15.13:
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.
15.13:
Nitrosation of Enols
The nitrosation reaction is one of the methods of preparing 1,2-diketones. The enol tautomer of the starting ketone reacts with sodium nitrite in hydrochloric acid, generating the 1,2-diketone after hydrolysis.
Figure 1: Keto–enol tautomerization
As depicted in Figure 2, when treated with hydrochloric acid, sodium nitrite forms an oxonium ion. The expulsion of a water molecule from the oxonium ion produces a nitrosonium ion.
Figure 2: The chemical reaction of the formation of the nitrosonium ion
The electrophilic nitrosonium ion is attacked by the enol tautomer to give an unstable nitroso compound (Figure 3).
Figure 3: The chemical reaction of the formation of nitroso compounds
As shown in Figure 4, the tautomerization of the nitroso compound involves the transfer of the hydrogen atom from the carbon to the oxygen of the nitroso group, thereby forming a stable oxime. The stability of the oxime is due to the hydrogen bond between the oxime’s hydroxyl group and the ketone’s carbonyl oxygen. Hydrolysis of the oxime results in the formation of the 1,2-diketone as the final product.
Figure 4: The formation of a diketone from a nitroso compound via an oxime intermediate
The nitrosation reaction is regioselective, where the second carbonyl group is preferentially introduced at the more-substituted carbon.