Kin~tics and mechanism of oxidation of phosphinic, phenylphosphinic phosphorous acids by pyridinium bromochromate

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1 Indian J<ltmal of Chemistry Vol. 33AI July 1994, pp Kin~tics and mechanism of oxidation of phosphinic, phenylphosphinic phosphorous acids by pyridinium bromochromate and Anjali Grover, Seema Varshney & Kalyan K Banerji* Department of Chemistry, J.N.Y. University, Jodhpur Received 10 January 1994; revised and accepted 11 March 1994 The title reaction is of second order, first order with respect to each reactant. The reaction is alysed by H + ions and the H + dependence has the form kobs = a + b [H+]. The oxidation of phosp. 'c and phosphorous acids exhibits a substantial primary kinetic isotope effect. The reaction has b en studied in nineteen organic solvents and the effect of solvent is analysed using Taft's and S ain's multiparametric equations. The participation of the tautomeric forms of the phosphorus o yacids, in the oxidation process, has been discussed. It has been concluded that the trichlorinated fo m of the phosphorus oxyacid does not participate in the oxidation process and a suitable mechanis has been proposed. Pyrid' urn bromochromate (PBC) has been reported as a mild and selective oxidizing reagent in synthe c organic chemistry!. There seems to be only 0 e report on the mechanistic aspects of oxidation by PBC2. We have been interested in the kinetic of reactions of complexed Cr(VI) species and ha e reported the kinetics and mechanism of oxidati n of lower oxyacids of phosph6rus by pyrid' urn fluorochromate (PFC) and pyridinium chloro hromate (PCC)3,4. It was observed that the oxidati ns by PFC and PCC presented different kinetic pictures. Further, the lower oxyacids of phosp rou are reported to exist in two tautomeric fo rns5 and it is of interest to determine the nature f tautomer of the oxyacids involved in the oxidati n process. We report in this paper, the kinetic of the oxidation of phosphinic (PA), phenyl hosphinic (PPA), and phosphorous (POA) acids y PBC in DMSO as solvent. Mechanistic aspects are discussed. Materi s and Methods The oxyacids were commercial products (Fluka) an were used as supplied. PBC was prepared by the reported method!. Deuteriated phosphinic and p osphorous acids were prepared by repeatedly issolving the oxyacid in deuterium oxide (BAR, 99.4%) and evaporating water and the excess f deuterium oxide6 The isotopic purity of the de teriated PA and POA, as determined by NMR pectra, was 91 ± 4% and 93 ± 5% respectively. he solvents were purified by the usual metho S7. Stoichiometry The oxidation of lower oxyacids of phosphorus leads to the formation of corresponding oxyacids containing phosphorus in a higher oxidation state. Reaction mixtures were prepared containing a known excess of phosphinic or phosphorous acid. On the completion of reaction, amount of phosphorous acid formed in the oxidation of phosphinic acid and the residual reductant in the oxidation of phosphorous acid were determined by the literature method. To determine the stoichiometry of the oxidation of PPA, a known excess of PBC was treated with PPA and the residual PBC was determined spectrophotometrically at 356 nm after the completion of the reaction. The oxidation state of chromium in a completely reduced :t;eaction mixture, determined by iodometric titrations, was 4.07 ± Kinetic measurements The reactions were carried out under pseudofirst order conditions by maintaining a large excess of [oxyacid] over [PBC]. The solvent was DMSO, unless otherwise specified. The reactions were followed, at constant temperatures ( ± 0.1 K), by monitoring the decrease in [PBe] spectrophoto metrically at 356 nm. No other reactant or product has any significant absorption at this wavelength. The pseudo-first order rate constant, kobs, was evaluated from the linear (r= ) plots of log [PBC] against time. Duplicate kinetic runs showed that the rate constants were reprod-

