ADDITION OF NITROGEN COMPOUNDS TO ALDEHYDES AND KETONES

Ammonia and amines of the general formula RNH2 are nitrogen analogs of water and alcohols. They are more nucleophilic than water and alcohols and react faster with carbonyl groups of aldehydes and ketones. In these reactions, a nitrogen analog of a hemiacetal, called a hemiaminal, forms. 

The hemiaminal does not react with a second mole of amine to form the nitrogen analog of an acetal because it readily loses water to form a compound with a carbon-nitrogen double bond, called an imine. 

The mechanism for this dehydration resembles that of the conversion of an acetal to a carbonyl compound (Section 19.6). Although protonation of the more basic nitrogen atom of the hemiaminal does occur, this reaction leads only to the reverse of the reaction that formed it. However, protonation of the hydroxyl group opens up an alternate reaction pathway, which leads to the imine. 

Loss of a proton from the iminium gives the imine product. All of the reactions leading to imine are reversible. Therefore, an imine can hydrolyze to form an amine and a carbonyl compound. Le Chatelier s principle allows us to predict the conditions required to drive the reaction in the given direction. Removing water as it forms favors formation of the imine. Adding excess water favors imine hydrolysis. 

The overall reaction of a carbonyl compound with an amine is an addition-elimination reaction. In the addition step, the nitrogen atom bonds to the carbonyl carbon atom and a hydrogen atom bonds to the carbonyl oxygen atom. The addition product then loses a molecule of water in an elimination reaction to give an imine. 

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