Steric Effects in Nucleophilic Aromatic Substitution reactions with aromatic amines 12th communication on nucleophilic aromatic substitution reactions

Abstract

AbstractThe kinetics of the SNAr reactions of aniline and N‐methylaniline with a variety of substituted nitrochlorobenzenes in acetonitrile demonstrate that the formation of the intermediate σ‐complex is rate determining. The ratio of the rate constants of the aniline and the N‐methylaniline reactions (kA/kM) increases with increasing size of the 6‐substituent; with picryl chloride kA/kM reaches a value of over 20 000. The reaction of aniline with 4‐X‐2,6‐dinitrochlorobenzenes is subject to considerably larger para‐substituent effects than the corresponding reactions with N‐methylaniline. These results are interpreted in terms of two effects: (i) A primary steric effect, which renders the approach of N‐methylaniline to the substrate difficult. (ii) A shift towards earlier, more reactant‐like transition state structures caused by the primary steric effect. In early transition states the activating power of the electron‐withdrawing substituents in the substrate is expected to be relatively small. An early transition state for the slow N‐methylaniline reaction and a late transition state for the fast aniline reaction is in apparent contradiction to what would be expected on the basis of the Hammond postulate.