2 BANERJI et at.: KINETICS OF PHOSPHINIC, PHENYLPHOSPHINIC & PHOSPHOROUS ACIDS 623 ucible to within ± 3%. Simple and multivariate linear regression analyses were carried out by the least-squares method on a personal computer. Results The analysis of product formed in the oxidation of PA showed that 1.01 ± 0.05 mole of the product is formed for every mole of PBC consumed. Similarly in the oxidation of POA, amount of the oxyacid consumed per mole of PBC consumed is 1.01 ± In the oxidation of PPA, the amount of PBC consumed per mole of the oxyacid o~dized is 1.03 ± The overall reaction may be written as in Eq. (1). RPH(O)OH + Cr02BrO-PyH+ --+ RP(O)(OHh + CrOBrO-PyH+... (1) PBC undergoes a 2-electron change. This is in accord with the earlier observations with both PFC and PCC34. Chromium(IV) species are known to be less stable. However, the reduction products of both PCC8 and PFC9 have been well characterised to be Cr(IV) species. Rate laws The reaction is first order with respect to PBC. Further, the pseudo-first order rate constnat, kobs' is independent of initial [PBC]. The reaction is of first order with respect to the oxyacid also. The reaction is catalysed by hydrogen ions. The H + dependence has the form kobs = a + b [H +]. Addition of a radical scavenger, acrylonitrile, had no effect on the rate (Table 1). Kinetic isotope effect To ascertain the importance of the cleavage of a P - H bond in the rate-determining step, oxidation of deuteriated PA and POA was studied. Results showed the presence of a substantial primary kinetic isotope effect (at 303 K, kh/ ko was found to be 5.56 for PA and 5.08 for POA). Effect of temperature and solvents The reaction rates at different temperatures were determined and the activation parameters were calculated (Table 2). The oxidation of PPA was studied in 19 different organic solvents. The choice of solvents was limited by the solubility of PBC and its reaction with primary and secondary alcohols. There was no reaction with the solvents chosen. Kinetics were similar in all the solvents. The values of k2 are recorded in Table 3. Discussion The oxidation of PA resulted in the formation of POA. PA is oxidized at ca. five times the rate of oxidation of POA. To reduce the effect of further oxidation of POA on the kinetics and stoichiometry of the oxidation of PA, the concentration of oxyacid was always kept in large excess over the concentration of PBC. Table I-Rate PPA POA (mol [oxyacid] * [W] dm-3) constants 104 for kobs oxidation (s - I ) of the oxyacids by PBC at 303 K in DMSO PA 1Q3[PBC]

3 624 INDIAN J CHEM, SEe. A, JULY 1994 Table 2-A~id Temperature dependence ]()3k2(dm'mol-1s-l) and the activation parameters of the fj.h* oxidation of oxyacidsfj.s* of phosphorus (kjmol-l) (j mol-i K-I) ± K ± ±2 131 ± ±23 by PBC in DMSO fj.g* (kj mol-i) 86.9± ± ± 1.0 Table 3- Effect of solvents on the oxidation of phenylphosphinic acid by PBC at 303 K Solvent lo4 k2 Solvent 104, k2 (dm' mol-i S-I) (dm) mo!