Kinetics and mechanism of the oxidation of substituted benzyl alcohols by pyridinium chlorochromate

摘要

1978 639 Kinetics and Mechanism of the Oxidation of Substituted Benzyl Alcohols by Pyridinium Chlorochromate By Kalyan K. Banerji, Department of Chemistry, University of Jodhpur, Jodhpur, India The oxidation of benzyl alcohol by pyridinium chlorochromate is first order with respect to both the oxidant and the alcohol. The reaction is catalysed by acid, the catalysed reaction being nearly first order in acidity. The kinetic isotope effect, k,/k,, is 5.07 at 30 "C. The reaction does not induce polymerisation of acrylonitrile. The reaction constant p for the uncatalysed and acid-catalysed oxidation of benzyl alcohol and eight monosubstituted benzyl alcohols have the values -1.70 f0.08 and -1.45 f0.06, respectively at 25 "C. Probable mechanisms are dis-cussed. PYRIDINIUMCHLOROCHROMATE (PCC) is a complex of chromium trioxide, pyridine, and hydrochloric acid and is known as Corey's reagent.1 Corey and Suggsl reported that this complex converts alcohols into carbonyl compounds smoothly at room temperature in yields well over 80.There seems to be no report about the mechanism of this important reaction. The present communication reports the kinetics of oxidation of sub-stituted benzyl alcohols by PCC and evaluates the reac- tion constant. Mechanistic aspects are discussed. RESULTS The oxidation of benzyl alcohol (BA)by PCC in 1 : 1 (v/v) niethylene chloride-nitrobenzene results in the formation of benzaldehyde. No detectable oxidation of benzaldehyde could be observed under the experimental conditions.Stoiclzeio~etry.-Excess of PCC was allowed to react with 0.005h1-benzyl alcohol at various acidities and the excess of CrVl estimated. For some runs, benzaldehyde was estim- ated, using an excess of the alcohol. Values of APCC/ ABA and APCC/ABenzaldehyde in Table 1 suggest the overall reaction (1). 3 PhCH,OH + 2 CrV1--+ 3 PhCHO + 6 H+ + 2 CrIlI (1) TABLE1 Stoicheiometry of the oxidation of benzyl alcohol by PCC TsOH/M APCC/ABA APCC/ABenzaldehyde 0.0 0.62 0.1 0.65 0.3 0.70 0.0 1.20 0.2 1.06 0.4 1.14 Rate Laws.-The rate laws and other experimental data were obtained for all the alcohols investigated. As the results are similar, only those of benzyl alcohol are repro- duced.The reaction is found to be cleanly first order with respect to the oxidant as regards time (as evidenced by constancy of first-order rates at different times) and concentration (as evidenced by the time-order rate coefficient being in-dependent of the initial concentration of the oxidant) (Table 2). The order with respect to the alcohol is also one (Table 3). The reaction is catalysed by acid. The catalysed reaction shows a near first-order depen-dence on acidity, the actual order being 0.87 5 0.02 (Table 4). Because of the non-aqueous nature of the sol- TABLE2 Oxidant depenclence of the reaction rate; BA 0.01h1, t 25 OC 103CrVI/ni 1.0 2.0 3.0 4.0 5.0 106kk,/s-' 7.50 7.50 7.23 7.77 7.61 TABLE3 Dependence of the reaction rate on substrate concentration ; Crvl 0.002MM,t 25 "c 102BA/~ 1.00 2.00 4.00 6.00 8.00 106kJs-1 7.50 15.2 30.6 44.0 60.0 TABLE 4 Dependence of the reaction rate on acidity; BA 0.0131, CrvI 0.0021~1,t 25 "C TsOH/M 0.05 0.10 0.20 0.30 0.40 1O6k1/s-'-11.2 15.8 25.0 32.8 41.0 vents, a constant ionic strength could not be maintained.However, the chromic acid oxidation of benzyl alcohol in aqueous acetic acid is not affected by changes in ionic strength. The rate of oxidation of ma-dideuteriobenzyl alcohol and benzyl alcohol at 30 "C are 103K 4.22 and 21.4 l2 mo1P s-1 respectively. The kinetic isotope effect kH/kD is 5.07 at 30 "C. Effect of Solvent Com$osition.