An enol is exactly what the name implies: an ene-ol. It has a C=C double bond and an OH group joined directly to it. In the case of dimedone, the enol must be formed by a transfer of a proton from the central CH₂ group of the keto form to one of the OH groups, a reaction known as enolization.

Notice that there is no change in pH—a proton is lost from carbon and gained on oxy gen. It is a strange reaction in which little happens: the only change is the transfer of one proton and the shift of the double bond. Interconversions like this are given the name tautomerism.

Tautomerism Any reaction that simply involves the intramolecular transfer of a proton, and nothing else, is called a tautomerism. Here are two other examples.

This sort of chemistry was discussed in Chapter 8, where the acidity and basicity of atoms were the prime considerations. In the first case the two tautomers are the same and so the equilibrium constant must be exactly 1 (the mixture must be exactly 50:50).

 In the second case  the equilibrium will lie on one side or the other depending on the nature of R.

مواضيع ذات صلة

The Robinson annelation

Intramolecular aldol reactions

How to control aldol reactions of aldehydes

How to control aldol reactions of esters

Specifi c enol equivalents from 1,3-dicarbonyl compounds

Conjugated Wittig reagents as specific enol equivalents

Silyl enol ethers in aldol reactions

Lithium enolates in aldol reactions

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization