Kin~Tics and Mechanism of Oxidation of Phosphinic, Phenylphosphinic and Phosphorous Acids by Pyridinium Bromochromate

Vol.Indian 33AI J

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

Anjali Grover, Seema Varshney & Kalyan K Banerji* Department of Chemistry, J.N.Y. University, Jodhpur 342 005 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 phos• p . '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 mechan• is has been proposed.

Pyrid' urn bromochromate (PBC) has been re• Stoichiometry ported as a mild and selective oxidizing reagent in The oxidation of lower oxyacids of phospho• synthe c organic chemistry!. There seems to be rus leads to the formation of corresponding oxy• « only 0 e report on the mechanistic aspects of ox• acids containing phosphorus in a higher oxida• idation by PBC2. We have been interested in the tion state. kinetic of reactions of complexed Cr(VI) species Reaction mixtures were prepared containing a and ha e reported the kinetics and mechanism of known excess of phosphinic or phosphorous acid. oxidati n of lower oxyacids of phosph6rus by On the completion of reaction, amount of phos• pyrid' urn fluorochromate (PFC) and pyridinium phorous acid formed in the oxidation of phos• chloro hromate (PCC)3,4. It was observed that the phinic acid and the residual reductant in the oxi• oxidati ns by PFC and PCC presented different dation of phosphorous acid were determined by kinetic pictures. Further, the lower oxyacids of the literature method. To determine the stoichi• phosp rou are reported to exist in two tautom• ometry of the oxidation of PPA, a known excess eric fo rns5 and it is of interest to determine the of PBC was treated with PPA and the residual nature f tautomer of the oxyacids involved in the PBC was determined spectrophotometrically at oxidati n process. We report in this paper, the 356 nm after the completion of the reaction. The kinetic of the oxidation of phosphinic (PA), oxidation state of chromium in a completely re• phenyl hosphinic (PPA), and phosphorous (POA) duced :t;eaction mixture, determined by iodometric acids y PBC in DMSO as solvent. Mechanistic titrations, was 4.07 ± 0.18. aspects are discussed. Kinetic measurements Materi s and Methods The reactions were carried out under pseudo• The oxyacids were commercial products (Flu• first order conditions by maintaining a large ex• ka) an were used as supplied. PBC was prepared cess of [oxyacid] over [PBC]. The solvent was by the reported method!. Deuteriated phosphinic DMSO, unless otherwise specified. The reactions and p osphorous acids were prepared by repea• were followed, at constant temperatures ( ± 0.1 K), tedly issolving the oxyacid in deuterium oxide by monitoring the decrease in [PBe] spectropho• (BAR , 99.4%) and evaporating water and the to metrically at 356 nm. No other reactant or pro• excess f deuterium oxide6• The isotopic purity of duct has any significant absorption at this wave• the de teriated PA and POA, as determined by length. The pseudo-first order rate constant, kobs, NMR pectra, was 91 ± 4% and 93 ± 5% respect• was evaluated from the linear (r= 0.990-0.999) ively. he solvents were purified by the usual plots of log [PBC] against time. Duplicate kinetic metho S7. runs showed that the rate constants were reprod-

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ucible to within ± 3%. Simple and multivariate dependence has the form kobs = a + b [H +]. Addi• linear regression analyses were carried out by the tion of a radical scavenger, acrylonitrile, had no least-squares method on a personal computer. effect on the rate (Table 1).

