Indian Journal of ChemistryVol. 29A, October 1990, pp. 963-966

Kinetics of oxidation of phosphinic, phenylphosphinic and phosphorous ac-ids by bromamine- B in aqueous perchloric medium

Anu Moondra, Abha Mathur & Kalyan K Banerji"Department of Chemistry, University of Jodhpur, Jodhpur 342 001

Received 12 December 1989; revised and accepted 2 March 1990

The oxidation of phosphinic, phenylphosphinic and phosphorous acids by sodium N-bromoben-zenesulphonamide (BAB) in perchloric acid solution, results in the formation of corresponding phos-phorous acids of higher valence state. The reaction is first order in [BAB], [oxyacid] and [H+]. Oxi-dation of deuteriated phosphinic and phosphorous acids shows the presence of a substantial primarykinetic isotope effect. The reaction fails to induce polymerization of acrylonitrile. Added benzenesul-phonamide has no effect on the reaction rate. (PhS02NH2Br)+ has been postulated as the reactiveoxidizing species. It has been shown that the inactive tautomer of the phosphorous oxyacids,RHp(O)OH, participates in the oxidation process. A rate-determining step involving transfer of a hy-dride ion from the P- H bond to the oxidant has been proposed.

The study of the reactions of oxyacids of phos-phorus, in lower valence state is of interest bec-ause of the existence of the two tautomers,Im:OHjz and RPH(O)OH (see refs 1 and 2).Oxidation of oxyacids of phosphorus (I) and (ill)by. various transition metal ions has been repor-ted3·12• Except for oxidation of phosphorous acidby iodine 13 and that of phosphinic acid by chlora-mine-T", the kinetics of oxidation by halogens ortheir compounds do not appear to have been in-vestigated. In the chloramine-T oxidation 14 ofphosphorous acid, no attempt was made t deter-mine the nature of the reactive form of the substr-ate. The title investigation is in continuation ofour earlier interest in the oxidation by N-metallo-N-haloarylsulphonamides 15.

Materials and MethodsThe oxyacids of phosphorus were commercial

products (Fluka) and were used as such. Thestock solutions of phosphinic acid (1) and phenyl-phosphinic acid (2) were standardized by cerime-tryl6. The aqueous solution of phosphorus acid(3) was standardized by alkalimetry. Bromami-ne-B (or BAB) was prepared by the reportedmethod'? and standardized iodometrically. Perch-loric acid (E. Merck, GR) was used as a source ofhydrogen ions. Doubly distilled conductivity waterwas used for the preparation of solutions.

The P- H bond in 1 and 3 was deuteriated byrepeatedly dissolving the acids in deuterium oxide(BARe, 99.4% purity), and evaporating the ex-cess of deuterium oxide and water in vacuol8•

The isotopic purity of the deuteriated 1 and 3, asascertained by their PMR spectra, were 91 ± 5and 93 ± 4% respectively.

StoichiometryOxidation of oxyacids of phosphorus by BAB

leads to the formation of corresponding oxyacidsof phosphorus in which phosphorus is in the high-er valence state. Reaction mixtures were preparedcontaining a known excess of 2, BAB and perch-loric acid. On completion of the reaction, theamount of phenylphosphonic acid formed was de-termined by the method'" described below.

The reaction mixture was partially neutralizedto bring [perchloric acid] < 0.1 mol dm - 3. Aknown excess of iodine was added to it, and thepH was adjusted to 7.3 (phosphate buffer). Afterca. 1 hr, the solution was acidified with 6 moldm - 3 acetic acid and excess of iodine was imme-diately titrated against a standardized thiosulphatesolution to a starch end point. Several such deter-minations indicated that ~[2]1~[BAB] = 1.06 ± 0.08. Hence the overall reactionmay be represented by Eq. (1 ).

RP02H2 +PhS02NBr- +H20-+(R=H,PhorOH)

RP03H2 +PhS02NH2 + Br : . .. (1)

Kinetic measurementsThe reactions were carried out under pseudo-

first order conditions ([substrate] > > [BABj) at a

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INDIAN J CHEM, SEC A, OCTOBER 1990

fixed temperature (±0.1 K). The reactions werefollowed by determining unreacted [BAB] at dif-ferent time intervals, iodometrically. The pseudo-first order rate constants, k., were evaluated fromthe linear plots of log [BAB] against time. Thereactions were followed upto almost 80% comple-tion. Duplicate kinetic runs indicated that the rateconstants were reproducible to within ± 4%. Thethird order rate constants, k3' were obtained fromthe relation Is = kj/[H+] [substrate]. Preliminaryexperiments showed that the reactions are notsensitive to ionic strength (0.1-2.5 mol dm t '),

hence no attempt was made to keep the ionicstrength constant.

