Thermodynamic properties of tert-butylbenzene and 1,4-di-tert-butylbenzene

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Abstract

Heat capacities, enthalpies of phase transitions, and derived thermodynamic properties over the temperature range 5 < (T/K) < 442 were determined with adiabatic calorimetry for tert-butylbenzene (TBB) {Chemical Abstracts Service registry number (CASRN) [98-06-6]} and 1,4-di-tert-butylbenzene (DTBB) {CASRN [1012-72-2]}. A crystal to plastic crystal transition very near the triple-point temperature of DTBB was observed. New vapor pressures near the triple-point temperature are also reported for DTBB for the liquid and crystal states. These new measurements, when combined with published results, allow calculation of the thermodynamic properties for the ideal gas state for both compounds. The contribution of the tert-butyl group to the entropy of the ideal gas is determined quantitatively here for the first time based on the calorimetric results over the temperature range 298.15 < (T/K) < 600. Comparisons with literature values are shown for all measured and derived properties, including entropies for the ideal gas derived from quantum chemical calculations.

Introduction

Tert-butyl benzenes are manufactured and used on a large scale worldwide. In 1999, the Gesellschaft Deutscher Chemiker (German Chemical Society) estimated that more than 2 · 105 kg of tert-butylbenzene alone were released through emissions from hydrocarbon fuels, manufacturing waste, and processing of fragrances [1]. Member companies of the Design Institute for Physical Properties (DIPPR®) [2] selected tert-butylbenzene and 1,4-di-tert-butylbenzene for property studies [3], [4] over a wide range of temperatures for development of general correlations needed for modeling the behavior of these important compounds. Enthalpies of formation for both compounds in the condensed state have been determined [5], [6]; however, before the present work, entropies for the condensed phases were unavailable, making derivation of Gibbs free energies of formation impossible for any phase.

Recently, properties in the ideal gas state for a series of tert-butyl aromatics and alkyl-tert-butyl aromatics were critically evaluated for the NIST WebThermo Tables (WTT) [7], [8] based on statistical calculations with computed molecular structures, rotational barriers, and fundamental vibrational frequencies, together with group-contributions methods [9]. The measurements reported here, in combination with property values reported in the literature, provide necessary validation for the computed values.

Heat capacities, enthalpies of phase transitions, and derived thermodynamic functions are reported here for tert-butylbenzene and 1,4-di-tert-butylbenzene (Chemical Abstracts registry numbers [98-06-6] and [1012-72-2], respectively) over the temperature range 5 < (T/K) < 442. New vapor pressure measurements near the triple-point temperature are also reported for 1,4-di-tert-butylbenzene for the liquid and crystal states. These new results are combined with previously published [3], [4] vapor pressures, densities, critical properties, and high-temperature heat capacities for the liquid phase to derive thermodynamic properties for the ideal gas state for both compounds. The contribution of the tert-butyl group to the ideal gas entropies is determined quantitatively here based on the calorimetric results for the temperature range 298.15 < (T/K) < 600. Experimental values will be shown to be in good accord with those computed for the NIST WebThermo Tables. Comparisons of the new experimental property results with literature values are also shown.

Section snippets

Materials

Details of the sample origin and purification for tert-butylbenzene (TBB) [4] and 1,4-di-tert-butylbenzene (DTBB) [3] were described previously. Compounds were purchased from the Aldrich Chemical Company and were purified by spinning-band distillations. Gas + liquid chromatographic analyses showed mole fraction purities in excess of 0.995. Purities determined with fractional melting studies (described later) were mole fraction 0.9994 for TBB and 0.9999 for DTBB.

Physical constants and standards

Values of molar masses are reported

Adiabatic heat-capacity calorimetry for tert-butylbenzene (tbb)

Measurements of enthalpy increments and derived heat capacities for TBB were made between the temperatures 5 K and 442 K, and included determination of the triple-point temperature Ttp and the enthalpy of fusion. Crystallization of TBB was initiated by slow cooling (approximately 1 mK · s−1) of the liquid sample. The sample supercooled approximately 20 K before crystallization began. Complete crystallization was ensured by maintaining the sample under adiabatic conditions in the partially melted

Comparisons with literature properties for phase transitions

Normal melting temperatures for TBB and DTBB have been reported many times in the literature, usually for chemical identification. These values are in accord with those of this research, but with much larger uncertainties than those reported here. The enthalpy of fusion for TBB was reported by Huffman et al. [27] ΔcrlHm=8.395kJ·mol-1 in 1930 with an early adiabatic calorimeter and by Timmermans [28] (ΔcrlHm=8.37kJ·mol-1). The value reported by Huffman et al. [27] is in remarkably good agreement

Acknowledgements

We acknowledge the contributions of An (Andy) Nguyen for the vapor-pressure measurements, and Aaron P. Rau for vapor-transfer of the samples prior to the property measurements. The authors thank Dr. Ala Bazyleva (Belarusian State University, Minsk, Belarus) for providing details of the quantum chemical and statistical calculations, as well as for additional statistical calculations used in this article for the tert-butylbenzenes. The authors acknowledge the financial support of the Office of

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      For instance, comparing the heat capacity (values at T = 298.15 K) of the solid phases of benzene (considering an estimation for a hypothetical solid-state at 298.15 K since the melting temperature of benzene is ≈ 279 K) [37] and 1,4–di–tert–butylbenzene [46], the increment per each t-Bu group added to benzene was estimated as 109 J·K−1·mol−1; as another example, the increment per each t-Bu group added to the biphenyl was estimated as 107 J·K−1·mol−1 [47–49]. For the liquid phase, various experimental determinations can be used to derive the contribution of the t-Bu group for the heat capacity (values in J·K−1·mol−1) at T = 298.15 K: a) tert-butylbenzene [51] vs. benzene [50] – increment of 102; b) 1,4–di–tert–butylbenzene [46] vs. benzene [50] – increment per t-Bu group of 106; c) 2–methoxy-2-methylpropane [53] vs. methanol [52] – increment of 107; d) 2–ethoxy–2–methylpropane [55] vs. ethanol [54] – increment of 106; e) tert-butyl acetate [57] vs. acetic acid [56] – increment of 108. These data reveal that the contribution of the tert-butyl group to the heat capacity is similar for different systems.

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