Mechanisms of aromatic nucleophilic substitution by the ambident thiocyanate ion

Abstract

Sulphur/nitrogen reactivity ratios in a series of aromatic nucleophilic substitution reactions of ambident thiocyanate ion have been determined. There are profound differences from the pattern found in S_N2 reactions at a saturated carbon atom. Abnormal transition states, involving interactions between entering and leaving group, are likely in the bond-breaking step of the intermediate complex in reactions of thiocyanate ion with 1-fluoro-2,4-dinitrobenzene and with 2,4- dinitrophenyl 4-toluenesulphonate. The nitro-substituted aryl thiocyanates are shown to be tri-functional electrophiles, with reactive centres at aromatic carbon, at cyanide carbon, and at sulphur. Aryl 4-toluenesulphonates are bifunctional electrophiles with reactive centres at aryl carbon and sulphonyl sulphur. The site of attack by nucleophiles depends on the nature of the nucleophile. The sulphur/nitrogen reactivity ratio of ambident SCN^-, and the electrophilic reactivity of tri- and bi-functional substrates, are in most instances consistent with the Hard and Soft Acids and Bases principle. Exceptions to the principle in some instances reveal differences between the S_NAr and S_N2 mechanisms, and in others indicate abnormal transition states.