Kinetic studies of neighbouring group participation in organic halides and sulphonates

Abstract

The effect of anchimeric assistance by neighbouring groups on the rates and products for the solvolysis of organic halides and sulphonates has been widely studied,(^1) but the activation parameters of such internal nucleophilic substitutions (S(_N)i) have received relatively little attention. This thesis describes experiments aimed at obtaining this information for the effect of neighbouring hydroxyl, methoxyl and phenyl groups on reactions with aqueous acetone. Compounds containing these substituents but reacting by the normal bimolecular mechanism (s(_N)2) were also examined, and data(^2) for the unimolecular reactions (S(_N)l) of substrates with structurally similar reaction centres allowed a comparison to be made of the entropies and heat capacities of activation associated with these three mechanisms. The chlorohydrins, HO(CH(_2)(_n)CH(_2)Cl, (I; n = 1-5) showed the enhanced rates and the abnormal reaction products expected for the S(_N)i mechanism when n = 3,4, as previously reported for reaction in water(^3). Entropies and heat capacities of activation for these compounds were intermediate between the values of the parameters for the solvolysis of structurally similar substrates by mechanisms S(_N)l and S(_N). The corresponding methoxy-bromides, MeO(CH(_2))(_n)CH(_2)Br, (II; n = 1-5) showed very similar kinetic features when initial rates were measured, but subsequent reactions of the intermediate produced in the first step of the S(_N)i process (n = 3,4) made it difficult to obtain rate coefficients with sufficient accuracy for the determination of accurate activation parameters. Similar results had already been reported for the reactions of MeO(CH(_2))(_2)CH(_2)0Bs, (III; n = 1-5) but approximate rate coefficients could be assigned to the individual reaction steps in the present work. Anchimeric assistance by a phenyl group in the reactions of C(_6)H(_5))(_3)CCH(_2)Cl and (C(_6)H(_5))(_3)CCH(_2)OTs was accompanied by more positive activation parameters than in similar S(_N)l and 5(_N)2 reactions. The results allowed some estimate to be made of the effect of individual leaving groups on the values of the entropies and heat capacities of activation for reaction by mechanism S(_N)i.