A reaction of the type C₆H₆ → C₆H₅CHO, used to introduce aldehyde groups onto benzene rings. It is used in the industrial manufacture of benzaldehyde. A mixture of hydrogen chloride and carbon monoxide is passed through benzene using a Lewis acid such as aluminium chloride as a catalyst. An intermediate H-C≡OC⁺ is formed and electrophilic substitution takes place on the benzene ring. There is a variation of the reaction, sometimes simply called the Gattermann reaction, in which the –CHO group is substituted onto the benzene ring of a phenol. In this reaction hydrogen cyanide is used rather than carbon monoxide. A mixture of zinc cyanide and hydrochloric acid produces the hydrogen cyanide along with zinc chloride to act as the catalyst. The electrophile is HCNH⁺ which produces an imine intermediate

This then hydrolyses to the aldehyde HOC₆H₄CHO. If an organic cyanide (nitrite) RCN is used, a ketone is produced. The reaction was discovered by Ludwig Gattermann and J. C. Koch in 1897. The use of hydrogen cyanide was reported by Gattermann in 1907.