The Vilsmeier reagent is an organic compound with the formula [(CH<sub>3</sub>)<sub>2</sub>NCHCl]Cl. It is a salt consisting of the N,N-dimethyl­iminium cation ([(CH<sub>3</sub>)<sub>2</sub>N=CHCl]<sup>+</sup>) and chloride anion. Depending on the particular reaction, the anion can vary. In typical POCl<sub>3</sub>-based reactions, the anion is PO<sub>2</sub>Cl<sub>2</sub><sup>âÂÂ</sup>. The iminium cation [(CH<sub>3</sub>)<sub>2</sub>N=CHCl]<sup>+</sup> is the reactive component of interest. This iminium species is a derivative of the imidoyl chloride CH<sub>3</sub>N=CHCl. Analogues of this particular reagent are generated when tertiary amides other than DMF are treated with POCl<sub>3</sub>.
The salt is a white solid that is soluble in polar organic solvents. Vilsmeier reagent is the active intermediate in the formylation reactions, the Vilsmeier reaction or Vilsmeier-Haack reaction that use mixtures of dimethylformamide and phosphorus oxychloride to generate the Vilsmeier reagent, which in turn is attacked by a nucleophilic substrate and eventually hydrolyzes to give formyl. It is a source of "O=CH<sup>+</sup>".