Results Kinetic isotope effect The analysis of product formed in the oxidation To ascertain the importance of the cleavage of a of PA showed that 1.01 ± 0.05 mole of the pro• P - H bond in the rate-determining step, oxidation duct is formed for every mole of PBC consumed. of deuteriated PA and POA was studied. Results Similarly in the oxidation of POA, amount of the showed the presence of a substantial primary kin• oxyacid consumed per mole of PBC consumed is etic isotope effect (at 303 K, kH/ ko was found to 1.01 ± 0.03. In the oxidation of PPA, the amount be 5.56 for PA and 5.08 for POA). of PBC consumed per mole of the oxyacid o~d• ized is 1.03 ± 0.03. The overall reaction may be Effect of temperature and solvents written as in Eq. (1). The reaction rates at different temperatures were determined and the activation parameters RPH(O)OH + Cr02BrO-PyH+ --+ were calculated (Table 2). The oxidation of PPA RP(O)(OHh + CrOBrO-PyH+ ... (1) was studied in 19 different organic solvents. The choice of solvents was limited by the solubility of PBC undergoes a 2-electron change. This is in PBC and its reaction with primary and secondary accord with the earlier observations with both alcohols. There was no reaction with the solvents PFC and PCC34. Chromium(IV) species are chosen. Kinetics were similar in all the solvents. known to be less stable. However, the reduction The values of k2 are recorded in Table 3. products of both PCC8 and PFC9 have been well characterised to be Cr(IV) species. Discussion The oxidation of PA resulted in the formation Rate laws of POA. PA is oxidized at ca. five times the rate The reaction is first order with respect to PBC. of oxidation of POA. To reduce the effect of fur• Further, the pseudo-first order rate constnat, kobs' ther oxidation of POA on the kinetics and stoichi• is independent of initial [PBC]. The reaction is of ometry of the oxidation of PA, the concentration first order with respect to the oxyacid also. The of oxyacid was always kept in large excess over reaction is catalysed by hydrogen ions. The H + the concentration of PBC.

0.1 Table I-Rate17.90.168133.00.480.0*0.960.120.818.80.120.30.120.27.1215.00.1330.120.03.208.900.480.00.720.019.453.80.4040.240.120.120.51.200.6360.4070.40225.672.00.5400.480.480.012.935.90.3980.067PPAPOA12.513.012.635.324.20.23535.60.27836.00.325(mol[oxyacid]0.033.089.30.012.135.00.4126.4518.00.1420.09.065.1011.90.100[W] dm-3)(moldm-3) constants104 forkobs oxidation(s - I) of the oxyacids by PBC at 303 K in DMSO (mol dm-3) PA 1.04.02.08.01.0 1Q3[PBC] *contained 0.001 mol dm -3 acrylonitrile 624 INDIAN J CHEM, SEe. A, JULY 1994

by PBC in DMSO

Table 2-A~id Temperature dependence]()3k2(dm'mol-1s-l) and the activation parameters of thefj.H* oxidation of oxyacidsfj.S* of phosphorus fj.G* (kJmol-l) (j mol-I K-I) (kJ mol-I) 32312.52.254.9521.43034.609.003137.4216.745.7±0.7-1400.561.2529351.836.339.2± K ± 0.6 1.3-131-153±2 ±±2 3 PA 2.67 86.9±0.6 PIA 84.5 ±0.5 90.6 ± 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-DimethoxyethaneTetrahydrofurant-ButylAcetophenoneAceticEthylDioxane7.76 disulphideacetate alcohol12518.6 acid12.64.500.6333.114.5Toluene3.164.4728.24.000.127.9420.91.3522.442.72.40 Dimethylformamide1,2-DichloroethaneDichloromethaneNitrobenzeneCyclohexaneButanoneBenzeneAcetoneDMSO

served H+ dependence suggests that the R z = 0.9343; sd = 0.19; n = 17; tp = 0.20 reaction ollows two mechanistic pathways, one log kz = - 4.76 + 2.50 (± 0.22).1l'* acid-inde endent and another acid-dependent. + 0.26 (± 0.18),8 ... (5) The aci catalysis may well be attributed to a protonati n of PBC (Eq. 2) to yield a protonated R z = 0.9098; sd = 0.22; n = 17; tp = 0.23 Cr(VI) s ecies which is a stronger oxidant and log kz = - 4.71 + 2.57 (± 0.23).1l'* .. '. (6) electrop Ie. Formation of a protonated Cr(VI) species s earlier been postulated in the reac• rZ = 0.8972; sd = 0.22; n = 17; tp = 0.24 tions of s cturally similar PCCl . log kz = - 3.96 + 0.73 (± 0.55),8 ... (7) + PyHOCr zBr + H+ ~ PyHOCr(OH)OBr ... (2) rZ = 0.1064; sd = 0.65; n = 17; tp = 0.85

