Etard reaction

Oxidation of an arylmethyl group to the corresponding aryl aldehyde using chromyl chloride. 

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The mechanism of the Etard reaction is not completely known. An insoluble complex is formed on addition of the reagents, which is hydrolyzed to the aldehyde. The complex is probably a kind of acylal, but what the structure is not fully settled, though many proposals have been made as to its structure and as to how it is... 

The name Etard reaction is often applied to any oxidation with chromyl chloride, for example, oxidation of glycols (19-7) and Alkenes (19-10). 

Oxidation of Arylmethanes - Electrochemical Alternative Routes to the Etard Reaction... 

Etard reagents (chromyl chloride and some derivatives) suffer from the problem that occasionally they can exhibit a lack of selectivity and low yields. They are useful in the selective oxidation of aromatic side-chains to a carbonyl group, aldehyde or ketone but in many instances, the formation of the initial complex is slow and yields are low because of difficulties in the work-up which lead to undesired over-reaction. Attempts have been made to solve the problems by the use of sonication [134]. A simple preparation of the liquid reagent was proposed and the Etard reaction itself together with the hydrolytic step were conducted under sonication, with some success (Scheme 3.25). 

Etard reaction org chem Direct oxidation of an aromatic or heterocyclic bound methyl group to an aldehyde by utilizing chromyl chloride or certain metallic oxides. a tar re,ak-sh3n ... 

Chromyl chloride converts the aromatic methyl group into an aldehyde this reaction is known as the Etard reaction h For example, toluene can be oxidized to benzaldehyde. 

Mechanism The mechanism of the Etard reaction involves the formation of dichromium species, which on hydrolysis gives an aldehyde (Scheme 7.35). 

Chromyl chloride, Cr202Cl2, a dark-red liquid (mp -96.5 °C, bp 117 °C, d 1.911), is prepared from chromium trioxide or sodium dichromate, hydrochloric acid, and sulfuric acid [665]. The reagent is used in solutions in carbon disulfide, dichloromethane, acetone, tert-butyl alcohol, and pyridine. Oxidations with chromyl chloride are often complicated by side reactions and do not always give satisfactory yields. The mechanism of the oxidation with chromyl chloride, the Etard reaction, is probably of free-radical nature [666]. Complexes of chromyl chloride with the compounds to be oxidized have been isolated [666, 667, 668]. 

Oxidations with chromyl chloride (Etard reaction) of aromatic compounds with longer chains are not always suitable because the reactions result in more than one product [673]. 

CrOs. Chromium oxide can also oxidize benzylic positions., as in the Etard reaction (see sec. 3.8.E). When 391 was treated with chromium trioxide in acetic acid, an 85% yield of 392 was obtained, as part of Pan s synthesis of (-)-6,7-dehydrofeiruginyl methyl ether. l... 

This reagent can also be used for the oxidation of methylated aryl derivatives to the corresponding aryl aldehyde, in what has become known as the Etard reaction. Tillotson and Houston found that the Etard reaction is catalyzed by small amounts of alkene, added to or present in the reaction medium. The reaction involves addition of chromyl chloride to a carbon disulfide or carbon tetrachloride solution of the arene. A dark brown, insoluble, and explosive intermediate usually precipitates. Dilute sulfurous acid is added to decompose the precipitate to the aldehyde. Toluene is converted to benzaldehyde and ethylbenzene was oxidized to phenylacetaldehyde with this reagent. 

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