ResultsThe reaction is first order in [BAB], [substrate]

and [H +]. Individual kinetic runs followed first or-der kinetics. Further, k, is independent of initial[BAB] (Table 1).

The rate of oxidation is not affected by the ad-dition of benzenesulphonamide (BSA), one of theproducts of the reaction (Table 2).

The oxidation reaction, under nitrogen atmos-phere, failed to induce polymerization of acryloni-trile. In control experiments, without the substr-ate, no reaction between BAB and acrylonitrilewas observed.

The oxidation of deuteriated 1 and 3 indicatedthe presence "Ifsubstantial primary kinetic isotopeeffect (kHlko = 4.83 and 5.02 respectively). Therates of deuteriated 1 and 3 were corrected forthe presence of protio compounds.

The rates were determined at different temper-atures and the activation parameters were evaluat-ed (Table 3).

DiscussionIn an acidified solution of BAB, the possible

oxidizing species are PhS02NHBr, HOBr,PhS02NBr2 and BAB itself (see ref. 20). Sinceadded BSA has no effect on the reaction rate, anequilibrium in which BSA is one of the products

is ruled out. This in effect also rules outPhS02NBr2 and HOBr as the oxidizing species.N-Haloarylsulphonamides are weak acids-?(pK "" 5) and even in the lowest concentration ofperchloric acid used, almost all of BAB will existas PhS02NHBr. A further increase in the aciditydoes not change the [PhS02NHBr] noticeably.Therefore, PhSO: NHBr is postulated as the reac-tive oxidizing species in the present reaction.

Table 1 - Rate constants of oxidation of phosphorous acid byBAB at 303 K

[H3P03] 103[BAB] [W] 105 k,(mol dmt) (mol dm v) (rnot dm=') (S-I)

0.1 1.0 0.2 8.030.1 2.5 0.2 7.860.1 5.0 0.2 7.950.1 7.5 0.2 8.000.1 10.0 0.2 7.900.2 5.0 0.2 16.00.4 5.0 0.2 31.5

0.6 5.0 0.2 47.00.8 5.0 0.2 63.31.0 5.0 0.2 79.70.2 5.0 0.1 7.920.2 5.0 0.3 23.60.2 5.0 0.5 40.00.2 5.0 0.7 55.20.2 5.0 0.9 71.4

0.2 5.0 1.0 80.0

Table 2 - Effect of added benzenesulphonamide on oxidationof phenylphosphinic acid by BAB

[BAB] 0.005 mol dm - 3; [PhPHP2] 0.10 mol dm - 3; [H +] 0.10mol dm - 3; temp. =298 K

W[BSAj 105 k, W[BSAJ 105 k,(moldm-3) (s -I) (mol dm r ') (S-I) Of

0.0 17.5 7.5 17.5

2.5 18.1 10.0 18.3

5.0 17.2

Table 3 - Rate constants and activation parameters of oxidation of oxyacids of phosphorus by BABl!J.G"Acid 104 k; (dm" mol '? S-I) l!J.H" l!J.S"

298 303 308 313 318 K (kJ mol " ') (Jmol-I K-I) (kl mol ' ')

Phosphorus 24.5 39.7 57.8 91.4 143 66.2 -73 87.9

Phenylphosphinic 176 273 370 515 747 53.1 -101 83.0

Phosphinic 5.80 10.0 14.7 23.2 35.4 67.8 -80 91.4

Average error: l!J.H", ± 1.0 kJ mol-I; l!J.S", ± 3.3 J mol-I K -I; l!J.G", ±O.9 kJ mol-I

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MOONDRA et at.: OXIDATION OF PHOSPHINIC, PHENYLPHOSPHINIC & PHOSPHOROUS ACIDS, BY BROMAMINE-B

The linear increase in the rate with an increasein the acidity points to a protonation ofPhS02NHBr in the pre-equilibrium (Eq. 2) togive a stronger oxidant and electrophile.

K,PhS02NHBr+ H30+ +:!: (PhS02NH2Br)+ + H20

... (2)

Formation of similarly protonated species instructurally related chloramine-T has been sup-ported by radiochemical studies?'.

