Inductive and field effects in aromatic substitution. Part VIII. Kinetics of nitration and bromination of the 'onium salts of dithia- and diazacyclophanes

Abstract

The reactivities, with respect to benzene, of certain open-chain ions (I; X = NMe₃ or SMe₂) and bridged ions (II; X = NMe₂ or SMe) have been compared in nitration and bromination with special reference to the rate retardation caused by bridging [(I)→(II)]. This retardation decreases with an increase in the degree of methylation of the poles and in the length (m) of the methylene chain. For the compounds studied here, the retardation is greater when n= 2 than when n= 1 provided that the length of the bridge and the extent of alkylation of the poles are held constant. The retardation is attributed mainly to the change in the form of the molecular cavity with a further contribution, when n > 1 from the change in the distance of the poles from the aromatic ring.