Real keto-enol tautomerism

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

This is where we meet the famous double helix immortalised by many books, articles, television programmes and of course films. The DNA double helix is the key element of DNA three dimensional structure. In the 1950s, when James Watson and Francis Crick first proposed the double helix as the key piece of DNA three dimensional structure, they generated enormous scientific and popular excitement, since for the first time the inheritance of genetic information could be understood explicitly in terms of a real chemical structure In order to appreciate this structure, there is a requirement to understand more about the heterocyclic bases (see Section 1.4.1) and their unrivalled capacity for specific hydrogen bonding. All these bases are aromatic but paradoxically prefer keto/amine to enol/imine tautomeric forms... 

It s important to recognize the difference between this conjugation and the tautomerism that interconverts the keto and enol forms of a carbonyl compound, which is a real equilibrium between two different structures and must be represented by equilibrium arrows. 

The dispute was in fact resolved in 1896 by Claisen [37], who isolated acetyldiben-zoylmethane as two separate solid forms, each with different melting points and chemical properties (interaction with metallic salts). Claisen correctly diagnosed them as the enol and keto forms having the structures 36b and 36a, respectively. More important still was the observation that, if either the keto or the enol form was heated in a solvent such as alcohol, or fused in the absence of solvents, a mixture was obtained from which both the keto and enol forms could be isolated. As result of this discovery, the pseudomerie/ortisomerie theory about the real existence of the isomers was proven to be correct. Ironically, the term tautomerism came into use to describe the process. In some natural way, according to the early reviews [38-46], tautomerism was considered and it is still considered in most of the cases as an equilibrium between forms coexisting in solution, and was defined as one of the most difficult subjects of experimental science. ... 

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