Electrophilic aromatic substitution. Part XIII. Kinetics, isomer yields, and the consequences of ipso-attack in the nitration of toluene and polymethylbenzenes in aqueous sulphuric acid, and their significance for the mechanism of aromatic nitration

Abstract

Toluene, o-xylene, m-xylene, 1,2,4-, and 1,2,3-trimethylbenzene give yields of mononitro-isomers which vary with the percentage of sulphuric acid in a way which depends on (a) the medium dependence of the relative reactivities of the various substituted and unsubstituted positions in the molecule and (b) the partitioning of Wheland intermediates formed at substituted (ipso) positions between rearrangement and nucleophilic capture. Nitration of all the polymethylbenzenes studied occurs at closely similar rates, the encounter rate between the aromatic compound and the nitronium ion, yet positional selectivity does not disappear under these conditions, showing the necessity of including in the kinetic scheme an intermediate preceding Wheland intermediate formation.