- Varrentrapp reaction
The Varrentrapp reaction is an
organic reaction involving thechemical decomposition of an α,β-unsaturated acid into two other acid fragments by action of moltenalkali . This reaction pioneered by F. Varrentrapp in 1840 [F. Varrentrapp, Ann., 36, 196 (1840)] has been of some importance in structure elucidation of certainfatty acid s but has no practical synthetic use ["Organic reactions in strong alkalis-I : Fission of ethylenic acids (the varrentrapp reaction)" Tetrahedron, Volume 8, Issues 3-4, 1960, Pages 221-238 R. G. Ackman, Patrick Linstead, B. J. Wakefield and B. C. L. Weedon doi|10.1016/0040-4020(60)80031-2] ["A Mechanism for the Cleavage of Unsaturated Acids with Molten Alkali" William A. Bonner, Robert T. RewickJ. Am. Chem. Soc. ; 1962; 84(12); 2334-2337. DOI|10.1021/ja00871a013] .The original 1840 Varrentrapp reaction concerned the conversion of
oleic acid topalmitic acid ,acetic acid andhydrogen gas:the reaction conditions are harsh: medium molten
potassium hydroxide at temperatures in the range of 250 to 300°C . Likewisecinnamic acid is converted tobenzoic acid .The
reaction mechanism for this reaction is outlined below. Although thealkene group in the fatty acid can be located at more than one position in the chain, this does not affect the nature of te reaction products. This is because the double bond always migrates to the position next to the carboxylic acid group prior to cleavage. The next step isnucleophilic addition of ahydroxyl anion to the double bond of 1 followed by proton migration to intermediate 3. The next step is cleavage toenolate 4a andaldehyde 4b. The enolate on acidic workup will form acetic acid. The aldehyde continues to react with another equivalent of base formingcarboxylate 7 together with hydrogen. The hydride transfer is akin theCannizzaro reaction .References
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