Indian Journal of ChemistryVol. 28A, March 1989, pp. 250-252

Kinetics & mechanism of oxidation ofdimethyl, methyl phenyl & diaryl sul-ph oxides by N-bromocaprolactam

C Srinivasan* & P Pandarakutty JegatheesanSchool of Chemistry, Madurai Kamaraj University,

Madurai 625021

Received 11 January 1988; revised 9 May 1988;accepted 8 June 1988

The oxidation of dimethyl (OMSO) and methyl phenyl(MPSON) sulphoxides by N-bromocaprolactam (NBC)follows zero order kinetics in [sulphoxide], but that ofdiphenyl sulphoxide (OPSO) is first order in [OPSOlHCI04 accelerates the rate of all the oxidation reactions.However, added caprolactam retards the oxidation ofonlyOPSO.

Though N-bromocaprolactam (NBC) has been usedin the asymmetric oxidation of sulphides to sulphox-ides 1, there appears to be no kinetic study of thisreaction. Certain interesting results obtained in thekinetics of N-bromosuccinimide (NBS) oxidationsof alkyl aryl and diaryl sulphoxides? prompted us tostudy the kinetics of oxidation of dimethyl sulphox-ide (DMSO), methyl phenyl sulphoxide (MPSO) anddiphenyl sulphoxide (DPSO) by NBC

NBC3, MPS04 and diaryl sulphoxides" were ob-tained by known methods. DMS05 and acetic acid"were purified as described in literature, All the otherchemicals used were of AR grade,

Kinetic measurements were carried out in aque-ous acetic acid (50% v/v for DMSO and MPSO and90% v/v for diaryl sulphoxides) in dark-colouredvessels in the presence of mercuric acetate (as a sca-venger of molecular bromine) under pseudo-firstorder conditions ([sulphoxide1 ~ [NBC], 10 : 1 ratio).Ionic strength was maintained with sodium perch-lorate. The kinetics were followed by estimating theunreacted NBC iodometrically.

The stoichiometry in all the presently studied oxi-dation reactions was 1 : 1 with corresponding sul-phone as the reaction product (see Eq. 1)

+R-SO- R' + NBC + H30->R - S02 - R' + Caprolactam +H + + HBrR = Ph or CH3; R' = CH3 or Ph

Mechanism of NBC oxidation of DMSO and MPSOThe good linear plots (r~0.995) of log [NBC]

versus time show first order dependence in [NBC].

... (1)

250

The zero order dependence in [substrate 1is evidentfrom invariance of rates with change in [DMS01o or[MPSO 10(Table 1). In both the cases the rates arenot influenced by the variation in ionic strength,[mercuric acetate 1and [caprolactam]. The rate is ac-celerated by added HCI04 and the slopes(DMSO = 0.99; MPSO = 0.96) of the linear plots oflog k, versus log [H+] establish first order depend-ence on Il-l "]

As the oxidation has been carried out in the pres-ence of mercuric acetate, NBC, NBCH+, HOBrand H20Br+ may be envisaged as the possible oxi-dising species in analogy with NBS7. Since the rate isunaffected by the addition of caprolactam, the hy-drolysis and/or disproportionation of NBC toHOBr and H20Br+ is ruled out. The first order de-pendence each in [NBC] and [H +] suggests that

Table 1- Effect of varying [sulphoxide] on rate of oxidationof various sulphoxide

[NBC] = 1 x 10-3 mol dm-3; [Hg?+] = 5 X 10-3 mol dm-3

102 [Sulphoxide] 104 k, 102 k2(moldm-3) (S-I) (dm3mol-'s-l)

DMSO'1.0 11.20 ± 0.452.0 10.90 ± 0.514.0 JO.90 ± 0.287.0 11.40 ± 0.299.0 11.90 ± 0.35

MPSOb1.0 8.87 ± 0.322.0 8.78 ± 0.064.0 8.98 ± 0.336.0 8.72 ±0.30

DPSO'1.0 2.19 ± 0.02 2.19±0.022.0 4.33 ± 0.05 2.17 ±0.D34.0 8.79 ± 0.37 2.20 ± 0.097.0 13.30 ± 0.21 1.90 ± 0.0310 20.90 ± 0.50 2.09 ± 0.05

a 1= 2.5 x JO- 2 mol dm - 3; solvent = 50% aq acetic acid (v/v);temp. = 32°C.b/= 2.5 x 10-2 mol drn ":'; solvent = 50%aq acetic acid (v/v);temp. = 30°C.'[HCI041 = 1.0 x JO-I mol dm ""; 1= 1.35 x JO-I mol dm " ':[Caprolactam 1 = 5 x 10- 4 mol dm - 3; solvent = 90% aq aceticaeid; temp. = 35°C.The error quoted in k is the 95% confidence level of student's Itest.

