Kinetics and mechanism of oxidation of aliphatic aldehydes by 2,2' -bipyridinium chlorochromate

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1 Indian Journal of Chemistry Vol. 39A, November2000, pp Kinetics and mechanism of oxidation of aliphatic aldehydes by 2,2' -bipyridinium chlorochromate Vinita Kumbhat, Pradeep K Sharma & Kalyan K Banerji* Department of Chemistry, J.N.V. University, Jodhpur , India Received 21 February 2000; revised 26 June 2000 Oxidation of six aliphatic aldehydes by 2,2'-bipyridinium chlorochromate (BPCC) in dimethyl sulphoxide (DMSO) leads to the formation of corresponding carboxylic acids. The reaction is first order each in BPCC and the aldehyde. The reaction is catalysed by hydrogen ions. The hydrogen-ion dependence has the form: kuhs = a + b[h+]. The oxidation of deuteriated acetaldehyde, MeCDO, exhibits a substantial primary kinetic isotope effect (kh/k 0 = 5.90 at 298K). The oxidation of acetaldehyde has been studied in nineteen different organic solvents. The solvent effect has been analysed using Taft's and Swain's multiparametric equations. The rate constants correlate well with Taft's o* values; reaction constants being negative. A mechanism involving transfer of hydride ion has been suggested. 2,2'-Bipyridinium chlorochromate (BPCC), has been used as an mild and selective oxidant in synthetic organic chemistry'. Some reports about the kinetics and mechanism of oxidation by BPCC are available in the literature 2-5. However, the kinetics of oxidation of aliphatic aldehydes by BPCC has not been investigated. We have been interested in the study of kinetics and mechanism of reactions of complexed Cr(VD species and have already reported the kinetics of oxidation of aldehydes by pyridinium halochromates 6 ' 7, as well as by quinolinium tluorochromate 8. In the present article, we report the kinetics of oxidation of six aliphatic aldehydes by BPCC in dimethyl sulphoxide (DMSO) as solvent. Mechanistic aspects are discussed. Materials and Methods All the aldehydes were commercial products and were used as such. BPCC was prepared by the reported method' and its purity checked by an iodometric method. Solutions of formaldehyde were prepared by heating paraformaldehyde and passing the vapours in DMSO. The amount of HCHO in DMSO was determined by chromotropic acid method 9 p-toluenesulphonic acid (TsOH) was used as a source of hydrogen 1ons. Deuteriated acetaldehyde (MeCDO) was obtained from Sigma Chemicals. Solvents were purified by the usual methods 10. Product analysis The product analysis was carried out under kinetic conditions. In a typical experiment, acetaldehyde ( 4.4 g, 0.1 mol) and BPCC (2.93 g, 0.0 I mol) were dissolved in DMSO (100 ml) and the reaction mixture was allowed to stand for ca. 24 h to ensure completion of the reaction. It was then rendered alkaline with NaOH, filtered and the filtrate was reduced to dryness under pressure. The residue was acidified with perchloric acid and extracted with diethyl ether (5x50 ml). The ether extract was dried (MgS04) and treated with I 0 ml of thionyl chloride. The solvent was allowed to evaporate. Dry methanol (7 ml) was added and the HCI formed was removed in a current of dry air. The residue was dissolved in diethyl ether (200 ml) and the ester content was determined colorimetrically as Fe (III) hydroxymate by the procedure of Hall and Schaefer''. Several determinations indicated a I: I stoichiometry. The oxidation state of chromium in a completely reduced reaction mixture, determined by iodometric titrations was 3.91 ± Therefore, the overall reaction can be written as follows: RCHO + Cr02CIO-bpyH+ ~ RCOOH + CrOClO-bpyH+... (I) Kinetic measurements Reactions were carried out under pseudo-first order conditions by keeping an excess (x 15 or greater) of [aldehyde] over [BPCC]. The solvent was DMSO,

