Indian Journal of ChemistryVol. 23A, March 1984, pp. 239-240

Kinetics of Chlorination ofCertain Aromatic Compounds

Using N-Chlorosuccinimide

S MOHAMED FAROOK*t, S SIVAKAMASUNDARI &S VISWANATHAN

Department of Chemistry, University of Madras, Madras 600025

Received \0 October 1983; accepted 5 December 1983

Kinetics of chlorination of anisole, p-cresol, p-chlorophenol andacetanilide by N-chlorosuccinimide (NCS) have been studied in dryacetic acid in the presence of LiC\. Kinetic results reveal that NCSchlorination of these substrates fol\ows a mechanism which isdifferent from that of chlorination by molecular chlorine orbromination by structurally related N-bromosuccinimide (NBS).

The kinetics of chlorination of organic substrates byNvchloroacetauilide! and oxidation by N-chlorosuccinimide! (NCS) are well studied, but notmuch.' has been reported on the kinetics ofchlorination by NCS. The title work has beenundertaken with a view to comparing the results ofchlorination by NCS with those of chlorination bymolecular chlorine+" and bromination by N-bromosuccinimide (NBS)6, which structurally re-sembles NCS.

Acetic acid (BDH, LR) was purified by literaturemethod. NCS was recrystallised from dry benzene,m.p, 148-49'. LiCI was prepared from lithiumcarbonate, recrystallised, dried at 1300 and kept in adessicator. Anisole, p-cresol and p-chlorophenol werepurified by distillation and acetanilide byrecrystallisation.

No reaction occurred between anisole and NCSwhen the reaction was carried out in dry acetic acid for6 hr at 50. Though acids like HCI04 and H2S04 didinitiate the reaction, the kinetic runs were notreproducible and the reaction was instantaneous whenLiC) was added along with these acids. However, in thepresence of LiCI alone, the reaction took place at ameasurable rate and data were reproducible.Hence all the runs were carried out in the presence ofLiCI (0.05 mol dm -3).

Kinetic runs were carried out in specially devisedtubes to avoid loss of chlorine. The reactions wereinitiated by mixing thermally equilibrated mixtures ofsubstrate and LiCI with NCS, quenching it with KIafter the desired interval of time and estimating theunreacted NCS iodometrically. Blank runs weretPresent address: Department of Chemistry, Avvaiyar GovernmentCollege for Women, Karaikal 609602.

carried out during each experiment. NCS solution wasquite stable in dry acetic acid and no free chlorinecould be detected (A.max=317.5nm) spectrophotom-etrically even after 8 to 10 hr.

The kinetic results can be summarised as follows:(i) The total order of the reaction was one as found bythe fractional life period method. Integral first orderplots were linear upto 90% of the reaction and kobs

x 104 remained constant at 4.17 ± 0.03 s -I for a widevariation in reactant concentrations (from 0.05 to0.0015 mol dm -3). (ii) Order in substrate was found tobe zero for all the four substrates studied. (iii) Thereaction was first order each in NCS and LiCl.(iv) Added neutral salts like NaCI04 retarded the rateof the reaction. (v) The effect of solvent on the reactionwas studied using several acetic acid-water and aceticacid-nitrobenzene mixtures as solvents. Whilenitrobenzene had no effect on the rate of the reactionadded water retarded the reaction. (vi) Addedsuccinimide (lOO-fold variation in concentration) didnot have any effect on the rate ofthe reaction. (vii) Thestoichiometry of the reaction for all the substrates wasfound to be I: 1. The products of chlorination ofanisole, acetanilide and p-chlorophenol were isolatedand indentified by direct comparison with authenticsamples as p-chloroanisole, p-chloroacetanilide and2,4-dichlorophenol respectively. (viii) Polymerisationtests completely ruled out the possibility of radicalintermediates. (ix) Activation energy remainedconstant for the different substrates (Table 1).

The kinetic data reveal that the chlorination of thesubstrates studied presently fotlows a mechanismwhich is different from that of chlorination bymolecular chlorine or bromination by NBS. In the caseof chlorination by molecular chlorine+" the reaction issecond order, first order in chlorine and first order insubstrate. The rate-determining step is the formationof a a-complex between the substrate and chlorine.The structural changes in the substrates are directlyreflected in the activation energy. In the present case

Table I-Activation Parameters for the Chlorination byNCS

[NCS] =, [substrate] =0.00625 mol dm -3; [LiCl] =0.05 mol dm -3;solvent = 100% Acetic acid

Substrate kobo x 104(s -I) E.------------kJmol-'

20° 30°Anisole 1.86 3.99p-Cresol 1.88 4.05p-Chlorophenol 1.44 3.29

40°8.478.966.88

50°C15.2117.9214.27

56.1758.7359.31

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INDIAN J. CHEM., VOL 23A, MARCH 1984

not only the total and individual orders are different,but the activation energy also remains constant (58.07± 1.9kJ mol -1). Since the order in substrate is zeroand the activation energy remains unaffected forwidely differing substrates, the rate-determining stepmay not involve substrate molecule. The mechanismmay be given by Eqs (1) and (2)

NCS + LiCl solvent ~Succinimide + Li + +CI2 ... (I)slow

ArH+CI2~ ArCI+H+ +CI- ... (2)last

That the first step is not an equilibrium step is provedby the fact that addition of even loo-fold excess ofsuccinimide does not alter the rate. The formation ofmolecular chlorine in reaction (1) has been confirmedin a separate experiment in the absence of substrate.The slow evolution of chlorine in the reaction wasdetected spectrophotometrically by recording theabsorbance at 317-5 nm at regular time intervals.

In the case of bromination by NBS though theactivation energy remains constant the observedindividual orders are zero in NBS and one in substrateirrespective of the substrate and the total order is one.Also addition of bromide to NBS liberates molecularbromine instantaneously contrary to the steadyevaluation of Cl2 by NCS in the presence of chloride.The difference in NBS and NCS halogenationmechanisms ma y be due to the difference inelectronegativities 7 of the halo substituents. Thisrenders the rupture of N - Br bond in NBS easier thanthat of N -CI bond in NCS. Thus step (1) becomesslow and rate-determining in NCS chlorination.

240

The involvement of molecular chlorine as activeoxidising species in the chlorination by NCS is provedby the fact that in the absence of LiCI and the presenceof added AgCI04 (7.3 x 1O-3moldm -3) which is ascavanger" of CI-, no reaction occurs even in thepresence of2moldm -3 HCI04.

It is also found that only in case of reactivesubstrates like anisole, p-cresol etc. the chlorinationfollows the above mechanism. With substrates like p-bromoacetanilide and p-chloroanisole, the mechanismis entirely different. In these cases the rate depends onthe [substrate] and there is accumulation of chlorine inthe system since step (2) is not fast compared to step (I).Hence the mechanism ofNCS chlorination depends onthe nature of the substrate.

Since added nitrobenzene does not affect the rate,the rate retardation by added water is not due todielectric effect but is believed to be due to specificsolvation effect. Water may directly solvate the halideions thereby reducing their nucleophilic reactivitytowards NCS.

The authors thank the UGC, New Delhi for theaward of a teacher fellowship to one of them (S M F).References1 Carr M 0 & England 8 D, Proc chem Sac. (\958) 350.2 Srinivasan N S & Venkatasubramanian N, Tetrahedron, 30(1974)

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