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 C6H5OH → HOC6H4CH=NH 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.
C6H5OH → HOC6H4CH=NH