Kinetics and Mechanism of the Oxidation of Oxalic andFormic Acids by 2,2'-Bipyridinium ChlorochromateKavita Loonker, Pradeep K. Sharma and Kalyan K. Banerji*Department of Chemistry, J.N.V. University, Jodhpur 342 005, IndiaThe oxidation of formic and oxalic acids by 2,2'-bipyridiniumchlorochromate (BPCC) involves initial formation of an anhydride inter-mediate and its subsequent decomposition via a symmetrical cyclic transition state.2,2'-Bipyridinium chlorochromate has been used as a mildand selective oxidizing reagent.1 Here we report the kineticsof oxidation of oxalic acid (OA) and formic acid (FA) by2,2'-bipyridinium chlorochromate (BPCC) in dimethyl sulf-oxide (DMSO) as a solvent.The mechanistic aspects are dis-cussed. The oxidation of these acids by pyridinium uoro-,chloro- and bromo-chromates (PFC, PCC, PBC) have beenpreviously reported from this laboratory.6¡Ó8The reactions were studied under pseudo-rst-order con-ditions by keeping an excess (15 or greater) of the organicacid over BPCC.The solvent was DMSO, unless otherwisespecied. The reactions were followed by monitoring thedecrease in the concentration of BPCC at 365nm for up to80% reaction extent. Pseudo-rst-order rate constants, kobs,were evaluated from linear plots (r> 0.990) of log[BPCC]against time.The oxidation of organic acids by BPCC resulted in theformation of carbon dioxide. The overall reaction may,therefore, be written as eqns (1) and (2).COOH2 O2CrClO£¾bpyH£¾42CO2 2H2O OCrClO£¾bpyH 1HCOOH O2CrClO£¾bpyH£¾4CO2 H2O OCrClO£¾bpyH 2The reactions were found to be rst order with respect toBPCC.Michaelis¡ÓMenten type kinetics were observed withrespect to the organic acids. This leads to the postulation ofthe following overall mechanism [eqns (3) and (4)] and therate law (5).organic acid BPCC £¾£¾* )£¾£¾Kcomplex 3complex£¾4k2 products 4£¾dBPCC=dt k2KBPCC organic acid=1 Korganic acid 5The dependence on the concentration of the organic acidwas studied at dierent temperatures and the values of Kand k2 were calculated from the double reciprocal plots.Thethermodynamic parameters for the complex formation andthe activation parameters for the decomposition of thecomplex were calculated.To ascertain the importance of the cleavage of thea-CH bond in the rate-determining step, the oxidation ofa-deuterioformic acid (DCO2H) was studied. The resultsshowed that the formation constants of the formic acid¡ÓBPCC complex for ordinary and deuteriated acids do notdier much.The decomposition of the complex showedthe presence of a substantial primary kinetic isotope eect(kH/kD=5.77 at 303 K).The addition of acrylonitrile had no eect on the reactionrate. This indicates that a hydrogen abstraction mechanism,giving rise to free radicals, is unlikely.Solvent Eect.The oxidation of formic acid was studiedin 19 dierent solvents. There was no reaction with the sol-vents chosen and the kinetics were similar in all the solvents.It was observed that the formation constant, K, of the for-mic acid¡ÓBPCC complex does not vary much with the sol-vent but there is a considerable variation in the values of k2.The correlation of k2, in 18 solvents (CS2 was not con-sidered, as the complete range of solvent parameters wasnot available), in terms of the linear solvation energy re-lationship of Kamlet et al.10 is not signicant.The data on the solvent eect were analysed in terms ofSwain's equation12 of the cation- and anion-solvating con-cept of the solvents [eqn.