Abstract
Rate data for the SN2 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 SN2 reactions are solvated by hydrogenbonding interactions with methanol. Rate constants for SNAr 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 SNAr reactions are quite different from those for SN2 reactions. Changes in Arrhenius parameters, brought about by change from methanol to dimethylformamide, are different for SN2 and SNAr 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.
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