-i S-I) Chloroform 1, Carbon 2-Dimethoxyethane Tetrahydrofuran t-butyl Acetophenone Acetic Ethyl Toluene Dioxane disulphide acetate alcohol acid served H+ dependence suggests that the reaction ollows two mechanistic pathways, one acid-inde endent and another acid-dependent. The aci catalysis may well be attributed to a protonati n of PBC (Eq. 2) to yield a protonated Cr(VI) s ecies which is a stronger oxidant and electrop Ie. Formation of a protonated Cr(VI) species s earlier been postulated in the reactions of s cturally similar PCCl. + PyHOCr zbr + H+ ~ PyHOCr(OH)OBr... (2) R z = ; sd = 0.19; n = 17; tp = 0.20 log kz = (± 0.22).1l'* (± 0.18),8 R z = ; sd = 0.22; n = 17; tp = 0.23 log kz = (± 0.23).1l'* rz = ; sd = 0.22; n = 17; tp = 0.24 log kz = (± 0.55),8 rz = ; sd = 0.65; n = 17; tp = (5).. '. (6)... (7) So/vent The r e constants of oxidation, kz, in seventeen solv nts (CSz and acetic acid were not considered the complete range of solvent parameters wer not available) were correlated in terms of linear solvation energy relationship (LSER) of Kamlet a d Taftlt (Eq.3). logkz= +p.1l'*+b,8+aa... (3) In Eq. ( ),.1l'*represents the solvent polarity,,8 the hydr gen bond acceptor acidities and a is the hydroge bond donor basicities. ~ is the intercept ter. The results of correlation analyses terms of Eq. (3), a biparametric equation involving.1l'* d,8, and separately with.1l'*and,8 are given bel w [Eqs (4)-(7)]. log kz = l'*+ 0.11, a... (4) (±0.21) (±0.18) (±0.29) Here n is the number of data points and tp is Exner's statistical parametertz. Kamlet and Taft's triparametric equation explain> 93% of the effect of solvent on the oxidation. However, by Exner's criterionlz, the correlation is not even satisfactory (ct. Eq. 4). The major contribution is of solvent polarity. It alone accounted for ca. 90% of the data. The data on the solvent effect were also anaj lysed in terms of Swain's equation13 of cationand anion-solvating concept of the solvents (Eq.8) log kz = aa + bb + C... (8) 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

4 BANERJI et at.: KINETICS OF PHOSPHINIC, PHENYLPHOSPHINIC & PHOSPHOROUS ACIDS solvents were analysed in terms of Eq. (8), separately with A and B and with (A+ B). log k2 = 0.45 (± 0.04) A (± 0.03) B (9) R2 = ; sd = 0.03; n = 19; 1/J= 0.03 log k2=0.15 (±0.90)A (10) r2 = ; sd = 0.72; n = 19; 1/J= gk2=2.69(±0.13)B (11) r2 = ; sd = 0.11; n = 19; 1/J= 0.11 log k2 = 2.00± 0.28 (A+B) (12) r2 = ; sd= 0.37; n = 19; 1/J=0.38 The rates of oxidation of PPA in different solvents show an excellent correlation if Swain's equation (Eq. 9) is used with the cation-solvating power playing the major role. In fact, the cationsolvation alone accounts for > 98% of the data. The solvent polarity, represented by (A+ B), also accounted for ca. 75% of the data. Reactive reducing species Lower oxyacids of phosphorus are reported4 to exist in two tautomeric forms (Eq. 13). The tricoordinated tautomer (B) is thought to be produced as an intermediate in the exchange of phosphorous bonded hydrogen with deuterium and tritium. The value13 of the equilibrium constant, Kt> is ofthe order of RPH(O)OH +t RP(OH)2 RPH ( 0 )OH + PBC k. Products... (13) (A) (B) (R = H, Ph, or OH) Hence two alternative mechanisms can be postulated. Assuming only the pentacoordinated tautomer (A) as the reactive form, the main reactijon will be as shown in Eq. (14), which leads to rate law (15).... (14)... (15) where [oxyacid]o represents the initial concentration of the oxyacid. Equation (15) is reduced to Eq. (16) as 1 ~ Kt. Rate = k.