The effects of solvent changes and of electrolyte additions on the stabilities of initial and transition states in nucleophilic substitution reactions

Abstract

This thesis describes the effects of solvent changes and electrolyte additions on the stabilities of the initial and transition states in the solvolysis (or ionisation) of organic chlorides. Very little relevant information was available when the present work began, but many workers (^1b, 32,42,43,50a,e,f,j.) had studied the effects of changes in the reaction conditions on the rate coefficient. The present experiments were carried out in aqueous acetone using 4-phenyl:4'-nitrodiphenylmethyl chloride (I),4-nitrodiphenylmethyl chloride (II) and 4-nitrobenzylchloride (III) as substrates. Compound (III) undergoes S(_N)2 solvolysis, while (I) and (II) react entirely by the unimolecular mechanism, S(_N)l.(^17,86.) The rate of solvolysis of compound (III) was less sensitive to solvent changes than that of compounds (I) and (II); in agreement with earlier views. (^1) In bimolecular solvolysis, compound (III), an increase in the water content reduced the stability of both the initial and transition states, though not to the same extent. However, the stability of the transition state in S(_N)l reactions was almost independent of the solvent composition, although this did not apply to the corresponding entropy and enthalpy terms. These results show some differences from those obtained from a study of the effects of solvent changes in the solvolysis of tert.-butyl chloride in aqueous ethanol. (^31, 55b, g, 85.) The effects of added electrolytes on the rate of ionisation of compound (III) in 70% (v/v) aqueous acetone depended markedly on their nature, in agreement with other recent reports of specific electrolyte effects.(^50) In general, the stability of the initial state was more sensitive to the nature of the electrolyte than the stability of the transition state. These results are qualitatively consistent with the view that two separate effects control the stabilities of these two states, and hence the rate: the ionic-strength effect,(^1) which is independent of the nature of the electrolyte and the salt-induced medium effect. (^50a, e., 58) The magnitude of the latter effect depends on the nature of the electrolyte, since it can be considered to arise from changes in the "effective" solvent composition caused by the varying degrees of solvation of different electrolytes. The smaller sensitivity of the transition state to the nature of the electrolyte is then consistent with its virtually constant stability as the composition of solvent is altered.