Q.Write Gattermann - Koch reaction.

The Gattermann–Koch reaction is a special Friedel–Crafts-type formylation that introduces a $-CHO$ group directly onto benzene, using $CO+HCl$ (which generates the formylating electrophile in situ) with anhydrous $AlCl_3$ and cuprous chloride as catalysts.

The Gattermann–Koch reaction is used to prepare aromatic aldehydes (specifically benzaldehyde and its simple derivatives) directly from the parent arene, by treating it with a mixture of carbon monoxide and hydrogen chloride gas, in the presence of anhydrous aluminium chloride and a small amount of cuprous chloride (which helps generate the reactive formyl cation from CO and HCl), under pressure: $C_6H_6 + CO + HCl \xrightarrow[\text{high pressure}]{anhyd.\ AlCl_3,\ CuCl} C_6H_5CHO + HCl$ Mechanistically, this is a Friedel–Crafts-type electrophilic aromatic substitution, where $CO$ and $HCl$ combine (assisted by the Lewis acid catalyst) to generate a formyl cation ($HC\equiv O^+$) in situ …