[oxyacid]o [PBC]... (16) Another mechanism can be written assuming the tricoordinated form (B) as the reactive reducing species, Eq. (17), which leads to rate Eq. (18). kb RP(OH)2 + PBC Products (17) Rate = Kt kb [oxyacid]o [PBC]/(l + Kt) (18) Rate law (18) can be reduced to Eq. (19) since l~kt Rate = Kt kb [oxyacid]o [PBC]... (19) Thus the two rate equations conform to the experimental rate law and are kinetically' indistinguishable. If Eqs (13) and (17) represent the mechanism of the oxidation of oxyacids of phosphorus then the experimental specific rate constant, k2 = Kt kb' The value of Kt is of the order of Therefore, the value of rate-limiting constant, kb, ranges between 108 and 109 dm3 mol-1 s -1. This value exceeds/equals the rate constants of diffusioncontrolled rate processesl4. Therefore, the participation of the tricoordinated form of the oxyacids in the oxidation process can be ruled out. The presence of a substantial primary kinetic isotope effect confirms the cleavage of a P- H bond in the rate-determining step. A preferential cleavage of a P- H bond, in the rate-determining step, is likely in view of the relatively high bond dissociation energy of the 0 - H bond. The mean value15 of the bond dissociation energy of a 0 - H bond is 460 kj mol- I, while that for a P-H bond16 is 321 kj mol-i. A one-electron oxidation, giving rise to free radicals, is unlikely in view of the absence of any effect of acrylonitrile on the reaction. There is no kinetic evidence for the formation of an intermediate adduct, though its formation in very small amounts cannot be ruled out. The mechanism in Scheme 1 accounts for all the experimental results., k RPH(O)OH CrBrOPyH ~ RP(O)OH + (HOCrOBrOPyHt... (20) + fast RP(O)OH + H20 -- RP(O)(OH}z + H+... (21) Scheme 1 A mechanism involving a hydride-ion transfer in the rate-determining step is also supported by the major solvents. role of cation-solvating power of the It is of interest to compare here the mode of oxidation of lower oxyacids of phosphorus by PFC3, PCC4, and PBC. The oxidation by PFC exhibited a Michaelis-Menten type kinetics with respect to the oxyacids. While the oxidation by both PCC and PBC presented similar kinetic pictures. The rate laws, H + dependence, and kinetic iso-

5 626 INDIAN J CHEM, SEe. A, JULY 1994 tope effe t, are similar in both the cases. In all the three oxi ations, excellent correlations were obtained in erms of Swain's equation 13 with the cation-solv ing power of the solvents playing the major rol. Though the data are not conclusive, it seems th t the mode of oxidation depends on the nature of the halogen present in the Cr(VI) species and n t on the nature of the base. Thanks are due to the UGC, New Delhi and the ACknOWI~dgement CSIR, New Delhi for financial support. Referenc s 1 Naraya n N & Balasubramanian T R, Indian J Chem, 25B(196) Narayan N & Balasubramanian T R, J chem Res (S), (1991) Monndr A, Mathur A & Banerji K K, J chem Sac, Dalton Tra, (1990) Seth M, Mathur A & Banerji K K, Bull chem Sac Japan, 63 (1990) Fratiello A & Anderson E W, JAm chem Sac, 85 (1963) Haight P, Rose M & Preer J, JAm chem Sac, 90 (1968) Perrin D D, Armstrong W L &. Perrin D R, Purification of organic compounds (Pergamon Press, Oxford) Bhattacharjee N M, Chaudhuri M K & Purakayastha S, Tetrahedron, 43 (1987) Brown H C, Gundu C R & K'ulkarni S U, J org Chern, 44 (1979) 2809; Synthesis, (1979) Banerji K K, J chem Soc, Perkin Trans, 2, (1978) Kamlet M J, Abboud J L M, Abraham M H & Taft R W, J org Chern, 48 (1983) 2877 and references cited therein. 12 Exner 0, Collect Czech Chem Commun, 31 (1966) Swain C G, Swain M S, Powel A L & Alunni S, J Am chem Soc, 105 (1983) Vetter K J, Electrochemical kinetics- Theoretical and experimental aspects (Academic Press, New York) (1967), p.slt. 15 Lovering E G & Laidler K J, Can J Chern, 38 (1960) Gunn S R & Green L G, J phys Chem, 65 (1961) 779.