The influence of steric effects on the kinetics and mechanism of S_NAr reactions of 1‐phenoxy‐nitrobenzenes with aliphatic primary amines in acetonitrile

Abstract

Rate constants are reported for the reactions of 1-phenoxy-dinitrobenzenes, 3, 1phenoxy-dinitrotrifluoromethylbenzenes, 4, with n-propylamine, and 1methylheptylamine in acetonitrile as solvent. The results are compared with results reported previously for n-butylamine, pyrrolidine, and piperidine. Decreasing ring activation leads to lower values of k 1 for nucleophilic attack although this may be mediated by reduced steric congestion around the reaction centre. Specific steric effects, leading to rate retardation, are noted for the ortho-CF 3 group. In general, reactant-bearing ortho-CF 3 group were subject to base catalysis irrespective of the amine nucleophile and values of k Am /k −1 are reduced as the size of the amine get bulkier. This is likely to reflect increases in values of k −1 coupled with decreases in values of k Am as the proton transfer from zwitterionic intermediates to catalysing amine becomes less thermodynamically favourable.