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 SN2 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 SNAr
and SN2 mechanisms, and in others indicate abnormal transition
states.
Nucleophilic Substitution (SN
2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent
