Final answer:
Primary amines react with carbonyl compounds to form imines because they have two hydrogens attached to the nitrogen, allowing for formation of a double bond with the carbonyl carbon.
Step-by-step explanation:
The reaction of primary amines with carbonyl compounds leads to the formation of imines, while secondary amines tend to form enamines.
The formation of imines or enamines is influenced by the structure of the amine. A primary amine has two hydrogens attached to the nitrogen, which allows it to form a double bond with the carbon of the carbonyl group, leading to an imine.
In contrast, a secondary amine has only one hydrogen attached and two carbons, which inhibits the direct formation of a stable double bond with the carbonyl carbon. Instead, a hydrogen from one of the carbons neighboring the nitrogen is removed in conjunction with the elimination of the hydroxyl group from the carbonyl, leading to the formation of an enamine.
The nitrogen atom in primary amines is more available for forming a double bond with the carbonyl carbon because it has a pair of unbonded electrons and additional hydrogens that can be lost as a part of the reaction mechanism.
Secondary amines, lacking an additional hydrogen, cannot form imines and instead form enamines after a hydrogen is abstracted from an adjacent carbon, creating a carbon-nitrogen double bond in a different arrangement. The difference in reactivity and the resulting structures is due to the amine's structure and the availability of nitrogen's lone pair of electrons for bonding.
Secondary amines form enamines because they lack an additional hydrogen and instead create a carbon-nitrogen double bond after a hydrogen is abstracted from an adjacent carbon. This outcome is dependent on the structure of the amine involved in the reaction.