THE EFFECT OF ORTHO SUBSTITUENTS ON THE MECHANISM OF AROMATIC NUCLEOPHILIC-SUBSTITUTION REACTIONS IN DIPOLAR APROTIC-SOLVENTS

The reactions of 2,6-dinitrophenyl phenyl ether and of 6-methyl-2,4-dinitrophenyl phenyl ether with piperidine, morpholine, butylamine and benzylamine are base catalysed in both dimethyl sulfoxide and acetonitrile. The reaction of 2-phenoxy-3,5-dinitropyridine with aniline is base catalysed in acetonitrile, but not in dimethyl sulfoxide, and its reactions with piperidine, morpholine, butylamine and benzylamine in acetonitrile are also base catalysed. The results are discussed in terms of the prevailing theories of aromatic nucleophilic substitution reactions. Increase in activation of the substrate increases the k2/k-1 and k3/k-1 ratios. For ortho substituents, steric/steroelectronic effects in the transition state reduce both k-1, the rate constant for the decomposition of the zwitterionic intermediate to reactants, and k2 and k3, the rate constants for its decomposition to products. When the substrate has two ortho groups the different behaviour of primary and secondary amines found with substrates containing only one ortho-nitro group is not observed.