Table 2- Effect of varying [HCI04) and [caprolactam) on rate ofoxidation of OPSO at 35°C

[OPSOl = 2 x 10-2 mol dm r ': [NBC) = 1 x 10-3 mol drn :";[Hg2+ ) = 5 x 10 - 3 mol dm - 3

104 kl 103 [Caprolactam]'(S-I) (mol dm-3)

1.81 ± 0.04 0.53.66 ± 0.26 1.06.37 ± 0.35 2.0

10.50 ± 0.74 4.014.40 ± 1.20 6.0

10 [HCI04f'(mol drn=']

1.02.03.04.05.0

104 kl(S-I)

4.330 ± 0.053.330 ± 0.111.970±0.171.030 ± 0.080.780 ± 0.07

'[Caprolactam) = 5 x 10-4 mol dm-3;

solvent = 75% aq acetic acid (v/v)b[HCl04)= LOx 10-1 moldm-l;solvent = 90% aq acetic acid (v/v)

Table 3-Second order rate constnats for NBC oxidation of4,4' -disubstituted diphenyl sulphoxides at 35°C

[Sulphoxide] = 1.0 x 10-2 mol drn ":';[NBC) = 1.0 x 10-3 mol dm :";[HCI04) = 1.0 x 10-1 mol dm-3;

[Hg2+] = 5 X 10-3 mol dm" ':[Caprolactam] = 5 x 10-4 mol drn "";/= 1.35 x 10-1 mol dm":solvent = 90% aq acetic acid (v/v)

Substituents at 102 k2 Substituents at 102 k24,4'-positions (dm ' mol- 1s : I) 4,4'-positions (drn ' mol- IS- I)

HMeF

2.190±0.056.030 ± 0.520.984 ± 0.11

CIBr

N02

0.480 ± 0.080.464 ± 0.090.015 ± 0.001

NBCH ", which is formed in the rate-determiningstep, is the reactive oxidising species. To account forthe observations the mechanism shown in Scheme 1is proposed.

•..low

NBCH+ + RSOCH, ~ RS02CH, + caprolactam

(R = CH, or PH)Scheme 1

To explain first order dependence in [NCS] and zeroorder dependence in [anisole], in the chlorination ofanisole by N-chlorosuccinimide, protonation ofNCS was considered to he the rate-determiningstep"

Mechanism of DPSO oxidationThe linear plot of log [NBS] versus time and the

constant values of the second order rate constants atdifferent [OPSO] (Table 1) reveal that the order in

NOTES

Table 4- Effect of varying solvent composition on rate ofNBC oxidation of various sulphoxides

[NBC) = 1 x 10-3; [Hg2+) = 5 X 10-3 mol dm-3

AcOH-H20 104 k, (s -I)% (v/v)

OMSO' MPSOb OPSOc

35-65 10.70 ± 0.7450-50 11.20 ± 0.45 8.87 ± 0.32 0.653 ± 0.0660-40 9.61 ± 0.39 0.771 ± 0.0770-30 6.23 ± 0.72 6.22 ± 0.18 1.010 ± 0.0780-20 3.52 ± 0.05 4.32 ± 0.29 1.490 ± 0.0990-10 2.09 ± 0.17 3.320 ± 0.15

,.bSame as given in Table I.cSame as in Table 1, except temp. = 30T.

each reactant is unity. The rate of oxidation ofDPSO is not affected by variation in ionic strengthand added mercuric acetate. Added caprolactam re-tards the rate (Table 2). The plot of 1/ k, versus [cap-rolactam] is linear exhibiting an inverse order de-pendence in [caprolactam]. The rate of the reactionincreases with increase in [HCl04] (see Table 2).The oxidation of DPSO has been carried out at high[acid] and under these conditions Landini et al? donot favour the correlation of rate constant with acid-ity function in the case of sulphoxides. Studies with4,4' -disubstituted diphenyl sulphoxides (Table 3)show that electron-releasing groups accelerate andelectron-withdrawing groups retard the rate. An ex-cellent correlation is obtained between log k2 and a(p = - 1.34; r = 0.997, s = 0.07) at 35°C.

The retarding influence of caprolactam suggeststhat the pre-equilibrium involves caprolactam asone of the products. The increase in rate with in-crease in [HCI04] indicates participation ofNBCH ". Therefore, H20Br+· appears to be thereactive oxidising species. A mechanism in whichthe rate-limiting step involves the reaction betweenOPSO and H20Br+ is suggested for the oxidation ofdiaryl sulphoxides (see Scheme 2)

-:

f.;,

NBCH + + H20 .= Caprolactam + H20Br+

+Ph2SO + H20Br+ 'I~\\ • Ph2SOBr + H20

+ .Ph2SOBr + H20~ Ph2SO; + HBr + H +

Scheme 2+

The involvement of Ph2SO(Br) in the slow step is

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INDIAN J CHEM, SEe. A, MARCH 1989

supported by the negative p-value. Scheme 2 leadsto the rate law

- d[NBC]

dtKIK2k[PhSOPh][MBC][H30+]

[Caprolactam]

= ki[PhSOPhJ[NBC]at constant [H+] and [caprolactam]

Effect of variation of solvent composition onNBC oxidation of sulphoxides

The data in Table 4 clearly indicate that in the oxi-dation of DMSO and MPSO the rates increase withincrease in polarity of the medium. In both the cases,protonation is the rate-limiting step and hence thepolar activated complex is stabilized by an increasein the dielectric constant of the medium.

On the other hand, the rate of oxidation of DPSOdecreases with increase in the polarity of the medi-um. Here DPSO is involved in the rate-limiting step.A sulphoxide is strongly solvated in proton-donat-ing solvents while the suiphones are poorly solvat-ed 10. Consequently the rate of oxidation of diarylsulphoxides to diaryl sulphones would decreasewith increase in the dielectric constant of the medi-

252

urn. The good correlation of log k, with liD with apositive slope (slope = + 13.9, r = 0.999 ands = 0.01) indicates that a positively charged 11 oxi-dant is involved in the mechanism.

The authors thank Prof. N. Arumugam for his in-terest in this work. P.PJ. thanks the CSIR, New Del-hi for the award of a junior fellowship.

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