Q.Write short notes on the following: (A) Reimer-Tiemann reaction (B) Etard reaction (C) Cannizzaro reaction OR (A) How will you convert methanal into ethanal? (B) How will you convert benzaldehyde into benzophenone? (C) How will you convert ethanol into ethanoic acid?

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 …