Abstract
This chapter reviews that the only way to rationalize tautomerism is using the concept of aromaticity and its opposite—that is, antiaromaticity. Because this concept cannot be unambiguously applied to small and large rings, the interpretation and rationalization of such tautomerism have not made much progress. In “microcycles,” the ring is too small to sustain the aromatic sextet efficiently and in “macrocycles,” with the ring size increasing, the polyenic character of the heterocycle predominates. Only for the tautomerism of porphyrins and phthalocyanins has major progress been achieved as is discussed comprehensively in this chapter.