The Étard reaction is a chemical reaction that involves the direct oxidation of an aromatic or heterocyclic bound methyl group to an aldehyde using chromyl chloride.^[1][2][3] For example, toluene can be oxidized to benzaldehyde.

It is named for the French chemist Alexandre Léon Étard (5 January 1852, Alençon – 1 May 1910).

Reaction mechanism

The reaction mechanism proceeds via an ene reaction with chromyl chloride, forming the precipitated Étard complex. The Étard complex is then decomposed by a [2,3] sigmatropic rearrangement under reducing conditions to prevent further oxidation to a carboxylic acid. Reducing conditions for the decomposition of the Étard complex are provided by saturated aqueous sodium sulphite. Typical solvents for the reaction include carbon disulfide, dichloromethane,^[4] chloroform, and carbon tetrachloride, with carbon tetrachloride being the most common. To obtain a highly purified aldehyde product, the Étard complex precipitate is often purified before decomposition in order to prevent reaction with any unreacted reagent. The reaction is normally carried out for a few days to several weeks and the yields are high.^[5][6]

Limitations

The Étard reaction is most commonly used as a relatively easy method of converting toluene into benzaldehyde. Obtaining specific aldehyde products from reagents other than toluene tends to be difficult due to rearrangements. For example, n-propylbenzene is oxidized to propiophenone, benzyl methyl ketone, and several chlorinated products, with benzyl methyl ketone being the major product.^[7][8] Another example arises from the Étard reaction of trans-decalin which results in a mixture of trans-9-decalol, spiro [4.5]decan-6-one, trans-1-decalone, cis-1-decalone, 9,10-octal-1-one, and 1-tetralone.^[9]

Other oxidation reagents like potassium permanganate or potassium dichromate oxidize to the more stable carboxylic acids.

Uses

Oxidation of toluene to benzaldehyde is quite a useful conversion. Benzaldehyde is routinely used for its almond flavor. The aldehyde is comparatively reactive and readily participates in aldol condensations. Benzaldehyde can serve as a precursor for various compounds, including dyes, perfumes, and pharmaceuticals. For example, the first step in the synthesis of ephedrine is condensation of benzaldehyde with nitroethane ^{[citation needed]}. Additionally, benzaldehyde is instrumental in the synthesis of phentermine.^[10] Unlike other oxidising agents (like KMnO₄ or CrO₃ etc.), chromyl chloride does not oxidise aldehyde to carboxylic acid.