(12)].log k2 aA bB C 12Here A represents the anion-solvating power of the sol-vent and B the cation-solvating power and C is the interceptterm. The rates of oxidation in the dierent solvents showan excellent correlation in terms of the Swain's equation[eqn. (12)], with cation-solvating power playing the majorrole.logK2 1:4320:04A 1:7220:03B £¾ 5:76R2 0:9970; sd 0:03; n 19 13The presence of a substantial kinetic isotope eectconrmed that an a-C0H bond is cleaved in the rate-determining step.The highly unfavourable entropy termobserved in the complex formation of the oxalic acid¡ÓBPCCJ. Chem. Research (S),1998, 66¡Ó67J. Chem. Research (M),1998, 0457¡Ó0471CO2HCO2H+OCrO¡VbpyH+O ClOCrO¡VbpyH+O ClC OC OO+ H2O(A)OCrO¡VbpyH+O ClC OC OOKk2 2 CO2 + CrOClO¡VbpyH+OH C OHOCrO¡VbpyH+O Cl+K OCrO¡VbpyH+HO ClC O O(B)HCrOClOO¡VbpyH+ OHC OHCO2 + H2O + CrOClO¡VbpyH+#Scheme 1 *To receive any correspondence.66 J.CHEM. RESEARCH (S), 1998reaction suggests that oxalic acid acts as a didentate ligand and forms a cyclic intermediate complex. In chromic acid oxidation also, the formation of a cyclic anhydride inter- mediate, oxalyl chromate, has been postulated.13 For the formic acid oxidation, the cation-solvating power of the solvents plays a relatively more important role. Therefore, formation of an electron-de¢çcient carbon centre in the transition state is indicated. Thus the decomposition of the BPCC¡¾formic acid complex may involve a hydride ion transfer via an anhydride intermediate (Scheme 1).However, there is no real evidence for a hydride-ion trans- fer. In a concerted cyclic process, the di€erence between proton, atom and hydride-ion transfer is very subtle and cannot be established experimentally. The large kinetic iso- tope e€ect simply shows that a hydrogen transfer is involved in the transition state.The involvement of a concerted cyclic process is supported by a study of the temperature dependence of the kinetic isotope e€ect. The data for protio- and deuterio-formic acids when ¢çtted in the familiar expression kH/kD=AH/AD exp(¢§DH*/RT) show a direct correspondence with the properties of a symmetrical transition state in which the di€erences in the activation energies for the protio and deuterio compounds are equal to the di€erences in the zero point energies of the corresponding C0H and C0D bonds (ca. 4.5 kJ mol¢§1) and the entropies of the activation of the respective reactions are almost equal.14,15 The reaction is catalysed by hydrogen ions. The hydrogen-ion dependence has the following form: kobs=a+ b[H+]. This suggests a reversible protonation of the anhydride with both the unprotonated and protonated forms being reactive. The protonated anhydride decomposes at a rate higher than the decomposition of the unprotonated anhydride (Scheme 2).Thanks are due to the University Grants Commission (India) and the Council of Scienti¢çc and Industrial Research (India) for ¢çnancial support. Techniques used: Spectrophotometry, correlation analysis References: 15 Equations: 16 Table 1: Rate constants for the oxidation of organic acids by BPCC in DMSO at 303K Table 2: Formation constants and thermodynamic parameters for the organic acid¡¾BPCC complexes in DMSO Table 3: Rate constants and activation parameters for the oxidation of organic acids by BPCC in DMSO Table 4: Dependence of the reaction rate on hydrogen ion concen- tration Table 5: Solvent e€ect on the oxidation of formic acid by BPCC at 303K Received, 29th July 1997; Accepted, 13th October 1997 Paper 7/05487D References cited in this synopsis 1 F.S. Guziec and F. A. Luzzio, Synthesis, 1980, 691. 6 R. Asopa, A. Mathur and K. K. Banerji, J. Chem. Res., 1992, (S) 152; (M) 1117. 7 S. Varshney, S. Kothari and K. K. Banerji, 1992, (S) 356; (M) 2901. 8 S. Rathore, P. K. Sharma and K. K. Banerji, J. Chem. Res. (S), 1994, 504. 10 M. J. Kamlet, J. L. M. Abboud, M. H. Abraham and R. W. Taft, J. Org. Chem., 1983, 48, 2877 and references cited therein. 12 C. G. Swain, M. S. Swain, A. L. Powell and S. Alumi, J. Am. Chem. Soc., 1983, 105, 502. 13 F. Hassan and J. Rocek, J. Am. Chem. Soc., 1972, 92, 9073. 14 H. Kwart and M. C. Latimer, J. Am. Chem. Soc., 1971, 93, 3770. 15 H. Kwart and J. H. Nickel, J. Am. Chem. Soc., 1973, 95, 3394. O Cr O.bpyH+ O Cl C O C O O H + (A) + H+ + 2 CO2 + HOCrClO.bpyH+ O Cr O.bpyH+ HO Cl C O OH + (B) + H+ H + CO2 + H2O + HOCrClO.bpyH+ Scheme 2 J. CHEM. RESEARCH (S), 1998 67