Indian Journal of ChemistryVol. 28A, November 1989, pp. 961-964

Oxidation of substituted phenyl methyl sulphides by N-chloroacetamide inacidic medium: A kinetic and mechanistic study

S'Perumal"

Department of Organic Chemistry, School of Chemistry, Madurai Kamaraj University..Madurai 625 021and

MGanesanDepartment of Chemistry, Thiagarajar Arts College, Madurai 625 009

Received 26 July 1988; revised 26 December 1988; accepted 16 January 1989

The kinetics of oxidation of several substituted phenyl methyl sulphides by Nschloroacetamide(NCA) yielding sulphoxides have been studied in acidic aqueous acetonitrile medium. The reaction dis-plays first-order dependence each in [sulphide],[NCAland [W]. The reaction rate is not influenced bythe addition of acetamide, mercuric acetate &I1dacrylamide. Presence of electron releasing substituentsin the phenyl ring enhances the rate while that of opposite ones retards it. A good Hammett correlationbetween the rate data and a constants is obtained with a p-value of - 3.29 at 313K (r= 0.999). The re-sults point to a polar mechanism involvingthe rate-limiting formation of a chlorosulphonium cation bythe electrophilic attack of a protonated NCA on the sulphide sulphur. The chlorosulphonium cationpresumablyhydrolyses to sulphoxidein a fast step.

The kinetics of oxidation of organic sulphides byN-halo oxidants such as bromamine-T", chlora-mine-T', Nebromoacetamide' and N-bromosuccini-mide" have been investigated. The nature of activeoxidising species and the mechanism depend onthe nature of the halogen atom, the groups att-ached to the nitrogen and the reaction conditions.In our recent study' on the N-bromoacetamideoxidation of substituted phenyl methyl sulphides inthe presence of Hg(II) we have found that Hg(II)introduced into the reaction mixture to trap thebromide ion catalyses the reaction and the reac-tion proceeds entirely through the Hg(II)-catalysedpathway. With a view to obtaining further insightinto the mechanism of oxidation of sulphides byN-halo oxidants we have now studied N-chloroa-cetamide (NCA) oxidation of sulphides. The reac-tion failed to occur in neutral medium even inpresence of Hg(II) demonstrating that complex be-tween NCA and Hg(II) is not involved in the reac-tion in contrast to NBA oxidation involvingNBA. ..Hg(II) complex'. However, as the reactionof sulphides with NCA has been found to proceedsmoothly in presence of HCI04 the reaction hasbeen studied in acidic medium and the results arepresented in the paper. It is to be noted that nosystematic kinetic investigation on the oxidation oforganic substrates by NCA has yet been reportedin the literature in contrast to several oxidations

by N-bromosuccinimide4,5 and N-bromoaceta-mide+".

Materials and MethodsAll the reagents employed were of the highest

purity available. N-Chloroacetarnide, prepared? inthe laboratory, was kept always in a blackenedcontainer. Solutions of NCA were prepared afreshbefore each kinetic run and assayed iodometrical-Iy. Acetonitrile was purified by a standard proce-dure". All the sulphides were prepared by knownmethods-P-'? and their purity checked by TLC.Doubly distilled water was employed in all kineticruns.

Kinetic procedureThe kinetic measurements were performed in

50% (v/v) acetonitrile-water mixture under pseu-do-first order conditions, viz. [sulphide] ~ [NCAl atconstant ionic strength (maintained by NaCI04)

and constant [H+] (maintained by HCI04). The in-itial [sulphide] was in the range of 0.01-0.08 moldm-3 and the [NCAl in the range of 0.0005-0.004mol drn"". The runs were conducted in blackenedvessels and the kinetics followed upto 70-80%completion of the reaction by estimating uncon-sumed NCA iodometrically using starch as an in-dicator. The pseudo-first order rate constants(kobs) obtained from the slopes of the linear log

961

INDIAN] CHEM, SEe. A, NOVEMBER 1989

[NCAj versus time plots (r ~ 0.997) when dividedby [sulphide] afforded the second order rate con-stants.

Stoichiometry and product analysisReaction mixture containing an excess of [NCA]

over [MPS] was allowed to react at 313 K for 24hr in the presence of 0.025 mol dm ":' HCI04.

