Nucleophilic aromatic substitution promoted by lipophilic quaternary 'onium salts under phase-transfer conditions and in low polarity anhydrous solvents. A kinetic study

Abstract

A kinetic study of aromatic nucleophilic substitutions by N₃^– and SCN^– has been carried out under phase-transfer catalysis (p.t.c.) conditions and in anhydrous chlorobenzene in the presence of hexadecyltributylphosphonium azide and thiocyanate. Under p.t.c. conditions, reactions follow a pseudo-first-order kinetic equation, and k_obs is linearly related to the nucleophile concentration in the organic phase. In most reactions, the rate-determining step is attack by the nucleophile on the substrate. However in a few cases the process is controlled by diffusion of the anions through the interface. Passing from p.t.c. conditions to chlorobenzene solution leads to 5–7- and 2–3-fold increases in reaction rates with N₃^– and SCN^–, respectively. This behaviour is analogous to that observed in the aliphatic series, and is probably due to the limited number of water molecules accompanying the anions in the organic phase under p.t.c. conditions. Nucleofugicity scales are related to the polarizability of the nucleophile, and with N₃^– and SCN^– they are F Br ≳ I ≳ Cl and Br > I > F ≳ Cl, respectively. Reaction products, including those derived from successive decomposition of N₃ and SCN groups, have been identified.