text: Imidoyl chlorides are useful intermediates in the syntheses of several compounds, including imidates, thioimidates, amidines, and imidoyl cyanides. Most of these syntheses involve replacing the chloride with alcohols, thiols, amines, and cyanates, respectively. Imidoyl chlorides can also undergo Friedel-Crafts reactions to install an imine groups on aromatic substrates. If the nitrogen of the imidoyl chloride has two substituents, the resulting chloroiminium ion is vulnerable to attack by aromatic rings without the need for a Lewis acid to remove the chloride first. This reaction is called the Vilsmeier–Haack reaction, and the chloroiminium ion is referred to as the Vilsmeier reagent.  After attaching the iminium ion to the ring, the functional group can later be hydrolyzed to a carbonyl for further modification. The Vilsmeier-Haack reaction can be a useful technique to add functional groups to an aromatic ring if the ring contains electron-withdrawing groups, which make using the alternative Friedel-Crafts reaction difficult.
  Imidoyl chlorides can be easily halogenated at the α carbon position. By treating imidoyl chlorides with hydrogen halide, will cause all α hydrogens to be replaced with the halide. This method can be an effective way to halogenate many substances. Imidoyl chlorides can also be used to form peptide bonds by first creating amidines and then allowing them to be hydrolyzed to the amide. This approach may prove to be a useful route to synthesizing synthetic proteins.
  Imidoyl chlorides can be difficult to handle. Imidoyl chlorides react readily with water, which makes any attempt to isolate and store them for long periods of time difficult. Further, imidoyl chlorides tend to undergo self-condensation at higher temperatures if the imidoyl chloride has an α CH group. At even higher temperatures, the chlorine of the imidoyl chloride tends to be eliminated, leaving the nitrile. Because of these complications, imidoyl chlorides are typically prepared and used immediately. More stable intermediates are being sought, with substances such as imidoylbenzotriazoles being suggested.