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Iminium ions are positively charged chemical species with the general formula [R3R'N=CR''R''']+, where R, R', R'', and R''' represent organic substituents, and the central carbon atom is bonded to three other atoms and carries a formal positive charge. This structure features a nitrogen atom with a double bond to a carbon atom, and the nitrogen atom bears a full positive charge. They are derived from amines and are key intermediates in a variety of organic reactions.

The formation of iminium ions typically occurs through the reaction of a secondary amine with an aldehyde or ketone, usually under acidic conditions. The amine acts as a nucleophile, attacking the carbonyl carbon. This is followed by dehydration, eliminating a molecule of water and forming the double bond between the nitrogen and carbon atoms, resulting in the iminium ion. Tertiary amines can also form iminium ions, but they typically involve a different mechanism, often through elimination reactions.

Iminium ions are highly reactive electrophiles due to the positive charge on the carbon atom. This makes them susceptible to nucleophilic attack. This electrophilic character is central to their role in many important organic transformations.

Prominent reactions involving iminium ions include the Mannich reaction, where they act as electrophiles to alkylate enols or enolates, forming beta-amino carbonyl compounds. They are also intermediates in other reactions such as the Pictet-Spengler reaction, used in the synthesis of tetrahydroisoquinolines, and various cycloaddition reactions. The stability of iminium ions can vary depending on the nature of the substituents and the reaction conditions.