Role of Anhydride in the Ketonization of Carboxylic Acid: Kinetic Study on Dimerization of Hexanoic Acid

Ketonization of hexanoic acid (CH₃(CH₂)₄COOH) to produce 6-undecanone ((CH₃(CH₂)₄)₂CO) was performed and the reaction pathway was investigated through a kinetic study. Unlike studies suggesting β-keto acid as an undetectable intermediate of ketonization, hexanoic anhydride ((CH₃(CH₂)₄)­COOCO­(CH₂)₄CH₃) was observed to form as a result of the condensation of two hexanoic acid molecules by the loss of a water molecule. In order to investigate the role of hexanoic anhydride on the ketonization reaction, this kinetic study compared the performances of the reaction rate equations under different models for the reaction mechanism. Results indicate that ketonization occurs by the condensation of two hexanoic acid molecules producing hexanoic anhydride, followed by decarboxylation to produce 6-undecanone. By contrast, the formation of a β-keto acid is not observed in any experimental attempt.