Nucleophilic Addition of Ammonia and Its Derivatives

Ammonia and the amines may be regarded as nitrogen analogs of water and alcohols. Do they add to aldehydes and ketones In fact, they do, giving products corresponding to those just studied. We shall see one important difference, however. The products of addition of amines and their derivatives lose water, furnishing either of two new derivatives of the original carbonyl compounds imines and enamines. 

Upon exposure to an amine, aldehydes and ketones initially form hemiaminals, the nitrogen analogs of hemiacetals. In a subsequent, slower step, hemiaminals of primary amines lose water to form a carbon-nitrogen double bond. This function is called an imine (an older name is Schiff base) and is the nitrogen analog of a carbonyl group. 

Imine Formation from Amines and Aldehydes or Ketones 

The mechanism of the elimination of water from the hemiaminal is the same as that for the decomposition of a hemiacetal to the carbonyl compound and alcohol. It begins with protonation of the hydroxy group. (Protonation of the more basic nitrogen jnst leads back to the original carbonyl compound.) Dehydration follows to the intermediate iminium ion, which is then deprotonated to furnish the imine. 

Processes such as imine formation from a primary amine and an aldehyde or ketone, in which two molecules are joined with the elimination of water (or other small molecules such as alcohols), are called condensations. 

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