And 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). 

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... 

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]. 

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. 

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

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

Reaction XC1. (a) Oxidation of Aromatic Hydrocarbons to Aldehydes by the action of Chromyl Chloride in Carbon Disulphide Solution. (Etard.) (A. Ch., [5], 22, 225.)—In this reaction the hydrocarbon and chromyl-chloride are both dissolved in carbon disulphide, and the solutions carefully mixed. An explosive intermediate compound is precipitated, and this is separated and decomposed with water to give the aldehyde. The yields are very good, but the method is not often used owing to the inconvenience of working with carbon disulphide and the dangerous nature of the intermediate compounds. 

Triethanolamine (TEOA) is added to tbe initial reaction mixture to reduce [RuCbpy) ] as it is generated thus the back reaction (producing [Ruibpy) ] and is I etarded. The reduced methylviologen MV reduces water to hydrogen in the presence of collodial platinum and is oxidized back to thereby completing... 

This reaction was initially reported by Etard in 1880. It is the oxidation of hydrocarbons by hexavalent chromium compounds (chromyl chloride, i.e., chromatic acid chloride) to form a mixture of alcohol, carbonyl compounds (aldehyde or ketones), chloro-ketones, or aldehydes and the starting material. Therefore, this reaction is generally known as the Etard... 

Benzaldehyde.—The aldehydes of the aromatic seiies may also be obtained by the oxidation of a methyl side-chain with chromium oxychloride. The solid brown product, C,H,.CH.)(CrO,CL)2, formed by adding C1O2CIJ to toluene, dissolved in carbon bisulphide, is decomposed with water, and benzaldehyde sepaiates out (Etard). Other methods for pie-paring aromatic aldehydes are (i) the Fiiedel-Crafts reaction, in which a mixture of carbon monoxide and hydrogen chloride aie passed into the hydrocaibon in presence of aluminium chloride and a little cuprous chloride,... 

After his return he lived on his inherited wealth and did research work in his private lahoratory. He participated in the Curie couple s research by doing rectrometric measurements, he studied high-temperature reactions, and, in the field of organic chemistry, ethers and ter penes (Etard 1904). 

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