Experimental standard molar enthalpies of formation of some methylbenzenediol isomers
Introduction
The interest in the study of compounds with antioxidant properties has exponentially grown in the last decade. Among these compounds, special interest has been dedicated to phenolic type compounds. Our Research Group has been carrying out an extensive work on the energetics of substituted phenols for which we already reported our studies on alkylsubstituted phenols [1], [2], aminophenols [3], [4], 3-nitrophenol [5], fluorophenols [6], chlorophenols [7], [8], bromophenols [9], cyanophenol [10], thiophenols [10], [11], and alkylsubstituted catechols (1,2-benzenediol) [12]. In the present work we extend our studies to the methyl isomers of resorcinol (1,3-benzenodiol) and of hydroquinone (1,4-benzenodiol).
The resorcinol can be obtained by fusing many resins (galbanum, asafoetida, etc.) with potassium hydroxide [13], or it may be prepared synthetically from, for example, the fusion of many ortho- and para-compounds of the aromatic series with potassium hydroxide or through other specific reactions [13].
The resorcinol and its derivatives have many medical applications ranging from external use as an antiseptic and disinfectant in the treatment of chronic skin diseases such as acne, psoriasis, and eczema [14] to the internal use under pills format for treatment of gastric ulcers, or as analgesic and haemostatic. In large doses it is a poison causing giddiness, deafness, salivation, sweating, and convulsions [13].
Resorcinol is an important chemical intermediate in speciality chemicals manufacturing, such as light screen agents used to protect plastics from exposure top UV light, being also used on the manufacture of dyestuffs, pharmaceuticals, flame retardants, agriculture chemicals, fungicidal creams, explosive primers, antioxidants, and a colouring agent for certain chromatography experiments [13], [15].
The presents paper reports the thermochemical study of the 2-methylresorcinol (2-methyl-1,3-benzenediol), 4-methylresorcinol (4-methyl-1,3-benzenediol), 5-methylresorcinol (5-methyl-1,3-benzenediol), and methylhydroquinone (2-methyl-1,4-benzenediol) (figure 1). The unsubstituted 1,2-, 1,3-, and 1,4-benzenediols are historically and trivially known as catechol, resorcinol, and hydroquinone, respectively. In this work, those compounds and their substituted derivatives will be named using the latter nomenclature.
In a previous article [1] we reported the standard molar enthalpies of formation in the gas-phase of 3-methylcatechol and 4-methylcatechol as {−(299.3 ± 1.6) and −(298.4 ± 1.6)} kJ · mol−1, respectively, as well as values of gas-phase enthalpies of formation for 3-isopropylcatechol, 4-terbutylcatechol, 3-methyl-6-isopropylcatechol, and 3,5-diterbutylcatechol. In the literature, it was also found a study concerning one methyl derivative, the crystalline methylhydroquinone, for which the standard molar enthalpy of combustion in the condensed phase has been determined as −3492 kJ · mol−1 [16]. No other thermochemical data have been found for the others methyl-benzenediol derivatives.
The standard () molar energies of combustion, in oxygen, at T = 298.15 K, of the four methyl-benzenediols studied in the present work were obtained by static-bomb combustion calorimetry, while the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were derived from the temperature variation of the vapour pressures of the crystalline samples measured at several temperatures, using the Knudsen mass-loss effusion technique, with values of the heat capacity differences between the gaseous and the crystal phases for each compound estimated by group contribution values. The derived standard molar enthalpies of formation in the gaseous state are interpreted in terms of structural contributions to the energetics of the substituted benzenediols and compared with the same parameters estimated from the Cox Scheme [17].
Section snippets
Materials and purity control
The 2-methylresorcinol, CAS [608-25-3], 4-methylresorcinol, CAS [496-73-1], 5-methylresorcinol, CAS [504-15-4], and methylhydroquinone, CAS [95-71-6], were obtained from Aldrich Chemical Co. with the assessed minimum purity of 0.97 to 0.99 (mass fraction). They were purified by repeated vacuum sublimations at 0.1 Pa background pressure, and their final purity was checked by gas chromatography, performed on an Agilent 4890D Gas Chromatograph equipped with an HP-5 column, cross-linked, 5% diphenyl
Experimental enthalpies of formation
Detailed results for each combustion experiment performed for 2-, 4-, and 5-methylresorcinol and methylhydroquinone are given in the Supporting Information (table S2 to S5). Table 1 reports the results for one typical combustion experiment of each studied compound, where Δm(H2O) is the deviation of the mass of water added to the calorimeter from the mass assigned to ε(calor): 3119.6 g, ΔUΣ is the energy correction to the standard state and the remaining terms are as previously defined [26], [27]
Conclusions
The standard gas-phase molar enthalpies of formation, at T = 298.15 K, of 2-methylresorcinol, 3-methylresorcinol, 4-methylresorcinol, and methylhydroquinone have been obtained, both by experimental techniques and by the empirical methodology developed by Cox. The experimental enthalpies of formation and of sublimation were indirectly obtained from, respectively, static-bomb combustion calorimetry and mass-loss Knudsen-effusion technique experiments. The enthalpies of formation estimated by the Cox
Acknowledgments
Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Química, University of Porto. A.I.M.C.L.F. thanks FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of the post-doctoral fellowship (SFRH/BPD/27053/2006).
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