So/vent Here n is the number of data points and tp is Exner's statistical parametertz. The r e constants of oxidation, kz, in seven• teen solv nts (CSz and acetic acid were not con• Kamlet and Taft's triparametric equation ex• sidered the complete range of solvent parame• plain> 93% of the effect of solvent on the oxida• ters wer not available) were correlated in terms tion. However, by Exner's criterionlZ, the correla• of linear solvation energy relationship (LSER) of tion is not even satisfactory (ct. Eq. 4). The major Kamlet a d Taftlt (Eq.3). contribution is of solvent polarity. It alone ac• counted for ca. 90% of the data. logkz= +p.1l'*+b,8+aa ... (3) The data on the solvent effect were also anaJ In Eq. ( ), .1l'*represents the solvent polarity, ,8 lysed in terms of Swain's equation13 of cation• the hydr gen bond acceptor acidities and a is the and anion-solvating concept ·of the solvents hydroge bond donor basicities. ~ is the inter• (Eq.8) cept ter . The results of correlation analyses terms of Eq. (3), a biparametric equation involv• log kz = aA + bB + C ... (8) ing.1l'* d,8, and separately with .1l'*and ,8 are Here A represents the anion-solvating power of given bel w [Eqs (4)-(7)]. the solvent and B the cation-solvating power. C is log kz = 4.86 + 2.67 .1l'*+0.11 ,8 + 0.61 a ... (4) the intercept term. (A + B) is postulated to repre• (±0.21) (±0.18) (±0.29) sent the solvent polarity. The rates in different

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solvents were analysed in terms of Eq. (8), separ• kb ately with A and B and with (A+ B). RP(OH)2 + PBC ------. Products (17)

log k2 = 0.45 (± 0.04) A + 2.73 (± 0.03) B - 5.05 Rate = Kt kb [oxyacid]o [PBC]/(l + Kt) (18) ... (9) Rate law (18) can be reduced to Eq. (19) since l~Kt R2 = 0.9981; sd = 0.03; n = 19; 1/J= 0.03 Rate = Kt kb [oxyacid]o [PBC] ... (19) log k2=0.15 (±0.90)A-3.17 ... (10) r2 = 0.0022; sd = 0.72; n = 19; 1/J= 1.00 Thus the two rate equations conform to the ex• 4 perimental rate law and are kinetically' indistin• 10gk2=2.69(±0.13)B-4.87 ... (11) guishable. r2 = 0.9770; sd = 0.11; n = 19; 1/J=0.11 If Eqs (13) and (17) represent the mechanism of log k2 = 2.00± 0.28 (A+B)-4.98 ... (12) the oxidation of oxyacids of phosphorus then the experimental specific rate constant, k2 = Kt kb' r2 = 0.7453; sd= 0.37; n = 19; 1/J=0.38 The value of Kt is of the order of 10 - 12. There• The rates of oxidation of PPA in different sol• fore, the value of rate-limiting constant, kb, ranges vents show an excellent correlation if Swain's between 108 and 109 dm3 mol-1 s -1. This value equation (Eq. 9) is used with the cation-solvating exceeds/equals the rate constants of diffusion• power playing the major role. In fact, the cation• controlled rate processesl4. Therefore, the partici• solvation alone accounts for > 98% of the data. pation of the tricoordinated form of the oxyacids The solvent polarity, represented by (A+ B), also in the oxidation process can be ruled out. accounted for ca. 75% of the data. The presence of a substantial primary kinetic isotope effect confirms the cleavage of a P- H Reactive reducing species bond in the rate-determining step. A preferential Lower oxyacids of phosphorus are reported4 cleavage of a P- H bond, in the rate-determining to exist in two tautomeric forms (Eq. 13). The tri• step, is likely in view of the relatively high bond coordinated tautomer (B) is thought to be pro• dissociation energy of the 0 - H bond. The mean duced as an intermediate in the exchange of phos• value15 of the bond dissociation energy of a 0 - H phorous bonded hydrogen with deuterium and tri• bond is 460 kJ mol- I, while that for a P-H tium. The value13 of the equilibrium constant, Kt> bond16 is 321 kJ mol-I. A one-electron oxidation, is ofthe order of 10- 12. 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 for• RPH(O)OH +t RP(OH)2 ... (13) mation of an intermediate adduct, though its for• (A) (B) mation in very small amounts cannot be ruled out. The mechanism in Scheme 1 accounts for all (R = H, Ph, or OH) the experimental results. Hence two alternative mechanisms can be postu• , k RPH(O)OH + 02CrBrOPyH ~ lated. Assuming only the pentacoordinated tau• + tomer (A) as the reactive form, the main reactijon RP(O)OH + (HOCrOBrOPyHt ... (20) will be as shown in Eq. (14), which leads to rate law (15). + fast RP(O)OH + H20 -- RP(O)(OH}z + H+ ... (21) Scheme 1 RPH( 0 )OH + PBC ------.k. Products ... (14) A mechanism involving a hydride-ion transfer ... (15) in the rate-determining step is also supported by the major role of cation-solvating power of the where [oxyacid]o represents the initial concentra• solvents. tion of the oxyacid. Equation (15) is reduced to It is of interest to compare here the mode of Eq. (16) as 1 ~ Kt. oxidation of lower oxyacids of phosphorus by PFC3, PCC4, and PBC. The oxidation by PFC ex• Rate = k.[oxyacid]o [PBC] ... (16) hibited a Michaelis-Menten type kinetics with re• Another mechanism can be written assuming spect to the oxyacids. While the oxidation by both the tricoordinated form (B) as the reactive reducing PCC and PBC presented similar kinetic pictures. species, Eq. (17), which leads to rate Eq. (18). The rate laws, H + dependence, and kinetic iso- 626 INDIAN J CHEM, SEe. A, JULY 1994

