Hydrogen cyanide, Friedel-Crafts reaction with

The preparation of a formyl-substituted aromatic derivative 3 from an aromatic substrate 1 by reaction with hydrogen cyanide and gaseous hydrogen chloride in the presence of a catalyst is called the Gattermann synthesis This reaction can be viewed as a special variant of the Friedel-Crafts acylation reaction. 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or Friedel-Crafts catalysts to introduce formyl or acyl substituents. The active electrophiles are believed to be dications resulting from diprotonation of CO, HCN, or the nitrile.64 The general outlines of the mechanisms of these reactions are given below. 

Acylations with Nitriles. With particularly reactive aromatic nuclei such as phenols, a Friedel-Crafts type reaction may be effected with nitriles in the presence of an acid catalyst such as hydrogen chloride, aluminum chloride, or zinc chloride. The reaction is usually called the IIouben-Hoesch synthesis.700 When hydrogen cyanide is employed, the process leads to aromatic aldehydes even with a number of aromatic... 

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