Azines—continued group

Table 11 summarizes the main results on the tautomerism of mono-hydroxy-, -mercapto-, -amino- and -methyl-azines and their benzo derivatives, in water. At first sight the equilibrium between 2-hydroxypyridine (71) and pyridin-2-one (72) is one between a benzenoid and a non-benzenoid molecule respectively (71a 72a). However, the pyridinone evidently has a continuous cyclic p- orbital system, containing six it- electrons, the usual aromatic count, if the carbonyl group contributes none. This assumption implies the formula (72b), from which by redistribution of electrons we arrive at (72c), which has the same benzenoid system as (71a). Further canonical forms (71b, 71c) can be drawn of (71) which correspond to the non-benzenoid forms of (72). The elusive property of aromaticity is therefore possessed by both tautomers, although not necessarily by both equally. When the carbonyl oxygen of (72) is replaced by less electronegative atoms, as in the imine tautomers of amino heterocycles, or the methylene tautomers of methyl derivatives, the tendency towards polarization in forms corresponding to (72b) and (72c) is considerably less, and the amino and methyl tautomers are therefore favoured in most instances. 

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