In organic chemistry, an iminium cation is a polyatomic ion with the general structure . They are common in synthetic chemistry and biology. Structure Iminium cations adopt alkene-like geometries. The central C=N unit is nearly coplanar with all four substituents. The C=N distances, which are near 129 picometers in length, are shorter than C-N single bonds. Cis/trans isomers are observed. Formation Iminium cations are obtained by protonation and alkylation of imines: RN=CR'_2 + H+ -> [RNH=CR'_2]+ RN=CR'_2 + R''+ -> [RR''N=CR'_2]+ They also are generated by the condensation of secondary amines with ketones or aldehydes: O=CR'_2 + R2NH + H+ <=> [R2N=CR'_2]+ + H2O This rapid, reversible reaction is one step in "iminium catalysis". More exotic routes to iminium cations are known, e.g. from ring-opening reactions of pyridine. Occurrence Iminium derivatives are common in biology. Pyridoxal phosphate reacts with amino acids to give iminium derivatives. Many iminium salts are encountered in synthetic organic chemistry. Reactions Iminium salts hydrolyse to give the corresponding ketone or aldehyde: [RR''N=CR'_2]+ + H2O -> [RR''NH2]+ + O=CR'_2 Iminium cations are readily reduced to the amines, e.g. by sodium cyanoborohydride. They are intermediates in the reductive amination of ketones and aldehydes. Named reactions involving iminium species Aza-Cope rearrangement Beckmann rearrangement Duff reaction Mannich reaction Pictet-Spengler reaction Stephen aldehyde synthesis Stork enamine alkylation Vilsmeier-Haack reaction and Vilsmeier reagent Iminylium ions Iminylium ions have the general structure R2C=N+. They form a subclass of nitrenium ions. See also Ammonium Imine Quaternary ammonium cation References Functional groups Chemistry suffixes