The Varrentrapp reaction is an organic reaction involving the chemical decomposition of an α,β-unsaturated acid into two other acid fragments by action of molten alkali. This reaction pioneered by F. Varrentrapp in 1840 has been of some importance in structure elucidation of certain fatty acids but has no practical synthetic use.
The original 1840 Varrentrapp reaction concerned the conversion of oleic acid to palmitic acid, acetic acid and hydrogen gas:
the reaction conditions are harsh: medium molten potassium hydroxide at temperatures in the range of 250 to 300 °C. Likewise cinnamic acid is converted to benzoic acid.
The reaction mechanism for this reaction is outlined below. Although the alkene group in the fatty acid can be located at more than one position in the chain, this does not affect the nature of the 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 is nucleophilic addition of a hydroxyl anion to the double bond of 1 followed by proton migration to intermediate 3. The next step is cleavage to enolate 4a and aldehyde 4b. The enolate on acidic workup will form acetic acid. The aldehyde continues to react with another equivalent of base forming carboxylate 7 together with hydrogen. The hydride transfer is akin the Cannizzaro reaction.
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