The oxyacids of phosphorus are weak acids",and in the presence of a strong acid like perchlor-ic acid, they would exist almost wholly in the un-dissociated form. Oxyacids of phosphorus are be-lieved to exist in two tautomeric forms 1,2(Eq. 3).

oII K2

R-~-OH ~~~':.':.~-_-~

Hinactive

R-P-OHI '•...OH

octive4

The active form is thought to be produced as anintermediate in the exchange of phosphorusbonded hydrogen with tritium and deuterium':".The value of the equilibrium constant, K2, is ofthe order of 10-12 (ref. 22). Two alternate broadmechanisms can be written, one assuming the in-active form as the reactive species (see Eq. 4)

k.RHR:O)OH + (PhS02NH2Br) + Products

... (4)

The rate law for the sequence of reactions (2-4)will have the form (5)

- d[BAB] = K1k..[BAB][RHP(O)OHlo[H+] ... (5)dt 1+ K2

where [RH.E~O)OH]odenotes the initial concentra-tion of the reductant. Since 1> > K2, Eq. (5) canbe written as (6).

Similarly the sequence of reactions (2), (3) and(7) is proposed for the oxidation of the 'active'form of the substrates, thus leading to the ratelaw (8).

k"-+ Products ... (7)

which can be reduced to Eq. (9), acknowledgingthat 1 > > K2

- t(BAB]/ dt= K1K2!G,[RHR:O)OHJo[BAB][H+]. .. (9)

Thus both the rate equations confirm to the ex-perimental rate law.

If the active of the substrate is the reactive spe-cies and the sequence of reactions (2), (3) and (7)represent the mechanism of the reaction, then theexperimental specific rate constant, Is, is given byEq. (10).

... (10)

The value of K2 is of the order of 10 - J2. The va-lue of KI is not known. However, KI must besmall « 10-3) as no levelling off of the rate is ob-served even at the higher range of acidity. Assum-ing a value of KI as 10-3, the value of the rate li-miting constant, kb' comes to be in the range of1012 and 1015 s - I. This rate constant, thus, ex-ceeds/equals the rate constants of diffusion-con-trolled processes+'. Therefore, one can rule outthe participation of the active tautomer of the ox-yacids of phosphorus in the oxidation process.

The presence of a substantial kinetic isotope ef-fect confirms the rupture of P- H bond in therate-determining step. A one-electron oxidation,giving rise to free radicals, is unlikely in view ofthe failure to induce polymerization of acryloni-trile. There is no kinetic evidence for the forma-tion of an intermediate complex. Formation of ahypobromite ester and its subsequent decomposi-tion can also be ruled out, in view of the nil effectof added BSA on the reaction rate. Hence it isproposed that the rate-determining step involves ahydride ion transfer to the oxidant (see Eqs. 11and 12).

OHI s~w •

O=P-H .• Br -NH2S02 Ph - - - - - -) RP(OlQH • HBr • PhS02 NH2 (11)IR

• losl +RP(O) OH + H20 -----) R P(O)(OH)2 + H (12)

The rates of oxidation of the oxyacids of phos-phorus follow the order: phenylphosphinicacid> phosphorous acid> phosphinic acid. Phenylgroup is capable of stabilizing the cationic speciesformed in reaction (11) by resonance. This mayaccount for the faster oxidation of phenylphos-phinic acid.

It is of interest to compare the mode of oxida-tion of oxyacids of phosphorus by BAB and other

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INDIAN J CHEM, SEC A, OCTOBER 1990

oxidants. The oxidations by Hg(II)24, Ag(II? andiodine" show a zero order dependence in oxidantand the rate-determining step is proposed to beconversion of inactive tautomer to the active one.The oxidation of phosphinic acid by permangan-ate? and vanadiumt'V )" involves the inactive form.Cerium(JV) in nitric acid" is reported to oxidizethe active form of phosphinic acid. The acid de-pendence of rate in the oxidation+ of phenyl-phosphinic acid by vanadium(V) has been ex-plained on the basis of simultaneous oxidation ofactive form (acid-independent path) and the inac-tive form (acid-dependent path). Sen Gupta etal.26 however, proposed that phosphorous acid isoxidized by vanadium(V) via its active form, butno evidence has been given for the same. Similar-ly in chromic acid oxidation also:", involvement ofthe acive form has been assumed without any kin-etic evidence. Recently, Sharma and Mehrotra'"reported that it was not possible, in the oxidationof phenylphosphinic acid by chromic acid, to pin-point the reactive form of the phosphorus com-pound. In view of the extremely small equilibriumconstant, the concentration of the active tautomerin solutions would be very small. Any reaction in-volving the active form only, is therefore, likely tobe very slow. In most other reports, including thatinvolving chloramine-T'", there is no mention oftautomerism and the predominant tautomer hasbeen assumed as the reactive species.

AcknowledgementThanks are due to the CSIR and UGC, New

Delhi for financial support.

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