Enolization is, in fact, quite a slow process in neutral solution, even in D₂O (the exchange described above might take place over a period of hours to days at room temperature), and we would catalyse it with acid or base if we really wanted it to happen fast. In the acid-catalysed reaction, the molecule is first protonated on oxygen and then loses a proton from C in a second step. We shall use a different example here to show that aldehydes form enols too, but acid or base will catalyse enolization of any carbonyl compound in the same way.

This is a more detailed mechanism for enolization than those we have been drawing because it shows that something (here a water molecule) must actually be removing the proton from carbon. Although this reaction will occur faster than the uncatalysed enolization, the equilibrium is not changed and we still cannot detect the enol spectroscopically. In the base-catalysed reaction the C–H proton is removed first by the base, say a hydroxide ion, and the proton added to the oxygen atom in a second step.

This is a good mechanism too because it shows that something must remove the proton from carbon and something (here a water molecule—we don’t, of course, have protons available in basic solution) must put the proton on the oxygen atom. Notice that both of these reactions are genuinely catalytic. You get the proton back again (in the form of H₃O⁺) at the end of the acid-catalysed mechanism, and you get the hydroxide ion back again at the end of the base-catalysed mechanism.

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