Etard Reaction, Gattermann-Koch and Toluene Oxidation: 3 Ways to Make Benzaldehyde

The Etard reaction turns toluene into benzaldehyde. You treat toluene (C6H5-CH3) with chromyl chloride (CrO2Cl2) in a solvent like CS2 or CCl4. The two Cr-O bonds add to the -CH3 side chain and form a brown solid called the chromium complex. This complex protects the carbon from being oxidised further. When you add water (acidic hydrolysis, H3O+), the complex breaks and gives benzaldehyde. The key point: chromyl chloride stops the oxidation exactly at the aldehyde, not the acid.

The intermediate is the brown chromium complex C6H5-CH(OCrOHCl2)2. It forms because two molecules of CrO2Cl2 add across the methyl carbon. Students often wrongly pick C6H5-CHCl2 (benzal chloride) here, but that comes from chlorination, not from Etard. In Etard the carbon is bonded to chromium-oxygen groups, and only after adding water do you get the -CHO group. NEET 2021 asked exactly this.

Gattermann-Koch makes benzaldehyde directly from benzene (not toluene). You pass carbon monoxide (CO) and hydrogen chloride (HCl) gas through benzene in the presence of anhydrous AlCl3 and cuprous chloride (CuCl or Cu2Cl2). This behaves like a Friedel-Crafts reaction where -CHO is added to the ring. So benzene becomes benzaldehyde. Remember: Gattermann-Koch starts from benzene, Etard starts from toluene.

Toluene has a -CH3 side chain. Strong oxidants like KMnO4/H+ or K2Cr2O7 oxidise it all the way to -COOH (benzoic acid). To stop at the aldehyde (-CHO), you must use a mild or controlled method. One route is chlorination: toluene + Cl2 in light gives benzal chloride (C6H5-CHCl2), a gem-dichloride. Hydrolysing this with water at 373 K gives benzaldehyde. If you over-chlorinate to C6H5-CCl3 (benzotrichloride), hydrolysis gives benzoic acid instead.

Because CrO2Cl2 first forms a stable chromium complex on the side-chain carbon. This complex is a solid that does not react further under those conditions. It locks the carbon so it cannot be oxidised more. Only when you add water does the complex break down to release the -CHO group. If you had used strong KMnO4 or K2Cr2O7 instead, there would be no protecting complex, so the reaction would run all the way to benzoic acid.

Benzal chloride (C6H5-CHCl2) has two chlorines on one carbon. Hydrolysis replaces both Cl with -OH, giving an unstable gem-diol that loses water to form benzaldehyde (C6H5-CHO). Benzotrichloride (C6H5-CCl3) has three chlorines on one carbon. Its hydrolysis gives benzoic acid (C6H5-COOH). So how many chlorines you add on the side chain decides whether you get the aldehyde or the acid.

From toluene. The Etard reaction needs a -CH3 side chain to oxidise, so it uses toluene (or other methylbenzenes). Benzene has no side chain, so it cannot undergo the Etard reaction. To make benzaldehyde from benzene, use Gattermann-Koch instead.

Carbon monoxide (CO) and hydrogen chloride (HCl) gas, with anhydrous AlCl3 as catalyst and cuprous chloride (CuCl) as co-catalyst. This mixture behaves like a Friedel-Crafts formylation, adding -CHO to the benzene ring.

KMnO4 (with H+) is a strong oxidant. It oxidises the -CH3 all the way to -COOH, giving benzoic acid, not benzaldehyde. To stop at the aldehyde you need a controlled method such as chromyl chloride (Etard) or side-chain chlorination followed by mild hydrolysis.

It is a brown solid chromium complex, C6H5-CH(OCrOHCl2)2. This brown adduct is why you can isolate it and then hydrolyse it with water to release benzaldehyde.

Both appear. Etard is asked mainly through its intermediate (the chromium complex, NEET 2021), and Gattermann-Koch shows up in match-the-column questions (NEET 2021). Learn the starting material and reagent for each and you can answer both.