The Gattermann–Koch reaction, named after the German chemists Ludwig Gattermann

Ludwig Gattermann

Ludwig Gattermann was a German chemist who contributed significantly to both organic and inorganic chemistry.- Early life :...

 and Julius Arnold Koch

Julius Arnold Koch

Julius Arnold Koch was an American chemist born in Germany. Koch graduated from University of Pittsburgh in 1884. Koch was the first dean of the School of Pharmacy at University of Pittsburgh and held this position until his retirement in 1932...

, in organic chemistry

Organic chemistry

Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 refers to a Friedel–Crafts acylation reaction in which carbon monoxide

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 and hydrochloric acid are used in situ

In situ

In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...

 with Friedel–Crafts catalyst, namely AlCl₃ to produce a benzaldehyde

Benzaldehyde

Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...

 derivative from a benzene derivative in one step. Benzaldehyde and many aromatic aldehyde

Aldehyde

An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s are conveniently synthesized by this reaction. Presence of traces of copper(I) chloride

Copper(I) chloride

Copper chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid...

 are also needed.
An example would be the conversion of toluene

Toluene

Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 to p-tolualdehyde

4-Methylbenzaldehyde

4-Methylbenzaldehyde is a simple aromatic aldehyde. It is commercially available, but may be prepared from the Friedel-Crafts formylation of toluene with carbon monoxide and hydrogen chloride under Gattermann-Koch conditions:...

.