in the trails-acid. However, the data against an involvement of the activated complex II is not conclusive. Freeman el al. 9. IS. 19 also have not been able to show conclusively whether the activated complex had a three- or a four-member cyclic structure.

# # r H H H H

R - ~:::::-. ~ - COOH I bf I

R-C~C-COOH '. b+.· : b- I

l 0 o 0

I \/ crOCJObpyH CrCIObpyH

(I) (II)

# # H H H H I I I I

R - C.:-:-:-.. C - COOH R - C+-C - COOH

I o ,0 a- 0 \ .... :/ \/

CrC10bpyH CrCIObpyH

(1lI) (I V)

The analysis of the solvent effect also supports the proposed mechanism. The fact that the reaction proceeds faster in more polar solvents is in accordance with the formation of a partially charged activated complex from two neutral molecules. The relatively major contribution of the cation-solvating power of the solvents supports the generation of an electron-deficient carbon centre in the transition state. Further, the relatively low magnitude of the regression coefficient, b, is consistent with the development of a partial positive charge in the activated complex. In the oxidation of benzaldehyde by pyridinium tluorochromateS

, where the formation of a carbocationic activated complex has been postulated, the value of b is l.73. To conclude, though both structures I and II are possible for the activated complex, the balance of evidence is in favour of I.

Acknowledgement Thanks are due to UGC, New Delhi for financial

support.

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1186 INmAN J. CHEM., SEC A, NOVEMBER 2001

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