Q.Convert toluene to benzaldehyde.

The Etard reaction uses chromyl chloride (CrO2Cl2) to stop the oxidation of a toluene-type methyl group at the aldehyde stage, giving benzaldehyde rather than benzoic acid.

C6H5-CH3 --(CrO2Cl2, CS2)--> [chromium complex] --(hydrolysis, H3O+)--> C6H5-CHO

Ordinary strong oxidants (like KMnO4 or acidic K2Cr2O7 under harsher conditions) push the oxidation all the way to the carboxylic acid (benzoic acid); the Etard reaction's controlled chromium complex intermediate lets the reaction be stopped cleanly at the aldehyde.

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Toluene's methyl side-chain can be oxidised in a controlled way directly to the aldehyde stage (Etard reaction), or first converted to the gem-dihalide and then hydrolysed, to give benzaldehyde without over-oxidation to benzoic acid.

Method 1 — Etard reaction: Ordinary strong oxidising agents (like $KMnO_4$ or acidic $K_2Cr_2O_7$) oxidise toluene's methyl group all the way to $-COOH$ (benzoic acid), overshooting the aldehyde stage. To stop precisely at the aldehyde, toluene is treated with chromyl chloride ($CrO_2Cl_2$) in carbon disulfide ($CS_2$), forming a chromium complex, which is then hydrolysed with water/dilute acid to liberate the aldehyde:

$C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\,CS_2} (\text{Cr complex}) \xrightarrow{H_3O^+} C_6H_5CHO$

Method 2 — side-chain chlorination then hydrolysis: Toluene undergoes free-radical chlorination at its side chain (methyl group) under photochemical conditions ($Cl_2/h\nu$); using excess $Cl_2$, chlorination is carried through to the dichloride stage (a gem-dihalide, i.e. two Cl on the same carbon):

$C_6H_5CH_3 \xrightarrow{Cl_2/h\nu} C_6H_5CHCl_2\;(\text{benzal chloride})$

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The oxidant in Etard's reaction is chromyl chloride, $CrO_2Cl_2$.

Concept. In Etard's reaction, toluene is oxidised to benzaldehyde using chromyl chloride ($CrO_2Cl_2$) in $CS_2$/$CCl_4$. It forms a brown chromium complex (Etard's complex) which on hydrolysis gives benzaldehyde, stopping the oxidation at the aldehyde stag …