INTRODUCTION:

Selective oxidation of organic compounds under non-aqueous conditions is an important reaction in synthetic organic chemistry. For that purpose, a number of different chromium (VI) derivatives have been reported^1-4. Benzimidazolium dichromate (BIDC) is also one of the compound used for the oxidation of alcohols and carbonyl compounds which is reported by Meng et al.⁵ in 1998. BIDC is neither light sensitive nor hygroscopic, and therefore it is more stable and easily stored as compared to other Cr(VI) reagents. There seems to be very few reports available on the kinetic and mechanistic aspects of oxidation by BIDC^6-9. We have been interested in the kinetic and mechanistic aspects of the oxidation by Cr (VI) species and several reports by halochromates and dichromates have already been reported^10-15.

Though the oxidation of alkenes by chromyl chloride and chromic acid has received much attention^16,17, there seems to be no report available on the oxidation of unsaturated acids by using Benzimidazolium dichromate (BIDC). We have, therefore, undertaken an investigation of the oxidation of maleic (MA), fumaric (FA), crotonic (CrA) and cinnamic (CiA) acids by BIDC in dimethyl sulphoxide (DMSO) as a solvent. Mechanistic aspects are discussed.

EXPERIMENTAL:

Materials: The unsaturated acids were commercial products and were used as supplied. BIDC was prepared by the reported method⁵and its purity was ascertained by an iodometric method. Solvents were purified by the usual methods of purification¹⁸. Due to non-aqueous nature of the medium, p- toluene sulphonic acid (TsOH) was used as a source of hydrogen ions.

Product analysis: Product analysis was carried out under kinetic conditions i.e. with an excess of the reductant over BIDC. In a typical experiment, the unsaturated acid (0.2 mol) and BIDC (4.542g, 0.01 mol) were dissolved in 100 ml of DMSO and was allowed to stand for ca.15 h to ensure completion of the reaction. The solution was then treated with an excess (200 cm³) of a saturated solution of 2,4-dinitrophenylhydrazine in 2 mol dm^-3HCl and kept overnight in a refrigerator. 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 found to be 2.46 g (86%) and 2.28 g (80%) respectively. The DNP was found identical (m.p. and mixed m.p.) with the DNP of acetophenone, acetone and pyruvic acid in the oxidation of cinnamic, crotonic and maleic/fumaric acids respectively. The oxidation state of chromium in completely reduced reaction mixtures, determined by an iodometric method, was 3.93 ± 0.10.

Kinetic measurements: The pseudo-first order conditions were attained by keeping an excess (×10 or greater) of the reductant over BIDC. The solvent was DMSO, unless specified otherwise. The reactions were followed at constant temperature (±0.1K). The reactions were followed by monitoring the decrease in the concentration of BIDC spectrophotometrically at 370 nm for at least three half-lives. The pseudo-first order rate constant, kobs, was evaluated from the linear (r²> 0.995) plots of log [BIDC] against time (Figure-1). The second order rate constant, k₂, was evaluated from the relation: k₂ = kobs / [reductant]. All experiments, other than those for studying the effect of hydrogen ions, were carried out in the absence of TsOH.

RESULTS AND DISCUSSION:

The rates and other experimental data were obtained for all the acids. Since the results are similar, only representative data are reproduced here.

Stoichiometry: After the workout of the product, the final product, 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 as shown in equation (1).

Table 1: Rate constants for the oxidation of Fumaric acid by BIDC at 308 K.

10³[BIDC] (mol dm^-3)	[FA] (mol dm^-3)	[TsOH] (mol dm^-3)	10⁴kobs s^-1
1.0	0.10	0.00	2.51
1.0	0.20	0.00	5.02
1.0	0.40	0.00	10.3
1.0	0.60	0.00	15.2
1.0	0.80	0.00	20.4
1.0	1.00	0.00	24.9
1.0	1.50	0.00	37.5
1.0	3.00	0.00	75.2
2.0	0.20	0.00	6.63
4.0	0.20	0.00	5.90
6.0	0.20	0.00	6.81
8.0	0.20	0.00	6.08
1.0	0.40	0.00	10.0^*
*contained 0.001 M acrylonitrile

Table 2: Dependence of the reaction rate on hydrogen ion concentration.

