Introduction to Enols

What is an α carbon?

Organic chemists use Greek letters alpha (α), beta (β), gamma(γ), delta (δ) to describes the 1st, 2nd, 3rd, and 4th carbons away from a carbonyl carbon.  This molecule has two α-protons on its α-carbon.

For example, the following is an α,β-unsaturated aldehyde since between the α and β carbons it is unsaturated.

Cyclohexanone has two α-carbons and four α-hydrogens.

Enols and Tautomerization

An enol has the following general structure.  They are alkenes with a hydroxy group on one of the unsaturated carbons. 

You may recall that enols are formed as intermediates in the hydration of alkynes.

All ketones, aldehydes, and other carbonyl-containing compounds are in equilibrium with their tautomeric enol forms.  These are two distinct compounds in equilibrium.  We refer to them as tautomers, and the reaction between them is referred to as a tautomerization.

For most aldehydes and ketones with a single carbonyl group, the equilibrium is in favor of the carbonyl form.  

Introducing another carbonyl in the beta (β) position and the enol form becomes predominant.  This enol is stabilized by hydrogen bonding of the OH to the other carbonyl O atom.

Phenol is actually a very stable enol since it is aromatic and the keto form is not.  Remember the Claisen rearrangement,