2 1170 INDIAN J CHEM, SEC. A, NOVEMBER 2000 Table I -Rate constants for the oxidation of acetaldehyde by BPCC at 298 K I0 3 [BPCC] [McCHO] [TsOH] I0 4 kuhs mol dm- 3 mol dm- 3 mol dm- 3 s-j * *contained mol dm- 3 acrylonitrile unless mentioned otherwise. All reactions were carried out in flasks blackened from the outside to prevent any photochemical reactions. The reactions were carried out at constant temperature (±0.1 K) and were followed up to 80% of the extent of reaction, by monitoring the decrease in [BPCC] at 365 nm. The pseudo-first order rate constant, kobs was computed from the linear least-squares plot of log [BPCC] versus time. Duplicate runs showed that the rate constants were reproducible to within ±3%. The second order rate constant, k 2, was calculated from the relation : k 2 = kob.l[aldehyde]. Results and Discussion The reactions are of first order with respect to BPCC. Further, the values of kobs are independent of the initial concentration of BPCC. The reaction is first order with respect to aldehyde also (Table 1 ). The rates of oxidation of six aliphatic aldehydes were determined at different temperatures and the activation parameters were calculated (Table 2). Kinetic isotope effect To ascertain the importance of the cleavage of aldehydic C-H bond in the rate- determining step, the oxidation of deuteriated acetaldehyde (MeCDO) was studied. The oxidation of deuteriated acetaldehyde exhibited a substantial primary kinetic isotope effect (Table 2). Induced polymerization of acrylonitrile The oxidation of aldehydes by BPCC, in an atmosphere of nitrogen, failed to induce the polymerization of acrylonitrile. Further, the addition of acrylonitrile had no effect on the rate (Table 1). Thus a one-electron oxidation, giving rise to free radicals is unlikely. Effect of acidity The reaction is catalysed by hydrogen ions. The hydrogen- ion dependence has the following form kobs = a + b [H+] (Table I). The values of a and b, for acetaldehyde, are 5.61±0.1 x I0-4s- 1 and 9.88±0.2 xto-4 mol- 1 dm\- 1 respectively (r 2 = ). The entropy and enthalpy of activation of the oxidation of six aldehydes are linearly related (r 2 = ). The value of isokinetic temperature evaluated 12 ' 13 from this plot is 1225±46 K. The correlation was tested and found genuine by Exners' criterian 14 The isokinetic temperature, calculated from Exner's plot of log k 2 at 288 K versus log k 2 at 318 K (r 2 = ) is 1280±44 K. The linear isokinetic correlation suggests that all the aldehydes are oxidized by the same mechanism. The observed hydrogen-ion dependence suggests that the reaction follows two mechanistic pathways, one acid-independent and the other acid dependent. The acid-catalysis may well be attributed to a protonation of BPCC to give a stronger oxidant and electrophile (2). Both BPCC and BPCCH+ are reactive species with the protonated form being more reactive. + bpyhocr0 2 CI + H+ <=> bpyhocr(oh)oci Solvent effect The oxidation of acetaldehyde was studied in 19 different organic solvents. The choice of solvents was limited due to the solubility of BPCC and its reaction with primary and secondary alcohols. There was no reaction with the solvents chosen. The kinetics were similar in all the solvents. The values k 2 at 298 K are recorded in Table 3. The rate constants, k 2, in eighteen solvents (CS 2 was not considered, as the complete range of solvent parameters was not available) were correlated in 000 (2)

3 KUMBHAT eta/.: KINETICS OF OXIDATION OF ALIPHATIC ALDEHYDES 1171 Table 2-Rate constants and activation parameters of the oxidation of aliphatic aldehydes by BPCC Subst k 2 ( dm 3 mol- 1 s- 1 ) tj.jt /).5. D.c K (kj mol- 1 ) (J mol- 1 K- 1 ) (kj mol- 1 ) H ± ±3 92.3±0.7 Me ± ±2 85.8±0.6 Et ± ±2 84.4±0.6 Pr ± ±2 84.2±0.5 Pr; ± ±2 83.2±0.5 CICH ±1.0-95±4 99.8±0.8 MeCDO ± ±2 90.2±0.4 kh/k Table 3-Effect of solvents on the oxidation of acetaldehyde by BPCC at 298 K Solvents 10 4 k2 s-1 Solvents 10 4 k2 s-1 Chloroform 15.5 I,2-Dichloroethane 19.6 Dichloromethane 17.4 DMSO 56.2 Acetone 18.2 N,N-Dimethylformamide 28.2 Butanone 13.5 Nitrobenzene 20.4 Benzene 6.76 Ethyl acetate 6.92 Acetic acid 2.14 Cyclohexane 0.62 Toluene 5.37 Acetophenone 23.4 THF 9.55 /-butyl alcohol 6.31 I,4-Dioxane 8.51 I,2-Dimethoxyethane 4.79 Carbon disulphide 2.45 terms of the linear solvation energy relationship (Eq. 3) of Kamlet et a/ (3) In this equation, 1r: represents the solvent polarity, ~ the hydrogen bond acceptor basicities and a is the hydrogen bond donor acidity. A 0 is the intercept term. It may be mentioned here that out of the 18 solvents, 12 have a value of zero for a. The results of correlation analyses in terms of Eq. (3), a biparametric equation involving n* and ~. and separately with rt and ~ are given below [Eqs (4) - (7)]. log k 2 = (±0.21) n (±0.89) ~ +0.26(±0.17)a... (4) R 2 = ; sd = 0.19; n = 18; 'I'= 0.31 log k 2 = (±0.21) 1t (±0.18) ~... (5) R 2 = ; sd = 0.21; n = 18; 'I'= 0.13 logk 2 = l.69(±0.2l)n*... (6) r 2 = ; sd = 0.22; n = 18; 'I'= 0.15 log k 2 = (±0.37) ~... (7) r 2 = ; sd = 0.45; n = 18; 'I'= 0.88 Here n is the number of data points and 'I' is the Exner's statistical parameter 16 Kamlet's 15 triparametric equation explains ca. 81% of the effect of solvent on the oxidation. However, by Exner's criterion 16 the correlation is not even satisfactory (cf. Eq. 4). The major contribution is of solvent polarity. It alone accounted for ca. 80% of the data. Both ~ and a play relatively minor roles. The data on the solvent effect were analysed in terms of Swain's equation 17 of cation- and anion-solvating concept of the solvents also [Eq. (8)]. log k 2 = aa + bb + C... (8) Here A represents the anion-solvating power of the

4 1172 INDIAN 1 CHEM. SEC. A, NOVEMBER 2000 Table 4--Temperature dependence of the reaction constant Temp./ K ± ± ± ±0.05 p sd 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. (8), separately with A and B and with (A+ B). Jog k 2 = 0.44 (±0.03) A (±0.02) B (9) R 2 = ; sd = 0.02; n = 19; 'I'= 0.04 log k 2 = 0.19 (±0.60) A (10) r 2 = ; sd = 0.47; n = 19; 'I'= 0.98 log k 2 = 1.73 (±0.08) B r 2 = ; sd = 0.09; n = 19; 'I'= (II) log k 2 = 1.33 ± 0.17 (A + B) ( 12) r 2 = ; sd = 0.22; n = 19; 'I'= 0.35 The rates of oxidation of acetaldehyde in different solvents showed an excellent correlation in Swain's equation [cf. Eq.(9)] with the cation-solvating power playing the major role. In fact, the cation-solvation alone accounts for ca. 96% of the data. The correlation with the anion-solvating power was very poor. The solvent polarity, represented by (A + B), also accounted for ca. 78% of the data. In view of the fact that solvent polarity is able to account for ca. 78% of the data, an attempt was made to correlate the rate with the relative permittivity of the solvent. However, a plot of log k 2 against the inverse of the relative permittivity is not linear (r 2 = ; sd = 0.34; "'= 0.56). Mechanism In aqueous solutions most aliphatic aldehydes exist predominantly in the hydrate form 18 and in many oxidations, in aqueous solutions, it has been postulated that the hydrate is the reactive species. However, owing to the non-aqueous nature of the solvent in the present reaction, only the free carbonyl form can be the reactive species. The rates of the oxidation of six aldehydes show an excellent correlation with Taft's cr substituent constants 19, the reaction constant being negative (Table 4). The negative polar reaction constant indicates an electron-deficient carbon centre in the transition state of the rate-determining step. The presence of a substantial primary kinetic isotope effect (kh/k 0 = 5.90 at 298 K), confirms that the aldehydic C-H bond is cleaved in the rate-determining step. The large negative value of the polar reaction constant together with the substantial deuterium isotope effect indicates that the transition state approaches a carbocation in character. Hence, transfer of a hydride ion from the aldehyde to the oxidant is suggested. The hydride ion transfer mechanism is also supported by major role of cation-solvating power of the solvents. There is no kinetic evidence for the formation of an intermediate in the present reaction, however, its formation in small amounts cannot be ruled out. An analysis of the temperature dependence of the kinetic isotope effect indicated the presence of a symmetrical transition state in the rate-determining step of the reaction. Kwart and Nickle 20 have shown that a study of the dependence of the kinetic isotope effect on temperature can be gainfully employed to indicate whether the transition state is symmetrical or not. The data for protio- and