The formation of enols is catalysed by acids and bases. The reverse of this reaction—the for mation of ketone from enol—must therefore also be catalysed by the same acids and bases. If you prepare simple enols in the strict absence of acid or base, they have a reasonably long lifetime. A famous example is the preparation of the simplest enol, vinyl alcohol, by heating ethane-1,2-diol (glycol—antifreeze) to very high temperatures (900 °C) at low pressure. Water is lost and the enol of acetaldehyde is formed. It survives long enough for its proton NMR spectrum to be run, but gives acetaldehyde slowly.

The spectrum illustrates the electronic effect of the oxygen atom on the double bond. The alkene proton next to OH (in green) is deshielded and the two alkene protons on the other carbon atom (in orange) are shielded as you would expect from the feeding of electrons into the double bond by the OH group. The coupling constants across the double bond are as expected too: a large trans coupling (14.0 Hz) and a smaller cis coupling (6.5 Hz). The very small geminal coupling is typical of a terminal double bond CH₂ group. Other enols can be made that are stable because it is very difficult for the carbon atom to be protonated. In the example on the right, the two substituted benzene rings crowd the enol and prevent approach of a protonating agent. The rings twist out of the plane of the double bond and shield both faces of the enol from attack by a proton.

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