Energetics of Cresols and of Methylphenoxyl Radicals

Abstract

Combustion calorimetry studies were used to determine the standard molar enthalpies of formation of o-, m-, and p-cresols, at 298.15 K, in the condensed state as Δ_fH^o_m(o-CH3C6H4OH,cr) = −204.2 ± 2.7 kJ/mol, Δ_fH^o_m(m-CH3C6H4OH,l) = −196.6 ± 2.1 kJ/mol, and Δ_fH^o_m(p-CH3C6H4OH,cr) = −202.2 ± 3.0 kJ/mol. Calvet drop calorimetric measurements led to the following enthalpy of sublimation and vaporization values at 298.15 K: Δ_subH^o_m(o-CH3C6H4OH) = 73.74 ± 0.46 kJ/mol, Δ_vapH^o_m(m-CH3C6H4OH) = 64.96 ± 0.69 kJ/mol, and Δ_subH^o_m(p-CH3C6H4OH) = 73.13 ± 0.56 kJ/mol. From the obtained Δ_fH^o_m(l/cr) and Δ_vapH^o_m/Δ_subH^o_m values, it was possible to derive Δ_fH^o_m(o-CH3C6H4OH,g) = −130.5 ± 2.7 kJ/mol, Δ_fH^o_m(m-CH3C6H4OH,g) = −131.6 ± 2.2 kJ/mol, and Δ_fH^o_m(p-CH3C6H4OH,g) = −129.1 ± 3.1 kJ/mol. These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by the B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3P86/cc-pVDZ, B3P86/cc-pVTZ, MPW1PW91/cc-pVTZ, CBS-QB3, and CCSD/cc-pVDZ//B3LYP/cc-pVTZ methods, were used to obtain the differences between the enthalpy of formation of the phenoxyl radical and the enthalpies of formation of the three methylphenoxyl radicals: Δ_fH^o_m(C6H5O•,g) − Δ_fH^o_m(o-CH3C6H4O•,g) = 42.2 ± 2.8 kJ/mol, Δ_fH^o_m(C6H5O•,g) − Δ_fH^o_m(m-CH3C6H4O•,g) = 36.1 ± 2.4 kJ/mol, and Δ_fH^o_m(C6H5O•,g) − Δ_fH^o_m(p-CH3C6H4O•,g) = 38.6 ± 3.2 kJ·mol-1. The corresponding differences in O−H bond dissociation enthalpies were also derived as DH°(C6H5O−H) − DH°(o-CH3C6H4O−H) = 8.1 ± 4.0 kJ/mol, DH°(C6H5O−H) − DH°(m-CH3C6H4O−H) = 0.9 ± 3.4 kJ/mol, and DH°(C6H5O−H) − DH°(p-CH3C6H4O−H) = 5.9 ± 4.5 kJ/mol. Based on the differences in Gibbs energies of formation obtained from the enthalpic data mentioned above and from published or calculated entropy values, it is concluded that the relative stability of the cresols varies according to p-cresol < m-cresol < o-cresol, and that of the radicals follows the trend m-methylphenoxyl < p-methylphenoxyl < o-methylphenoxyl. It is also found that these tendencies are enthalpically controlled.

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