Steric and electronic effects on the mechanism of nucleophilic substitution (SNAr) reactions of some phenyl 2,4,6-trinitrophenyl ethers with aniline and N-methylaniline in acetonitrile and in dimethyl sulfoxide

Abstract

Rate data are reported for the reactions of a series of 3′- and 4′-substituted phenyl 2,4,6-trinitrophenyl ethers, 4, with aniline in acetonitrile, leading to 2,4,6-trinitrodiphenylamine. The main reaction flux occurs through a base catalysed pathway involving formation of a zwitterionic intermediate, equilibrium constant K₁, and rate-limiting proton transfer to base, rate constant k_An. The effects of ring substituents on values of rate and equilibrium constants are discussed. In contrast the corresponding reactions in DMSO involve both uncatalysed and base catalysed pathways. Reactions of 4 with N-methylaniline are extremely slow, but values of rate constants for reaction of 4-nitrophenyl 2,4,6-trinitrophenyl ether were measured using ¹H NMR spectroscopy. The value of the parameter K₁k_An is lowered by a factor of 10⁵ for N-methylaniline relative to aniline in both acetonitrile and in DMSO. This reduction is attributed to increased steric hindrance both in formation of the intermediate and to proton transfer.