Thermodynamics of the lipase-catalyzed transesterification of 1-phenyl-1-alkanols and butyl acetate in organic solvents

The thermodynamics of the lipase-catalyzed transesterification reactions of butyl acetate and 1-phenyl-1-alkanols from C1 to C4 have been studied in organic solvents. Equilibrium measurements of the reactions with benzyl alcohol and (R)-(+)-1-phenyl ethanol were carried out in n-hexane, acetonitrile, 2-butanone, tert-butyl methyl ether, carbon tetrachloride and neat (no solvent added) at 298.15 K. The average value for the equilibrium constant and the standard molar Gibbs energy change ΔrG°m in these solvents for the reaction with benzyl alcohol (C1) are 0.29 and 3.1 kJ mol−1, respectively; for the reaction with (R)-(+)-1-phenyl ethanol (C2) the respective values are 0.11 and 5.4 kJ mol−1. The difference of 2.3 kJ mol−1 in the values of ΔrG°m between the C1 and C2 alkanols is attributed to increased steric hindrance associated with the additional methyl group in (R)-(+)-1-phenyl ethanol. In addition, the temperature dependence of the equilibrium constants for the reactions with C1 and C2 alkanols were also studied in n-hexane. The standard molar Gibbs energy ΔrG°m, enthalpy ΔrH°m, and entropy ΔrS°m changes at 298.15 K have been calculated from these results. It is seen that the temperature variation of the values of the equilibrium constants is small and that the values of ΔrH°m are zero within experimental error. The equilibrium constants for the reactions involving (R)-(+)-1-phenyl-1-propanol (C3) and (R)-(+)-1-phenyl-1-butanol (C4) were measured in n-hexane only and the values are both 0.11. The fact that the values of the equilibrium constants for the reactions involving C2–C4 are constant is consistent with the view that no additional steric hindrance is caused by adding more than one methylene group to the (R)-(+)-1-phenyl-1-alkanol reactant.