deuterio-acetaldehydes, fitted to the familiar expression khiko = AH/Ao exp(~ E.IR1) show a direct correspondence with the properties of a. I symmetnca trans1t10n state m w h. IC h th e activation energy difference (M.) for khiko is equal to the zero-point energy difference for the respective C-H and C-D bonds (ca. 4.5 kj/mol) and the frequency factors and the entropies of activation of the respective reactions are nearly equal. Similar phenomena have been observed earlier in the oxidation of diols by BPCC 23 and that of alcohols by PFC 24. Bordwell 25 has documented very cogent evidence against the occurrence of concerted one-step bimolecular processes by hydrogen transfer and it is evident that in the present study also the hydrogen transfer does not occur by an acyclic bimolecular process. It is well established that intrinsically concerted sigmatropic reactions, characterised by transfer of hydrogen in a cyclic transition state, are the only truly symmetrical processes involving a

5 KUMBHAT eta/.: KINETICS OF OXIDATION OF ALIPHATIC ALDEHYDES 1173 ~~ - + Q R_:-C + O=Cr-ObpyH ~ R-C-0-Cr-ObpyH I,, I II\ H 00 HOCI R-COOH + (HOCrCIObpyH]+ Scheme 1 linear hydrogen transfer 26 Littler 27 has also shown that a cyclic hydride transfer, in the oxidation by Cr(VI), involves six electrons and, being a Hiickel-type system, is an allowed process. Thus the mechanism is proposed to involve a nucleophilic attack on the carbonyl group by BPCC, in a fast pre-equilibrium step. The intermediate then picks up a proton to yield a chromate ester. The ester intermediate undergoes disproportionation in the slow step via a cyclic concerted symmetrical transition state leading to the product (Scheme I). It is of interest to compare here the mode of oxidation of aliphatic aldehydes by PFC 6, PBC 7, QFC 8 and BPCC. The oxidation by QFC and PFC presented a similar kinetic picture, i.e. Michaelis-Menten type kinetics, with respect to the reductants, while, the oxidation by PBC 7 and BPCC the reactions are of first order with respect to the reductants. It seems that the values of the formation constants for the intermediates are very small in these two cases. This resulted in the observed second order kinetics. Kinetic isotope effects, solvent effects and the dependence of the hydrogen ions are of similar nature in all these reactions, for which essentially similar mechanisms have been proposed. Acknowledgement Thanks are due to U G C, New Delhi, for the financial support. References I Guziec F S & Luzio FA, Synthesis, 1980, Rathore S, Sharma P K & Banerji K K, J chem Res, ( 1994) (S) 298 (M), Rathore S, Sharma P K & Banerji K K, Indian J Chern., 34B(I995) Loonkar K, Sharma P K & Banerji K K, J chem Res, ( 1997) (S) 242 (M) Loonkar K, Sharma P K & Banerji K K, J chem Res, (1998) (S) 66 (M) Agarwal A, Choudhury K & Banerji K K, J chem Res ( 1990) (S) Khanchandani R, Sharma P K & Banerji K K, Indian J Chern, 35A (1996) Khurana M, Sharma P K & Banerji K K, React Kinet Catal Lett, 61 (1999) Mitchell J Jr, Organic analysis Vol II (lnterscience, New York) 1954, Perrin D D, Armarego L & Perrin D R, Purification of II organic compounds, (Pergamon Press, Oxford), Hall R T & Schaefer W E, Organic analysis, Vol II (lnterscience, New York) 1954, Leffler J E, J org Chem, 20 ( 1955) 1202; J phys Chem, 29 (1964) Patterson R C, J org Che1n, 29 ( 1964) Exner 0, Collect Chem Czech Commun, 31 ( 1966) Kamlet M J, Abboud J L M, Abraham M H & Taft R W, 1 org Chem, 48 (1983) 2877 and references cited therein. 16 Exner 0, Collect Czech Chem Commun, 38 ( 1973) Swain C G, Swain M S, Powel A L & Alunni S, JAm chem Soc, I 05 (1983) Bell R P, Adv phys org Chem, 4 ( 1966) I. 19 Taft R W, Steric effects in organic chemistry, edited by M S Newman (Wiley, New York) 1956, Chapter Kwart H & Nickel J H, JAm chem Soc, 95 (1973) K wart H & Latimer M C, JAm chem Soc, 93 ( 1971) Kwart H & Slutsky J, J chem Soc Chem Commwz, (1972) Loonkar K, Sharma P K & Banerji K K, J chem Res, ( 1997) (S) 242; (M) Banerji K K, J chem Soc, Perkin Trans 2, (1988) Bordwell F G, Ace chem Res, 5 ( 1974) Woodward R W & HolTmann R, Angew Chem lnt Ed Eng, 8 (1969) LitHer J S, Tetrahedron, 27 ( 1971) 81.