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

The kinetics of the S_NAr 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 (k_A/k_M) increases with increasing size of the 6-substituent; with picryl chloride k_A/k_M 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.