Now for a reaction with nitrogen as an electrophile that illustrates enol reactivity and reminds us that tautomerism happens with functional groups other than the carbonyl. Let us suppose you have a carbonyl compound and wish to introduce another carbonyl group next to the first. One way you might go about it is like this:

The fi rst step involves the formation of the weak acid nitrous acid (HNO2 or, more helpfully, HONO) from the sodium salt and the strong acid HCl. Nitrous acid is itself protonated and then loss of water creates the reactive electrophile NO⁺.

This diatomic cation, isoelectronic with carbon monoxide, is electrophilic at nitrogen and attacks the enol of the ketone to form an unstable nitroso compound.

The nitroso compound is unstable because it can tautomerize with the transfer of a proton from carbon to the oxygen of the nitroso group. This process is exactly like enolization but with N=O in the place of the C=O group. It gives an oxime as the stable ‘enol’. The oxime’s O–H can form an intramolecular hydrogen bond with the ketone carbonyl group. Hydrolysis of the oxime reveals the second ketone.

If the ketone is unsymmetrical, this reaction will occur on the more substituted side, for the same reason that acid-catalysed enol bromination gives the more substituted α-bromocarbonyl compound.

● Enols and enolates generally react with electrophiles at carbon.

The nitroso group

The difference between the nitro and nitroso groups is one of oxidation state and conjugation. The much more stable nitro group has a trigonal nitrogen atom with no lone pair; the N=O bond is delocalized. The nitroso group has a trigonal nitrogen atom with a lone pair in the plane; the N=O bond is not delocalized. Both can form ‘enols but the equilibria are biased in different directions.

مواضيع ذات صلة

Nitration of benzene

Benzene and its reactions with electrophiles

enols and phenols

Reactions of silyl enol ethers with halogen and sulfur electrophiles

Reactions of enol ethers Hydrolysis of enol ethers

Enol and enolate reactions at oxygen: preparation of enol ethers

Silyl enol ethers

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