The amine reactant in the Mannich reaction plays a crucial role in determining the selectivity and outcome of the reaction. Primary amines are typically the most reactive, forming iminium ions readily, which then react with nucleophiles such as enolates or enols to produce β-amino carbonyl compounds. Secondary amines can also participate but may lead to sterically hindered products, while tertiary amines do not directly participate in the traditional Mannich reaction due to the lack of a hydrogen atom required for iminium ion formation. The choice of amine, along with the reaction conditions, can therefore control the product's stereochemistry and yield.