Étard reaction

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,{{cite journal|author=F. Freeman|title=Chromyl Chloride|journal=Encyclopedia of Reagents for Organic Synthesis|year=2004|doi=10.1002/047084289X.rc177|isbn=0471936235}} 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.{{Cite journal|author1=Necsoiu, I. |author2=Balaban, A. T. |author3=Pascaru, I. |author4=Sliam, E. |author5=Elian, M. |author6-link=Costin Nenițescu |author6=Nenitzescu, C. D. |year=1963|title=The mechanism of the Étard reaction|journal=Tetrahedron|volume=19|issue=7|pages=1133–1142|doi=10.1016/s0040-4020(01)98572-2}}{{Cite journal|author=Wheeler, Owen H.|year=1958|title=Étard Reaction: I. Its Scope and Limitation with Substituted Toluenes|journal=Canadian Journal of Chemistry|volume=36|issue=4|pages=667–670|doi=10.1139/v58-093|doi-access=free}}

Image:Etard rxn.svg

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.{{Cite journal|author1=Renţea, C. N. |author2=Necşoiu, I. |author3=Renţes, M. |author4=Ghenciulescu, A. |author5=Nenitzescu, C. D. |name-list-style=amp |year=1966|title=Étard reaction—III: Oxidation of N-propylbenzene and methylcyclohexane with chromyl chloride|journal=Tetrahedron|volume=22|issue=10|pages=3501–3513|doi=10.1016/s0040-4020(01)92538-4}}{{Cite journal|author1=Wiberg, K. B. |author2=Marshall, B. |author3=Foster, G. |name-list-style=amp |year=1962|title=Some observations on the Étard reaction|journal=Tetrahedron Letters|volume=3|issue=8|pages=345–348|doi=10.1016/s0040-4039(00)70878-1}} 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.{{Cite journal|author1=Renţea, C. N. |author2=Renţea, M. |author3=Necşoiu, I. |author4=Nenitzescu, C. D. |name-list-style=amp |year=1968|title=Étard reaction—VI: Oxidation of cis and trans-decaline with chromyl chloride|journal=Tetrahedron|volume=24|issue=13|pages=4667–4676|doi=10.1016/s0040-4020(01)98663-6}}

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

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|reason=The reaction product would be missing the needed alpha-hydroxyl group and would be a direct precursor to alpha-methylphenethylamine, which would require a bit of work to reach ephedrine. It is likely the first step of any major preparation of ephedrine would be a yeast fermentation, or an acylation reaction between benzene and propionyl chloride to propiophenone, which would require aminobromination and reduction of the ketone to an alcohol to produce an ephedrine.|date=February 2016}}. Additionally, benzaldehyde is instrumental in the synthesis of phentermine.{{Cite book|author1=Vardanyan, Ruben S. |author2=Hruby, Victor J. |name-list-style=amp |year=2006|title=Synthesis of Essential Drugs|edition=first|publisher=Elsevier Science|location= Amsterdam|isbn=978-0-444-52166-8}} Unlike other oxidising agents (like KMnO₄ or CrO₃ etc.), chromyl chloride does not oxidise aldehyde to carboxylic acid.

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Category:Organic redox reactions

Category:Name reactions