If you dissolve a simple carbonyl compound (for example, 1-phenylpropan-1-one, ‘propiophe none’) in D2O and run a series of 1H NMR spectra over a period of time, the signal for protons next to the carbonyl group very slowly disappears. If the compound is isolated from the solution afterwards, the mass spectrum shows that those hydrogen atoms have been replaced by deuterium atoms: there is a peak at (M + 1) ⁺ or (M + 2) ⁺ instead of at M+ Enolization usually means losing a proton from C and gaining one at O. But in D2O all of the ‘protons’ are in fact ‘deuterons’ (D⁺, or 2H⁺), so initially an enol with an ‘OD’ group form. This doesn’t matter, though, because when the enol form reverts to the keto form, it loses the D from O. But what does matter is that it also picks up a deuteron instead of a pro ton at C.

The process can now be repeated: either the D or the H could be lost this time, but eventually it is certain that the huge excess of D over H in the solvent will mean that both H atoms adjacent to the carbonyl group are replaced by D.

We can detect this exchange by the slow disappearance of the 2H signal for the protons on the carbon next to the carbonyl group. There are, of course, eight other hydrogen atoms in the molecule but they are not affected by enolization.

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