Assessing thermochemical data

This paper describes, with examples, a critical assessment of thermochemical data for some small molecules and free radicals. The available heats of formation, ΔfH° (all 298 K values). for simple alkyl hydroperoxides and di-alkyl peroxides were compared and new data are provided. The ΔfH° values, all ±5 kJ/mol, are: CH3OOH, −135; CH3CH2OOH, −168; n-C3H7OOH, −189; s-C3H7OOH, −205; t-C4H9OOH, −240; CH3OOCH3, −132; CH3CH2OOCH3, −165; C2H5OOC2H5, −198; n-C3H7OOn-C3H7, −240; s-C3H7OOs-C3H7, −272; t-C4H9OOt-C4H9, −342. These are consistent with established O–O bond dissociation energies and with additivity considerations. ΔfH° values for the corresponding alkoxy radicals are also addressed.A similar survey was applied to the homologous n-alkyl aldehydes, C2 to C8, for which recommended ΔfH° values, all ±1.5 kJ/mol, are: −166.5, −189, −207.5, −227, −248, −268 and −289, respectively. Particular attention was given to ΔfH°(CH3CO) = −10.3 ± 1.8 kJ/mol.The current NIST WebBook datum, ΔfH°(CS) = 280.3 kJ/mol, is arguably the best value, being consistent with related thermochemical data.Finally the ΔfH° values for the allylic free radicals CH2CHCH2, 174 ± 3 kJ/mol, CH2CHCH(OH), 4.5 ± 4 kJ/mol, and (CH2CH)2C(OH), 37 ± 4 kJ/mol, derived from experimental data and results of computational chemistry are described, together with some related homolytic bond strengths.