Tautomerism

The reversible formation of enols from enolizable ketones brings into focus a phenomenon termed as tautomerism. In a broad sense, two structural isomers, which differ in the relative positions of their atoms, and are in rapid equilibrium, are called tautomers. Their interconvertibility of tautomers is a chemical reaction, which involves making and breaking of a bond. Tautomers, being an equilibrium process, is symbolized by double arrows.

Tautomerism is classified into cationotropy and anionotropy depending upon whether atoms or groups of atoms shift as cations or anions. A great majority of cases of cationotropy, however, is prototropy in which protons are involved.

Prototropy:

The classical example of prototropy is a keto-enol tautomerism exhibited by ethyl acetoacetate. The compound forms a cyanohydrin with HCN, an oxime with hydroxylamine, and phenylhydrazone with phenylhydrazine. Therefore, it appears to be a saturated keto ester. 

On the other hand, it appears to be an unsaturated hydroxyl ester since it gives an intense red colour with ferric chloride, rapidly absorbs bromine, and reacts with diazomethane.

In principle, any molecule containing a keto grouping and a-hydrogen(s) is capable of undergoing tautomeric change to an isomeric enol molecule.

The enols forms of simple aldehydes and ketones have, however, not been isolated because of their easy conversion into a more stable carbonyl forms. However, in the presence of more than one carbonyl function on the same carbon in the molecule, the enol content becomes more significant, and the enol forms become isolable.

Acetone exists in equilibrium almost wholly in its keto form, whereas ethyl acetoacetate and acetylacetone have significant enough contents. This is to be expected since the electron-withdrawing effect of the carbethoxy or the acetyl substituent increases the acidity of the a-hydrogen. Since the carbonyl group of a ketone is an effective electron-withdrawing group, much more than the carbonyl group of an ester, acetylacetone has more enol content than ethyl acetoacetate. These substituents increase the stability of the enol form by the conjugative effect. In addition, intramolecular hydrogen bonding also occurs in the enol forms stabilizing them with respect to the keto forms by 6 kcal/mol it 25 kJ/mol.

Sometimes, conformation plays a dominant role in deciding the extent of enolization in certain a-diketones. Biacetyl, for instance, has very little enol content in contrast to 1,2-cyclopentanedione which exists mostly in the enolic form. The carbonyl groups of biacetyl are oriented in opposite directions so as to reduce dipole-dipole repulsion, whereas being a cyclic compound has no such choice, and has to resort to enolization in order to avoid their repulsion.

The percentage of enolization present in a keto-enol equilibrium varies markedly with the nature of the solvent. This is to be expected since the keto form of a diketone or a keto ester is more polar than the enolic form, and hence the percentage of the enol form should increase as the polarity of the solvent decreases.

Anionotropy:

There are a number of examples of tautomerism involving the movement of atoms or group of atoms or anions. These processes are collectively known as anionotropy. A simple example is the transformation of crotyl chloride into methylvinylcarbinyl chloride.

Either of these substances gives the reactions of both, and either can be converted into an equilibrium mixture. Other well-known examples include the conversion of 1-phenylallyl-p-nitrobenzoate (56) to 57 and that of geraniol (58) to linalool (59).