The stabilities of Meisenheimer complexes. Part 32. Rate-limiting proton transfer in the reactions of 1,3,5-trinitrobenzene with pyrrolidine and piperidine

Abstract

Rate and equilibrium measurements are reported for the reactions of 1,3,5-trinitrobenzene with pyrrolidine and with piperidine in dimethyl sulphoxide (DMSO) and in a DMSO–water mixture. These reactions lead to the formation of anionic σ-adducts via zwitterionic intermediates and it is shown that proton transfer is rate-limiting or partially rate-limiting. In DMSO the rate of proton transfer is an order of magnitude faster for the reaction with pyrrolidine than for the reaction with piperidine. However, the addition of water reduces this difference. Implications for the mechanism of base catalysis in nucleophilic aromatic substitution reactions are discussed.