[BIDC] = 0.001 mol dm⁻³; [Crotonic acid] = 0.1 mol dm⁻³; Temp. = 298 K
[TsOH] /mol dm⁻³	0.10	0.20	0.40	0.60	0.80	1.00
10⁴ kobs /s^-1	8.41	10.2	14.1	18.5	22.2	25.8

Effect of Temperature:

The rate of unsaturated acids by using BIDC was obtained at different temperatures between 288K and 318K.The values of rate constant (k₂) are recorded in Table-3. The log k₂values at different temperatures is linearly to the inverse of the absolute temperatures in all the cases (Figure- 2). It proves that, the Arrhenius equation is valid for this reaction.

Figure 2: Effect of temperature on rate (Arrhenius plot)

Table-4: Effect of solvents on the oxidation of Crotonic acid by BIDC at 308 K.

Solvents	10⁴k₂ / (dm³ mol^-1 s^-1)	Solvents	10⁴k₂ / (dm³ mol^-1 s^-1)
Chloroform	28.01	Toluene	8.09
1,2-Dichloroethane	34.55	Acetophenone	42.38
Dichloromethane	30.71	THF	15.08
DMSO	94.20	t-Butyl alcohol	12.19
Acetone	26.21	1,4-Dioxane	14.06
DMF	52.19	1,2-Diethoxyethane	7.20
Butanone	20.26	Acetic Acid	5.37
Nitrobenzene	34.49	Ethyl Acetate	10.38
Benzene	11.09	CS₂	4.37
Cyclohexane	1.16

Effect of Solvents: The rates of the oxidation of the unsaturated acids were studied in nineteen different organic solvents. The solubility of reagents and reaction of BIDC with primary and secondary alcohols limited the choice of solvents. There was no reaction with the solvents chosen for the study. Kinetics was similar in all the solvents. The values of k₂are recorded in Table 4. The rate constants, k₂, in eighteen solvents (CS₂was not considered, as the complete range of solvent parameters was not available for the same) were correlated in terms of the linear solvation energy relationship (4) of Kamlet et al²¹.

log k₂ = A₀ + pp* + bb + a a(4)

In this equation, p^* represents the solvent polarity, b the hydrogen bond acceptor basicities and a is the hydrogen bond donor acidity. A₀ is the intercept term. It may be mentioned here that out of the 18 solvents, 13 has a value of zero for a . In our correlation analyses, we have used the coefﬁcient of determination (R² or r²), standard deviation (SD) and Exner’s statistical parameter²², y, as the measures of the goodness of ﬁt. The results of correlation analyses in terms of equation (4), a biparametric equation involving p^* and b, and separately with p^* and b are given below by Equations (5) - (8)

log k₂ = - 3.87 + 1.56 (± 0.19) p^* + 0.26 (± 0.15) b- 0.10 (± 0.15) a (5)

R² = 0.849; SD = 0.18; n = 18; y = 0.39

log k₂ = - 3.89 + 1.57 (± 0.19) p^* + 0.27 (± 0.15) b (6)

R² = 0.844; SD = 0.18; n = 18; y = 0.40

log k₂ = - 3.85 + 1.65 (± 0.20) p^* (7)

r² = 0.810; SD = 0.19; n = 18; y = 0.44

log k₂ = - 2.96 + 0.54 (± 0.33) b (8)

r² = 0.144; SD = 0.41; n = 18; y = 1.10

Here, n is the number of data points. Kamlet's²¹triparametric equation explains ca. 84% of the effect of solvent on the oxidation. However, by Exner's criterion²²the correlation is not even satisfactory (cf. equation 5). The major contribution is of solvent polarity. It alone accounted for ca. 81% of the data. Both b and a play relatively minor roles. The data on the solvent effect were analysed in terms of Swain's equation²³of cation- and anion-solvating concept of the solvents also (equation 9).

log k₂= aA + bB + C (9)

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

log k₂ = 0.63 (± 0.07) A + 1.71 (± 0.05) B - 4.08 (10)

