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

Abstract

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.

Ketone Formation from Carboxylic Acids by Ketonic Decarboxylation: The Exceptional Case of the Tertiary Carboxylic Acids

“…[16] These results were clearly incompatible with the widely accepted b-keto acid mechanism and the authors proposedac arbon-centered anion as intermediate instead (Scheme2). [22][23][24][25][26][27][28][29] For the theoretical examination it was mandatory to "fix" the reactionc enters on ac rystalline surface to get reliable information and to avoid excessive freedom for reactants and catalysts/catalytic sites. Scheme 2, alternative pathway), which are not observed in general, this mechanism was modifieda nd the decarboxylation and carbon-carbon bond formationw ere proposed to occur in ac oncerted fashion.…”

“…This is manifested in the number of catalytic studies that have appeared recently [17][18][19][20][21] and in the experimental and theoretical evaluations of the mechanism. [22][23][24][25][26][27][28][29] For the theoretical examination it was mandatory to "fix" the reactionc enters on ac rystalline surface to get reliable information and to avoid excessive freedom for reactants and catalysts/catalytic sites. Furthermore, it was of paramount interestthat the catalyst was stable under the reaction conditions and the geometry was unchanged after the transformation.…”

“…Therefore, in this case the principal kinetic effect should be:t oc arry out the reactionu nder "less humid conditions" avoiding water desorption within the cata-lytic cycle. [22] The latter turned out to be highly endothermic on zirconium oxide. [25] The reaction via the b-keto-acid intermediate was favored over the concerted mechanism or radical reactions for substrates without steric impedimentsi nt he a-position.…”

“…As already noted, given the increasing focus on biomass transformations, the ketonic decarboxylation and its mechanism has become matter of interest again. This is manifested in the number of catalytic studies that have appeared recently and in the experimental and theoretical evaluations of the mechanism . For the theoretical examination it was mandatory to “fix” the reaction centers on a crystalline surface to get reliable information and to avoid excessive freedom for reactants and catalysts/catalytic sites.…”

“…The carboxylic acid anhydride may also be an intermediate, because both precursors for the β‐keto‐acid can be formed from one molecule of acid anhydride. Therefore, in this case the principal kinetic effect should be: to carry out the reaction under “less humid conditions” avoiding water desorption within the catalytic cycle . The latter turned out to be highly endothermic on zirconium oxide …”