Self-association of phenols in inert solvents. Apparent heat capacities of phenol, substituted phenols and aromatic alcohols in n-heptane

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Abstract
Apparent heat capacities have been measured for phenol, substituted phenols and aromatic alcohols in dilute n-heptane at 25 °C. The alcohols are phenol, o-cresol, m-cresol, p-cresol, 2,6-dimethylphenol, 3,5-dimethylphenol, 3,5-di-tert-butylphenol, 2,6-di-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 4-tert-butylphenol, 2-ethylphenol, 2-propylphenol, 2-phenylphenol, benzyl alcohol, 2-phenylpropan-2-ol and phenethyl alcohol. In addition, cyclohexanol, 3-ethylpentan-3-ol and hexan-1-ol were also studied. The associational part of the apparent molar heat capacity ϕc(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. For the 2,6-disubstituted phenols the ϕc maximum disappears, indicating the absence of self-association. The Treszczanowicz–Kehiaian theory for associated liquids was used to obtain the volumetric equilibrium constants and the enthalpy of hydrogen-bond formation (ΔH0). For phenols ΔH0 is lower than for other alcohols, indicating weaker hydrogen bonding. In contrast to alkan-1-ols, where tetramers are the predominant species, for the present alcohols as the steric hindrance increases tetramer population is severely reduced, dimers and trimers being more important.

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