Nitrosation of Enols

The enol tautomer of carbonyl compounds can undergo a nitrosation reaction, where a second carbonyl group is introduced to form a 1,2-diketone. This nitrosation reaction occurs through a multistep mechanism.

Initially, sodium nitrite in hydrochloric acid produces the weak acid 'nitrous acid', which, on protonation, gives an oxonium ion.

The loss of water from the oxonium ion generates a nitrosonium ion with an electrophilic nitrogen. So, the nitrogen is susceptible to a nucleophilic attack by the enol, yielding an unstable nitroso compound.

The nitroso compound undergoes tautomerization, where hydrogen attached to the nitrogen-bearing carbon is transferred to the oxygen of the nitroso group, producing an oxime. A hydrogen bond between the oxime's hydroxyl group and the ketone's carbonyl oxygen stabilizes the oxime.

Finally, hydrolysis of the oxime yields the 1,2-diketone product.

Unsymmetrical carbonyl compounds undergo nitrosation reactions at the more-substituted carbon atom, generating 2,3-diketones as the product.