Thermodynamic Exercises for the Kinetically Controlled Hydrogenation of Carvone

Carvone belongs to the chemical family of terpenoids and is the main component of various plant oils. Carvone and its hydrogenated products are used in the flavouring and food industries. A quantitative thermodynamic analysis of the general network of carvone hydrogenation reactions was performed based on the thermochemical properties of the starting carvone and all possible intermediates and end products. The enthalpies of vaporisation, enthalpies of formation, entropies and heat capacities of the reactants were determined by complementary measurements and a combination of empirical, theoretical and quantum chemical methods. The energetics and entropy change in the hydrogenation and isomerisation reactions that take place during the conversion of carvone were derived, and the Gibbs energies of the reactions were estimated. It was shown that negative Gibbs energies are recorded for all reactions that may occur during the hydrogenation of carvone, although these differ significantly in magnitude. This means that all these reactions are thermodynamically feasible in a wide range from ambient temperature to elevated temperatures. Therefore, all these reactions definitely take place under kinetic and not thermodynamic control. Nevertheless, the numerical Gibbs energy values can help to establish the chemoselectivity of catalysts used to convert carvone to either carvacarol or to dihydro- and terahydrocarvone, either in carvotanacetone or carveol.