-The acid-catalysed oxid- ation of benzyl alcohol was studied in solutions containing varying proportions of methylene chloride and nitrobenzene (Table 5).The increase in the proportion of nitrobenzene reduces the reaction rate. This accords with Corey's observation that the use of more polar solvents in which the 1 E. J. Corey and W. J. Suggs, Tetrahedron Letters, 1975, 2647. G. V. Bakore, K. K. Banerji, and R. Shanker, 2. Phys.Chem. (Frankfurt), 1965, 45, 129. reactants are soluble lead to inconveniently long reaction times.1 TABLE5 Dependence of the reaction ra,te on solvent composition ; BA 0.02~~ t 25 "CCrVIO.OO~M,TsOH 0.1~~ Nitrobenzene (yo) 20 30 50 60 70 D 14.2 16.8 22.0 24.5 27.1 1O5k,Is-' 8.73 5.37 3.16 2.31 1.88 The oxidation of benzyl alcohol, under nitrogen, failed to induce polymerisation of acrylonitrile.One-electron oxid- ation is thus unlikely, though not ruled out. In control experiments, with the alcohol absent, the concentration of PCC does not show any appreciable change during the period in which it is reduced to one-fifth in the presence of benzyl alcohol. The uncatalysed and acid-catalysed oxidation of mono-substituted benzyl alcohol were studied at different tempera- tures (Tables 6 and 7). The activation parameters (25-40 "C) were evaluated by the standard pr~cedure.~ The error limits in the values of AH*, AS*, and AF* (at 25 "C) are amp;5 kJ mol-l, amp;lo J mol-l K-1, and f7 kJ mol-l, respec- tively (Table 8). TABLE6 Rate constants for oxidation of substituted benzyl alcohols by pyridinium chlorochromate 105k/lmol-l s-l 7 Substituent 25 "C 30 "C 35 "C 40 "C 13 75.0 112 155 216 m-Me 97.7 151 200 277 p-Et 126 186 263 355 @-Me 148 200 274 383 p-OMe 225 302 380 523 p-cr 31.6 47.0 69.0 102 wl3r 15.2 26.3 40.7 70.0 .in-NO, 4.47 8.32 14.1 23.6 P-NO, 3.64 6.62 11.8 20.0 DISCUSSION A near constancy of the free energy of activation shows that the same mechanisni is operative for all the alcohols.J.C.S. Perkin I1 the involvement of a protonated chromium(v1) species in the rate-determining step, in the presence of an acid. Involvement of such species are well established in chromic acid oxidations.* The kinetic isotope effect (kR/kD5.07) indicates that the rate-determining step involves C-H cleavage from the alcohol carbon atom. The activation enthalpies and entropies of both the TABLE7 Rate constants for the oxidation of substituted benzyl alcohol by pyridinium chlorochromate, in the presence of TsOH 104k/12mol-l s-l r Substituent 25 "C 30 "C 35 "C 40 OC' H 158 214 285 386 m-Me 191 257 347 467 9-Et 251 339 447 675 p-Mep-OMe m-Brp-Cl 275 390 75.9 42.7 360 490 112 61.7 468 603 159 100 589 724 245 151 m-NO, P-NO, 15.1 12.5 25.1 22.4 45.7 38.9 79.5 69.2 uncatalysed and acid-catalysed oxidations are linearly related (r 0.990 and 0.987 respectively).The correl- ations were tested and found genuine by applying Exner's ~riterion.~The isokinetic temperature computed from the plots between AH* and AS* are 430 and 375 K respectively. Current views do not attach much physi- cal significance to isokinetic temperature,6 though a linear correlation is usually a necessary condition for the validity of the Hammett equation.Dielectric constants for the methylene chloride-nitrobenzene mixtures are not available but can be estimated approximately from the dielectric constants of the pure solvents.' The estimated dielectric constants for the solvent mixtures are given in Table 5. A plot of log k, against the inverse of dielectric constant gives a TABLE8 Activation parameters for oxidation of substituted benzyl alcohols by pyridinium chlorochromate IJncatalysed oxidation Acid-catalysed oxidation Substituent hH*/kJ mol-1 -AS*/ J mol-1 K-1 AF*/kJ mol-; 'AH*/kJ mol-1 -AS*/J mol-1 K-l AF*/kJ mol-i H 54.7 125 92.0 47.9 123 84.5 .in-RiIe P-Et 56.4 115 54.6 122 89.1 91.0 46.7 125 84.0 45.0 128 83.2 p-Mep-OMep-Clm-Br 51.7 130 43.1 155 59.9 110 78.5 58.5 90.4 91.3 92.8 95.9 44.0 131 83.0 36.4 154 82.3 58.9 84.1 84.0 72.5 52.0 88.0 m-NO, 87.1 39.8 98.9 82.4 27.0 90.4 P-NO, 88.9 35.1 99.3 86.2 15.9 90.9 The free energy of activation of the catalysed oxidation is consistently lower than that of the uncatalysed reaction but are of the same order and indicate that the mechanism of the two reactions are essentially similar.The increase in the oxidation rate with acidity suggests A. A. Frost and R. G. Pearson, ' Kinetics and Mechanism,' Wiley Eastern, New Delhi, 1970, p.99. K. B. Wiberg, ' Oxidation in Organic Chemistry, Part A,' Academic Press, New York, 1965, p. 69. 