Solvation of ions. Part VII. Solvation of transition states for SN2 and SNAr reactions of azide and thiocyanate ions in methanol and dimethylformamide

Abstract

Rate data for the S_N2 reactions of methyl halides with azide and thiocyanate ions, and of butyl halides with pyridine, in methanol and in dimethylformamide, are discussed. Transition states of S_N2 reactions are solvated by hydrogenbonding interactions with methanol. Rate constants for S_NAr reactions of N₃^– and SCN^– with the 1-halogeno-2,4-dinitrobenzenes and for N₃^– and C₆H₅S^– with some 4-halogenonitrobenzenes in methanol and in dimethylformamide, show that the transition states for S_NAr reactions are quite different from those for S_N2 reactions. Changes in Arrhenius parameters, brought about by change from methanol to dimethylformamide, are different for S_N2 and S_NAr reactions. The influence on reactivity of having different halogens and halogenoids both as leaving and as entering groups, in nucleophilic substitution reactions, is discussed. Relative reactivities are so varied that attempts at correlation have very limited application. Ion-pair dissociation constants cannot always be evaluated from rate data, because of ion-pairing in the transtion state.