Self-association of cyclohexanols in inert solvents. Apparent heat capacities of cyclohexanol and substituted cyclohexanols in n-heptane and n-decane

Abstract

Apparent heat capacities have been measured for cyclohexanol and substituted cyclohexanols in dilute n-heptane and n-decane at 25 °C. The alcohols are cyclohexanol, 2-ethylcyclohexanol, 2-tert-butylcyclohexanol, trans-2-phenylcyclohexanol, 2,6-dimethylcyclohexanol, 2,3-dimethylcyclohexanol, 3,4-dimethylcyclohexanol, 3,3,5-trimethylcyclohexanol and 3,3,5,5-tetramethylcyclohexanol. The associational part of the apparent molar heat capacity, ϕ_c(assoc), shows a maximum against concentration which decreases and moves to higher alcohol concentration as the hydroxyl group on the alcohol becomes increasingly hindered, effectively reducing the self-association capability. A comparison between cyclohexanols and the previously reported phenols (J. Chem. Soc., Faraday Trans., 1991, 87, 1733) indicates that cyclohexanols show a higher ability to self-associate owing to the larger flexibility of the cyclohexanol ring as compared to the rigid aromatic ring. Treszczanowicz–Kehiaian theory for associated liquids was used to obtain the volumetric equilibrium constants and the enthalpy change for the formation of hydrogen bonds, ΔH⁰. For cyclohexanols ΔH⁰ is higher than for the corresponding phenols. As with the phenols, the tetramer population is severely reduced, dimers and trimers being more important.