Question

Write chemical equations of the Etard reaction and the Gattermann-Koch reaction.

Hint:

Both make benzaldehyde: Etard oxidises toluene with \(CrO_2Cl_2\); Gattermann-Koch formylates benzene with \(CO+HCl/AlCl_3\).

Answer 1

Step 1: Goal of both. Each named reaction is a preparation of benzaldehyde, but starting from a different aromatic compound.
Step 2: Etard reaction (from toluene). The side-chain \(-CH_3\) of toluene is oxidised by chromyl chloride in \(CS_2\). An intermediate chromium adduct is isolated, and on hydrolysis it gives benzaldehyde.
\(C_6H_5CH_3 + CrO_2Cl_2 \rightarrow\) complex \(\xrightarrow{H_2O} C_6H_5CHO\)
Step 3: Gattermann-Koch reaction (from benzene). A mixture of carbon monoxide and hydrogen chloride, activated by anhydrous aluminium chloride and cuprous chloride, formylates benzene directly.
\(C_6H_6 + CO + HCl \xrightarrow{AlCl_3/CuCl} C_6H_5CHO\)
Step 4: Note. Etard uses an oxidising reagent on a methyl group; Gattermann-Koch is an electrophilic formylation of the bare ring. Both stop cleanly at the aldehyde.
\[\boxed{\text{benzaldehyde by both routes}}\]