Calorimetrische Untersuchungen. Ueber den Wärmewerth der Aether der Phenolreihe

Enthalpy of Formation of Anisole: Implications for the Controversy on the O–H Bond Dissociation Enthalpy in Phenol

“…Excluding one early report that is only of historical significance, 15 three combustion calorimetric studies of liquid anisole, leading to Δ c H m °(PhOCH 3 , l) and Δ f H m °(PhOCH 3 , l) (Table 1), were found in the literature. In 1941 Badoche 16 reported Δ c u°(PhOCH 3 , l) = −(34941.84 ± 1.15) J•g −1 at 290.15 K, which corresponds to Δ c H m °(PhOCH 3 , l) = −3782.15 kJ•mol −1 , based on the molar mass of anisole used in this work (see above).…”

“…As mentioned above, the large discrepancies between the enthalpies of formation − and vaporization − of liquid anisole directly taken from the literature can lead to significant differences in Δ f H m ° (PhOCH 3 , g) depending on the specific combination of Δ f H m ° (PhOCH 3 , l) and Δ vap H m ° (PhOCH 3 ) used to calculate Δ f H m ° (PhOCH 3 , g). This problem is at the core of the above-mentioned controversy (see Introduction section) regarding the most reliable values of the enthalpy of formation of the phenoxy radical, Δ f H m ° (PhO • , g) and of the PhO–H bond dissociation enthalpy. , The controversy was originated by the selection of different Δ f H m ° (PhOCH 3 , g) values (separated by 8.8 kJ·mol –1 ) in the calculation of Δ f H m ° (PhO • , g) from the experimental enthalpy of reaction at 298.15 K, given as DH °(PhO–CH 3 ) = 273.2 ± 1.7 kJ·mol –1 .…”

“…This quantity can be derived by combining experimental Δ f H m o (PhOCH 3 , l) and Δ vap H m o (PhOCH 3 ) results. There are, however, significant discrepancies between the published Δ f H m o (PhOCH 3 , l) − and Δ vap H m o (PhOCH 3 ) − data used by various authors to derive Δ f H m o (PhOCH 3 , g) at 298.15 K. This leads to substantially different Δ f H m o (PhO • , g) and DH °(PhO–H) values, depending on the particular combination of data chosen. For example, based on a review of literature data and on predictions from a group contribution scheme, Mulder et al selected Δ f H m o (PhOCH 3 , g) = −(76.7 ± 1.0) kJ·mol –1 , which diverges by 8.8 kJ·mol –1 from Δ f H m o (PhOCH 3 , g) = −(67.9 ± 0.8) kJ·mol –1 given in Pedley’s compilation .…”

Energetics of Cresols and of Methylphenoxyl Radicals

“…Also included in Table are the enthalpies of formation of the cresols in the solid or liquid state recalculated from the enthalpies of combustion previously reported in the literature. The values published by Stohmann, , Barker, and Swarts 14 are only of historical value, and are not mentioned in major thermochemical compilations. ,, In these cases, for example, no reliable purity assessment was made, and no standard state corrections or uncertainties were considered in the calculation of Δ c u ° values from which the Δ f H m ° data were derived. No standard state corrections or uncertainties were also taken into account in the work by Pushin, Andon et al, and Bertholon .…”

“…The three isomers of methylphenol ( 1 , 2 , and 3 ), commonly dubbed cresols, are important materials in the production of resins, polymers, antiseptics, antioxidants, and a variety of other chemicals used for agricultural, cosmetic, and pharmaceutical applications. , Their energetics are also implicated in many fundamental studies, such as the relationships of enthalpy of formation and O−H bond dissociation enthalpies, DH °(O−H), , with the molecular structure, the antioxidant activity of phenol derivatives, − the depolymerization of coal, , and the fate of atmospheric pollutants released to the environment. , It is therefore surprising that some significant discrepancies are found between the published enthalpies of formation, enthalpies of vaporization/sublimation, and DH °(O−H) values of cresols. Even excluding early results that are only of historical value, − it is possible to conclude that the published enthalpies of formation of o -cresol (cr), m -cresol (l), and p -cresol (cr), at 298.15 K, span ranges of 2.6 kJ·mol -1 , − 17.0 kJ·mol -1 , − and 6.3 kJ·mol -1 , − respectively. The enthalpies of sublimation of both o -cresol and p -cresol at 298.15 K have been determined only once, , and the reported values of the enthalpy of vaporization of m -cresol at 298.15 K vary between 46.3 and 71.2 kJ·mol -1 . ,− To our knowledge, no direct measurements of the gas phase O−H bond dissociation enthalpies of cresols exist.…”