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

Abstract

Significant discrepancies in the literature data for the enthalpy of formation of gaseous anisole, ΔfHmo(PhOCH3, g), have fueled an ongoing controversy regarding the most reliable enthalpy of formation of the phenoxy radical and of the gas phase O-H bond dissociation enthalpy, DHo(PhO-H), in phenol. In the present work ΔfHmo(PhOCH3, g) was reassessed using a combination of calorimetric determinations and high-level (W2-F12) ab initio calculations. Static-bomb combustion calorimetry led to the standard molar enthalpy of formation of liquid anisole at 298.15 K, ΔfHmo(PhOCH3, l) = −(117.1 ± 1.4) kJ·mol(-1). The corresponding enthalpy of vaporization was obtained as, ΔvapHmo(PhOCH3) = 46.41 ± 0.26 kJ·mol(-1), by Calvet-drop microcalorimetry. These results give ΔfHmo(PhOCH3, g) = −(70.7 ± 1.4) kJ·mol(-1), in excellent agreement with ΔfHmo(PhOCH3, g) = −(70.8 ± 3.2) kJ·mol(-1), obtained from the W2-F12 calculations. The ΔfHmo(PhOCH3, g) here recommended leads to ΔfHmo(PhO•, g) = 55.5 ± 2.4 kJ·mol(-)1 and DH°(PhO-H) = 368.1 ± 2.6 kJ·mol(-1).

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

“…and 55.5 6 2.4 kJ mol21 by Simoes et al69 for phenoxy radical agree well with the G4/BAC result of 54.1 kJ mol 21 . For the C 6 H 5 OAH bond, three experimental BDE values are available in the literature, which are 368.1 6 2.6 kJ mol 21 , 69 368.2 6 6.3 kJ mol 21 54 , and 365.3 6 3.2 kJ mol 21 .…”

Enthalpies of formation of the benzyloxyl, benzylperoxyl, hydroxyphenyl radicals and related species on the potential energy surface for the reaction of toluene with the hydroxyl radical

“…The phenoxyl radical (III) was also studied experimentally by Walker and Tsang [47] who found a formation enthalpy of 13.22 kcal/mol. A careful experimental and theoretical study performed recently by Simões et al [54], afforded a formation enthalpy of 13.26 ± 0.57 kcal/mol, very near to the previous one. Our own results were obtained using the non-isodesmic reactions TR + O2 → (III) + CH2O (18) T + OH • + O2 → (III) + H2O + CH2O…”

Benchmark thermodynamic properties of methylanisoles: Experimental and theoretical study

“…Using the enthalpies of reactions (5) and (6) obtained with the theoretical methods, together with the literature data for D f H m (g, T = 298.15 K) of benzene and toluene (respectively, (82.9 ± 0.9 and 50.1 ± 1.1) kJ Á mol À1 ) [15] and anisole (À70.7 ± 1.4 kJ Á mol À1 ) [14], the enthalpies of formation of all three methylanisoles were calculated (table 6).…”

“…They include uncertainties from the transpiration experimental conditions, uncertainties of vapour pressure, and uncertainties from temperature adjustment to T = 298.15 K. In order to assess consistency of the vaporisation enthalpies derived in this work for methylanisoles, we applied a simple group-additivity method for estimation of D g l H m (T = 298.15 K) of substituted benzenes [13]. In short, the difference between D g l H m (T = 298.15 K) of anisole (methoxybenzene) (46.41 ± 0.26) kJ Á mol À1 [14] and benzene (33.92 ± 0.06) kJ Á mol À1 [15], provides the increment DH(H ? OCH 3 ) = (12.5 ± 0.3) kJ Á mol À1 for substitution of an H atom by the methoxy-group in the benzene ring.…”