The excess of NCA remaining was estimated. Theresults show that one mole of MPS consumes onemoleofNCA.

The product from the reaction mixture of anactual kinetic run with MPS as a substrate was iso-lated and analysed by co-TLC with authentic sam-ples of MPS, methyl phenyl sulphoxide and methylphenyl sulphone. The reaction product gave onlytwo spots corresponding to the unchanged MPSand methyl phenyl sulphoxide, indicating that sul-phoxide is the only product and further oxidationto sulphone does not occur. Hence the overallreaction can be expressed by Eq. (1).

H+C6HsSCH3 + CH3CONHCI + H20-C6HsSOCH3+CH3CONH2+H+Cl- ... (1)

Results and DiscussionKinetic data were collected for all the substitut-

ed phenyl methyl sulphides. As the results were si-milar only those of methyl phenyl sulphide (MPS)are presented in detail. The rates of oxidation atvarious initial [MPSJ and [NCA] are listed in Table1. The reaction exhibits a clean first order de-pendence in NCA as evidenced by the good linearplots of log [NCAl versus time (r~ 0.997) and bythe constancy of kObs at varying [NCAl (Table 1).Further, the plot of kobs versus [MPS] is linearpassing through the origin revealing first order de-pendence in MPS and ruling out the possibility ofany substrate-independent path for oxidation.

The reaction rate increases with increase in[H+] at constant ionic strength (Table 2). A plot ofkobs versus [H+] is linear passing through the orig-in showing that the reaction proceeds completelythrough the acid-catalysed pathway and the orderin [H+] is unity. The reaction rate increases withincrease in ionic strength of the medium and inthe proportion of water in the solvent mixture(Table 2).

In an atmosphere of nitrogen, the addition ofacrylamide into the reaction mixture failed to af-fect the rate and no visible polymerisation of themonomer was noted. This rules out the intermedi-acy of free radicals in the reaction,

In an acidified solution ,of NCA, the probableactive oxidising species are the NCA itself, the

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Table 1- Pseudo first order (kob,) and second order rate con-stants for the oxidation of MPS by NCN

J02 [MPS] 103 [NCAj 104 kobsd(rnol dm " ") (rnol dm " ') S-I

1.00 1.00 9.42.00 1.00 19.03DO IDO 28.24.00 LOP 38.76.00 1.00 56.58.00 1.00 71.22.00 0.50 18.92.00 1.50 18.82.00 2.00 19.42.00 4.00 18.6

'Solvent: 50% (v/v) acetonitrile-water;[H+] = 0.025 mol dm - 3; temp 303 K

102 kz(drn! mol-I S-I)

9.4

9.59.4

9.79.4

8.99.59.4

9.79.3

[=0.15 mol dm ":'

Table 2-Influence of [H+), ionic strength and solvent compo-sition on the rate of oxidation of MPS by NCN

J02 [H+] 0.25 0.50 1.00 3.00 5.00(mol dm r ')

104 kob,(S-I) 0.90 1.69 3.74 11.5 19.1[(mol dm-3) 0.06 0.15 0.25 0.35104 kob,(S-I) 7.69 9.4 10.4 14.8% CH3CN - HzO (v/v) 20.0 30.0 40.0 50.0 60.0 70.0104 kob,(S-I) 22.9 19.1 14.5 09.4 08.4 08.1

'Solvent: 50% (v/v) acetonitrile-water; [MPS)=O.OI mol dm-3;

[NCAj=O.OOl mol dm': [H+]=0.025 mol dm-3; [=0.15 moldm - 3; temp = 303 K unless stated otherwise.

disproportionation product, viz. N,N-dichloroacet-amide (CH3CONCI2), the hydrolytic product(HOCI) and their protonated forms (see Eqs 2 and3). The strict first order dependence in NCA andthe absence of rate retardation by added aceta-mide (0.05 mol dm=') preclude the involvementof CH3CONCl2 (2) and HOCI (3) and their proto-nated forms in the rate-limiting step.

