Kinetic and equilibrium studies of σ-adduct formation and nucleophilic substitution in the reactions of ethyl thiopicrate with aliphatic amines in dimethyl sulfoxide

Abstract

Kinetic and equilibrium results are reported for the reaction of ethyl thiopicrate with butylamine, pyrrolidine and piperidine in dimethyl sulfoxide. The most rapid reaction involves attack at the unsubstituted 3-position to give anionic σ-adducts. The rate-limiting step in this process changes from nucleophilic attack by amine with butylamine to proton transfer from the zwitterionic intermediate with piperidine; pyrrolidine shows intermediate behaviour. Attack by amine at the 1-position results in displacement of the ethylthio group, although intermediates are not observed during this process. It is suggested that base catalysis observed in the reaction with pyrrolidine may result from rate-limiting proton transfer from the zwitterionic intermediate. ¹H NMR measurements using 0.1 mol dm^–3 solutions of substrate show that the displaced ethanethiolate ion may attack ring-carbon atoms of either unreacted substrate or substitution products.