Write short notes on Etard reaction and Gatterman-Koch reaction.

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Q. Write short notes on Etard reaction and Gatterman-Koch reaction.

Uttar Pradesh UpmspUP Board (UPMSP) Intermediate 2026Subjective· 4mImportance★★★★★

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Concept understanding — Etard Reaction

The Etard Reaction: From Intuition to Mechanism

Imagine you have a toluene molecule — a benzene ring with a methyl group ( $-CH_3$ ) attached. You want to stop oxidation at the aldehyde stage, getting benzaldehyde ( $C_6H_5CHO$ ). Normally, strong oxidants like $KMnO_4$ or $K_2Cr_2O_7$ would rip right through the methyl group all the way to benzoic acid ( $C_6H_5COOH$ ). That's too much.

The Etard reaction is a clever way to stop halfway. It uses a special reagent — chromyl chloride ( $CrO_2Cl_2$ ) — that forms a stable complex with the intermediate, preventing over-oxidation. Then you break that complex with water to release the aldehyde.

C_6H_5CH_3 \xrightarrow[1.\ CrO_2Cl_2,\ CS_2\ \text{or}\ CCl_4]{2.\ H_3O^+} C_6H_5CHO

The solvent ( $CS_2$ or $CCl_4$ ) is inert and keeps the reaction under control.

The Mechanism: Step by Step

Step 1 — Formation of the chromium complex.

Chromyl chloride is a strong electrophile at the chromium centre. It attacks the benzylic carbon (the one attached to the ring) of toluene. The methyl group gets oxidised, and a dark brown, insoluble complex called the Etard complex precipitates out. This complex has the structure:

C_6H_5CH(OCrCl_2OH)_2

Think of it as the aldehyde already formed but held captive by two chromium fragments. Because it's locked in this complex, it cannot be further oxidised.

Step 2 — Hydrolysis.

When you add water (or dilute acid), the complex breaks apart. The chromium fragments are released as chromium oxychloride byproducts, and benzaldehyde is set free.

Note

The overall reaction consumes 2 moles of $CrO_2Cl_2$ per mole of toluene. The chromium ends up as $CrO_2$ and $CrCl_3$ after hydrolysis.

Why This Works (and What Can Go Wrong)

The key insight: the Etard complex is insoluble in the reaction solvent. It falls out of solution, physically separating the intermediate from further oxidant. That's the trick — not a kinetic slowdown, but a precipitation-based stop.

Watch out

A common mistake is to think the Etard reaction gives benzaldehyde directly. It does not. The product is the complex; benzaldehyde appears only after hydrolysis. If you skip the water workup, you get nothing useful. …

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Both are ways to make benzaldehyde: Etard oxidises toluene's methyl group with $CrO_2Cl_2$ ; Gatterman–Koch formylates benzene using CO/HCl over $AlCl_3/CuCl$ .

(i) Etard reaction. Toluene is oxidised by chromyl chloride ( $CrO_2Cl_2$ ) in $CS_2$ (or $CCl_4$ ). The methyl group forms a stable chromium complex (an addition product), which on acid hydrolysis gives benzaldehyde. Controlled oxidation stops at the aldehyde (further oxidation to acid is avoided).

$C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\ CS_2} \text{[chromium complex]} \xrightarrow{H_3O^+} C_6H_5CHO$

…

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2024Set ANNUAL1 markMCQ
$Q. The oxidation of toluene to benzaldehyde by chromyl chloride is called$
(a) Etard reaction
(b) Reimer-Tiemann reaction
(c) Stephen's reaction
(d) Cannizzaro reaction
›Reveal solutionSolution

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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CBSE 2022Set ANNUAL1 mark
$Q. Convert toluene to benzaldehyde.$
›Reveal solutionSolution

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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CBSE 2018Set ANNUAL1 mark
$Q. Name an oxidising agent used in the Etard's reaction.$
›Reveal solutionSolution

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 …

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Q.Write short notes on Etard reaction and Gatterman-Koch reaction.

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