18.1: Enols and Enolates

The last major class of reactions involves C-C bond formation that proceeds through an intermediate known as an enolate. An enolate is simply a deprotonated enol, which is itself a hydroxyalkene. Enols are in equilibrium with the keto form (preferred), and the enolates themselves have resonance structures that place negative charge on oxygen and carbon, respectively, in an allyl-like arrangement.

The equilibrium between keto and enol forms is not necessarily rapidly established – there could be some barrier – but the overall movement of a proton through a $π$ system is known as tautomerization. The keto form is normally favored since the C=O bond is stronger than a C=C bond. One can promote tautomerization to the enol form by reducing the amount of electron density on the carbonyl. This would allow for better $σ$_C-H --> $π^{*}$_C-O overlap (since $π^{*}$_C-O will be lower). Thus, better $σ$-donation will result, more enol will be formed, and the proton that is moving will become more acidic. Another way of saying this is: as the electron deficiency of the carbonyl builds up, $σ$-donation is greater, and more enol forms.

The pK_a of the enolizable proton correlates with greater stabilization of the negative charge in the conjugate base. Consider the trend for both enol and pK_a below. The cut-off for keto-enol tautomerization is roughly a pK_a of 18-20. In 1,3-dicarbonyl compounds, the central atom’s pK_a is quite low due to the double resonance delocalization that can occur.

Let’s take a look at how this is manifest in some K_eq values.

Other examples:

We can promote enolization under both acidic and basic conditions:

1. acidic conditions

2. basic

Remember, also, that enol and enolates only form when $σ$_C-H is aligned with $π^{*}$_C-O. Bredt’s rule states that no enolization can occur on bridged bicyclic systems.