R² = 0.986; SD = 0.05; n = 19; y = 0.11

log k₂ = 0.40 (± 0.56) A - 2.91 (11)

r² = 0.029; SD = 0.45; n = 19; y = 1.28

log k₂ = 1.66 (± 0.12) B - 3.87 (12)

r² = 0.914; SD = 0.13; n = 19; y = 0.29

log k₂ = 1.35 ± 0.13 (A + B) - 4.05 (13)

r² = 0.849; SD = 0.17; n = 19; y = 0.39

The rates of oxidation of Crotonic acid in different solvents showed an excellent correlation in Swain's equation [cf. equation (10)] with the cation-solvating power playing the major role. In fact, the cation-solvation alone accounts for ca. 91 % of the data. The correlation with the anion-solvating power was very poor.

The solvent polarity, represented by (A + B), also accounted for ca. 84 % of the data. In view of the fact that solvent polarity is able to account for ca. 84 % of the data, an attempt was made to correlate rate with the relative permittivity of the solvent.

However, a plot of log k₂against the inverse of the relative permittivity is not linear (r²= 0.524; SD = 0.31; y = 0.74).

MECHANISM

The observed hydrogen ion dependence reveals that the reaction follows the two mechanistic pathways, one is acid dependent and the other being acid independent. The acid catalysis may wellbe contributed to a protonation of BIDC to give a protonated form of BIDC in pre-equilibrium (equation 14), with both the protonated and deprotonated forms being active oxidizing species.

C₇H₇ N₂)₂Cr₂O₇ + H⁺[(C₇H₇ N₂)₂OH Cr₂O₆]⁺(14)

The reactions of alkenes with various Cr (VI) 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 α-bond cleavage is not extensive in the formation of the activated complex. The formation of a rigid cyclic activated complex is indicated by the large negative values of the entropies of activation. The probable structures if the activated complexes are I, II, III and IV.

III is akin to the activated complexes of concerted cis-1,3-cycloaddition reactions, which are characterized by values of reaction constants close to zero²⁴. Therefore, an activated complex of type III is unlikely in view of the fact that the reactions of substituted styrenes²⁵and alkenes²⁶exhibit moderately large negative reaction constants (–1.99 and – 2.63 respectively).

In reactions involving fairly large degree of carbo-cationic character in the activated complex, reaction constant values of –3 to –5 have been observed^27,28. In the oxidation of trans–monosubstituted cinnamic acid by acid borate, Reddy and Sundaram²⁹observed that electron - withdrawing groups have a 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 styrenes²⁵by 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-3) showed that maleic acid is oxidized at a rate ca. 3 times that of fumaric acid. This mitigates against the formation of the activated complex II. The formation of a five-member cyclic activated complex is likely to be more vulnerable to steric factors. Sterically the attack of BIDC on the open face of maleic acid (having two hydrogens) is more facile than on fumaric acid. Had the activated complex been a five-membered cyclic structure, the difference in the rates of maleic and fumaric acids would have been much sharper.

A 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 trans-acid. However, the data against an involvement of the activated complex II is not conclusive.

Freeman et al ^16,25,26also have not been able show conclusively whether the activated complex had a three- or a five-member cyclic structure.

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 fluorochromate³⁰, where the formation of a carbocationic activated complex has been postulated, the value of b is 1.73. To conclude, though both structures I and II are possible for the activated complex, the balance of evidence is in favour of I.

CONCLUSION:

Reaction is proposed to proceed on the basis of consideration of structure I of the unsaturated acids, as the observational evidences are in favour of structure I.

ACKNOWLEDGEMENTS:

We are thankful to the authorities of R.D. and S.H. National College and S.W.A. Science College, Bandra (W), Mumbai, for providing us necessary facilities to carry out our research work and We are also thankful to Professor P. K. Sharma, Head, Department of Chemistry, J. N. V. University, Jodhpur and Professor Kalyan K. Banerji, Dean, Sciences, National Law University, Mandore, Jodhpur, for their valuable and critical suggestions.

REFERENCES:

1. G. Cainelli and G. Cardillo. Chromium oxidations in organic chemistry, (Springer- Verlag, Berlin). 1984, 19.

2. H. Firouzabadi and A. Sharifi. Synthesis. 1992, 999.

3. M. Li and M.E. Johnson. Synthetic Communication. 1995, 25, 533.

4. M.K. Mahanti and K.K. Banerji. J. Indian Chem. Soc. 2002, 79, 31.

5. Q.H Meng, J.C. Feng, N.S. Bian, B. Leu and C.C Li. Synthetic Communication. 1998, 28, 1097.

6. D. Pandey, S. Kothari. Oxidation Communications, 2009, 32(2), 371.

7. P. Kumar, D. Pandey, S. Kothari. Croatica Chemica Acta, 2011, 84(1), 53.

8. D. Pandey, S. Kothari. Journal of the Indian Chemical Society, 2012, 89(9), 1183.

9. D. Pandey, S. Kothari. Oxidation Communications, 2015, 38(3), 1221.

10. P. Purohit, S. Kumbhani, I. Shastri, K. K. Banerji and P. K. Sharma. Indian J. Chem., Sect. A, 2008, 47A,1671

11. D. Yajurvedi, S. Kumbhani, I. Shastri, M. Baghmar and P.K. Sharma. J. Indian Chem. Soc., 2008, 85, 459.

12. N. Soni, V. Tiwari, S. Kumbhani, I. Shastri, V. Sharma. J. Indian Chem. Soc., 2008, 85, 857.

13. V. Tiwari, S. Kumbhani, I. Shastri, V. Sharma. Indian J. Chem., 2008, 47A, 1520.

14. A. Choudhary, D. Yajurvedi, S. Kumbhani, I. Shastri, V. Sharma. J. Indian Chem. Soc., 2009, 86, 832.

15. M. Patel, L. Mathur, K. Jha, A. Kothari, I. Shastri, P.K. Sharma. Asian Journal of Chemistry, 2013,25,5

16. F. Freeman and K.W. Arlcdge. J. Org. Chem. 1972, 37, 2656.

17. A.K. Awasthy and J. Rocek. J. Am. Chem. Soc. 1969, 91, 991.

18. D.D. Perrin, W.L. Armarego, D.R. Perrin. Purification of organic Compounds. Pergamon. Press, Oxford, 1966.

19. K.B. Wiberg, P. Marshall and G. Foster. Tetrahedron Lett. 1962, 345.

20. R.T. Morrison and R.N. Boyd. Organic Chemistry. Prentice-Hall of India; New Delhi. 1989.

21. M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft. J. Org. Chem. 1983, 48, 2877.

22. O. Exner. Collect. Chem. Czech. Commun. 1966, 31, 3222.

23. C.G. Swain, S.H. Unger, N.R. Rosenquest and M.S. Swain. J. Am. Chem. Soc., 1983, 105, 492.

24. R. Huisgen, R. Grashey and J. Sauer. The chemistry of alkenes Ed. S. Patai, Interscience. London, 1964, page-644.

25. F. Freeman and N.J. Yamachika. J. Am. Chem. Soc. 1972, 94, 1214 and references cited therein.

26. F. Freeman, P.D. McCart and N.J. Yamachika. J. Am. Chem. Soc. 1972, 92, 4621.

27. J.R. Rolsten and K. Yates. J. Am. Chem. Soc. 1969, 91, 1969.

28. J.E. Dubois and A. Schawarcz. Tetrahedron Lett. 1967, 1964.

29. C.S. Reddy and E.V. Sundaram. Tetrahedron. 1989, 45, 2109.

30. S. Agarwal, K. Chowdhary and K.K. Banerji. J. Org. Chem. 1991, 56, 5111.

Received on 09.01.2018 Modified on 20.01.2018

Asian J. Research Chem. 2018; 11(1):153-158.

DOI:10.5958/0974-4150.2018.00032.9

10⁴k₂/ (dm³mol^-1s^-1) ∆H* - ∆S* ∆G* ___________ ________ _______________ USA 288 K 298 K 308 K 318 K (KJ mol^-1) (J mol^-1K^-1) (KJ mol^-1)
FA	9.24	15.2	24.9	42.3	35.9532	-178.0860	89.9178
CrA	32.1	53.5	94.2	155	37.7429	-161.5046	86.6822
MA	25.8	43.8	73.5	128	37.9758	-162.5148	87.2224
CiA	173	272	443	713	33.5143	-162.2022	82.6662