0. Exner, Coll. Czech. Chem. Comm., 1964, 29, 1094. straight line (Y 0.976) with a positive slope. This suggests an interaction between a positive ion and a dipole and confirms that the rate-determining step, in the presence of an acid, involves a protonated CrVI species. J. E. Leffler, J. Ovg. Chem., 1966, 31, 533. 7 C. N. R. Rao, 'A Handbook of Chemistry and Physics,' Affiliated East-yt Press, New Delhi, 1967, p. 169. E. S. Amis, Solvent Effects on Reaction Rates and Mech- anisms,' Academic Press, New York, 1967, p. 42. 1978 641 The rates of oxidation of substituted benzyl alcohols EXPERIMENTAL CJcorrelate well with Hammett's values, with negative Mateyiamp;.-The preparations and specifiactions of the reaction constants (Table 9).OH I Ph-C-H I H fastPhiHOH -PhCHO H 0 ' Ph-C nn t O=Cr-0" PyH' /bsol; Cl OH alcohols have been described earlier.ll The solvents were slow PhdHOH -k (HO)2 CrCl 0- PyH' (2) 4-H' SCHEME1 SCHEME2 The above results point to a hydride ion transfer in the rate-determining step. The hydride transfer may take place either directly (Scheme 1) or may involve the prior TABLE9 Temperature dependence of the reaction constant for the oxidation of benzyl alcohols by pyridinium chloro-chromate --P t/"C Uncatalysed Acid-catalysed 25 1.70 f0.08 1.45 f0.06 30 1.58 0.10 1.30 amp; 0.04 35 1.48 amp; 0.05 1.16 amp; 0.09 40 1.35 f0.06 1.01 f0.06 purified and dried in the usual manner.12 Toluene-+ sulphonic acid (TsOH) was used as a source of hydrogen ions.All reagents are of analytical grade. Product Analysis.-Benzaldehyde was characterized and estimated as its 2,4-dinitrophenylhydrazone. Kinetic Measurements.-The oxidant was prepared by the method of Corey and Suggs and the purity checked by estimating CrVI iodometrically. The reactions were arranged to be under pseudo-first-order conditions by keeping a large excess ( x 5 or greater) of the alcohol over PCC. The reactions were carried out at con-stant temperature (amp;O. 1 "C) and were followed iodometric-ally. The rate constants were evaluated from the plots of logCoxidant against time.The rate constants reported are the mean of at least duplicate runs and are reproducible to within amp;3. The solvent was always 1 : 1 (v/v) methylene chloride-nitrobenzene, unless mentioned otherwise. The reaction mixtures remain homogeneous in the solvent formation of a chromate ester (Scheme 2). The present data do not enable one to distinguish between the two mechanisms. Chromate ester formation is not likely to systemsbe susceptible to any considerable structural influen~e.~$l~ The large negative reaction constant can thus arise only from the differential effects of the substituents on the rate-determining step. @ N. C. Den0 and M. S. Newman, J. Amer. Chern. Soc., 1950,72, 3852. 10 U. Klanning and M. C. R. Symons, J. Ckem. Soc., 1961, 3204, and references therein. used. The usual initial concentration of the reactants were alcohol 0.01-0.20~~PCC ca. 0.002111, and TsOH 0.05-0.5~. Thanks are due to Dr. R. N. Mehrotra, University of Jodhpur, for helpful discussions. 7/1496 Received, 17th AGgust, 19771 l1 K. K. Banerji, J.C.S. Perkin 11, 1973, 435. l2 D. D. Perrin, W. L. Armarego, and D. R. Perrin, ' Purific-ation of Organic Compounds,' Pergamon Press, Oxford, 1966.