tope effe t, are similar in both the cases. In all the 4 Seth M, Mathur A & Banerji K K, Bull chem Sac Japan, three oxi ations, excellent correlations were ob• 63 (1990) 3840. 5 Fratiello A & Anderson E W, JAm chem Sac, 85 (1963) tained in erms of Swain's equation 13 with the ca• 519. tion-solv ing power of the solvents playing the 6 Haight P, Rose M & Preer J, JAm chem Sac, 90 (1968) major rol . Though the data are not conclusive, it 4809. seems th t the mode of oxidation depends on the 7 Perrin D D, Armstrong W L &. Perrin D R, Purification nature of the halogen present in the Cr(VI) spe• of organic compounds (Pergamon Press, Oxford) 1966. cies and n t on the nature of the base. 8 Bhattacharjee N M, Chaudhuri M K & Purakayastha S, Tetrahedron, 43 (1987) 5389. 9 Brown H C, Gundu C R & K'ulkarni S U, J org Chern, 44 (1979) 2809; Synthesis, (1979) 702. 10 Banerji K K, J chem Soc, Perkin Trans, 2, (1978) 639. Thanks are due to the UGC, New Delhi and 11 Kamlet M J, Abboud J L M, Abraham M H & Taft R W, J theACknOWI~dgement CSIR, New Delhi for financial support. org Chern, 48 (1983) 2877 and references cited therein. 12 Exner 0, Collect Czech Chem Commun, 31 (1966) 3222. 13 Swain C G, Swain M S, Powel A L & Alunni S, J Am Referenc s chem Soc, 105 (1983) 502. 1 Naraya n N & Balasubramanian T R, Indian J Chem, 14 Vetter K J, Electrochemical kinetics- Theoretical and ex• 25B(196)229. perimental aspects (Academic Press, New York) (1967), 2 Narayan N & Balasubramanian T R, J chem Res (S), p.Slt. (1991) 3 6. 15 Lovering E G & Laidler K J, Can J Chern, 38 (1960) 3 Monndr A, Mathur A & Banerji K K, J chem Sac, Dal• 2367. ton Tra , (1990) 2967. 16 Gunn S R & Green L G, J phys Chem, 65 (1961) 779.

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