2CH3CONHCl<= CH3CONCl2 + CH3CONH2... (2)

CH3CONHCl+ H20 <=CH3CONH2 + HOCI ... (3)

This leaves CH3CONHCI or its protonatedspecies as the most likely oxidising species. Thelinear increase in the reaction rate with increasein [H+] is clearly ~ favour of the protonatedNCA, i.e. CH3CONH2CI as the active oxidant(Eq. 4). The fact that the plot of kobs vs [H +] islinear (r= 0.999) with no levelling off at higher

... (4)

PERUMAL et af.: OXIDATION OF PHENYL METHYL SULPHIDES BY N-CHLOROACETAMIDE

[H+] suggests that the protonation equilibriumconstant, K, is of small magnitude. This is notsurprising as even in acetamide" the K-value is0.234 and Nvchloroacetamide is anticipated tohave a still lesser K-value.

All the above results are in accord with a rate-limiting electrophilic attack of CH3CONH2Cl onthe sulphide sulphur (Eq. 5). The absence of rate

C HOC H'j ~ + II k, 6 5 +r. CI-NH -C-CH - "S-CI+CH3CONH2 (5)/ V \j 2 3 Slow /

C~3 CH3

retardation by added acetamide excludes the pos-sibility of step (5) being an equilibrium.

The resulting chlorosulphonium cation in step(5) presumably undergoes a fast hydrolysis to thesulphoxide (Eq. 6).

The overall rate law is given by Eq. (7)

- d~CA] = k1[MPS][NCAH+]

= Kk1[MPS][NCA][H+]= kObs[MPS][NCA] at constant [H+] ... (7)

The above rate law is clearly in accord with theexperimental observations.

Substituent effectsWith a view to obtaining more information into

the nature of the transition state, the rates of oxi-dation of several substituted phenyl methyl sul-phides by NCA have been measured. The ratedata at 303, 313 and 32J K and the activationparameters computed from the slopes and inter-cepts of the Eyring's plots are listed in Table 3.Both entropies of activation and enthalpies of ac-tivation vary with the nature of the substituent.However, the plot of ~H t versus ~Sf is found tobe scattered.From the plot of log k2 (50°C) versuslog K 2 (30°C) the isokinetic temperature has beencalculated to be - 351.4 K. The entropies of acti-vation presented in Table 3 are largely negative inconsonance with a bimolecular. rate-limiting reac-tion involving an ordered transition state relativeto the reactants. The data in Table 3 further show

Table 3-Second-order rate constants (k2) and enthalpies andentropies of activation for the oxidation of p-substituted

phenyl methyl sulphides by NCN

Substituent 10z kz dH t - dS t(mol-I dm ' S-I) (kJ mol " ') (JK-I mol t ")

p-OMep-MeHp-F

p-Clp-COOMep-NOz

303K

66.532.09.426.811.74

0.2100.0205

313K 323K

122.0 203.055.2 81.318.1 33.912.5 24.53.40 7.26

0.540 1.230.0426 0.0837

145.3175.9139.7i42.7134.1105.6173.7

42.735.548.849.4

55.569.454.7

"Solvent: 50% (v/v) acetonitrile-water; [Sulphide] ~ [NCAl,1=0.15 mol dm-3; [H+]=0.025 mol dm " '

that the reaction rate increases with increase inthe electron donating character of the substituent.The rate data give an excellent correlation withHammett (J constants, the p-value at 313K being- 3.3 (r = 0.999). The large negative p-value indi-cates that the sulphide molecule acquires highelectron deficiency in the rate-limiting step. Al-most equal p-value ( - 3.2) has been encounteredin the bromine'? oxidation of alkyl phenyl sul-phides involving the formation of bromosulphoni-urn cation. Similar nucleophilic role by sulphidesand consequent negative p-values have been re-corded in several oxidations 1 - 3.9. The effect ofsolvent composition also is in accordance with themechanism. With increase in water content of themedium both the polarity and nucleophilicity ofthe solvent medium increases. This facilitates theformation of the chlorosulphonium cation with apolar transition state in the rate-limiting step. Thehigh p-value and the absence of influence by theradical trap on the rate are all consistent with thepolar mechanism advanced for this reaction.

AcknowledgementOne of the authors (M.G.) thanks the UGC,

New Delhi for financial support and the Principal,Aditanar College, Tiruchendur for facilities.

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