Elsevier

Fluid Phase Equilibria

Volume 397, 15 July 2015, Pages 131-140
Fluid Phase Equilibria

Isothermal vapor–liquid equilibria and excess molar enthalpy of 2-methylpyrazine (2MP) containing binary mixtures. Comparison with DISQUAC predictions

https://doi.org/10.1016/j.fluid.2015.03.033Get rights and content

Abstract

The vapour pressures of binary mixtures 2-methylpyridine with cyclohexane, n-heptane or toluene were measured by a static method in the range from 263.15 to 353.15 K. The pure components vapour pressures data and those of the mixtures were correlated with the Antoine equation. The excess enthalpies were measured at 303.15 K, by means of an isothermal calorimeter (C80 SETARAM model). The molar excess Gibbs energies, calculated from the vapour–liquid equilibrium data and the molar excess enthalpies compared satisfactorily with group contribution model (DISQUAC).

Introduction

In our previous papers, we have studied the thermodynamic properties of binary mixtures 2,6- or 3,5-dimethylpyridine with alkanes [1], toluene [2] and cyclohexane [3]. Results were successfully interpreted by using the quasi-lattice group contribution theory (DISQUAC) [4] confirming the presence of intermolecular effects in tertiary heterocyclic amines.

Pyrazines and its derivatives form an important class of compounds which are present in many natural flavors and complex organic molecules. Thus, the 2-methylpyrazine is used in food flavors and baccy. This molecule which is an intermediate for aldinamide and hydragogue has several commercial applications, such as in the synthesis of antituberculosis drug [5], [6].

Despite their structural and electronic similarity to benzene and pyridine, few experimental thermodynamic studies are reported in the open literature for pyrazines and derivatves. Experimental data on 2-methylpyrazine are scarce, they are limited to excess volume and the vapour–liquid equilibria at T = 353.15 K, for binary or ternary systems [6], [7], [8].

From a theoretical point of view, pyrazines with two nitrogen atoms in their ring are particularly interesting molecules for testing group-contribution models.

The purpose of the present work is to investigate VLE and excess enthalpy HE in binary mixtures containing 2-methylpyrazine + n-heptane, + cyclohexane and + toluene. The experimental data were described using the DISQUAC model [4].

Section snippets

Materials

The chemicals used in this study were 2-methylpyrazine, toluene, n-heptane and cyclohexane. The suppliers, formulas and purities of the pure components are shown in Table 1. All liquids were used as received and thoroughly degassed before measurements. Gas chromatography (GC) analysis of the chemicals revealed no significant impurities

Apparatus and procedure

The experimental HE data were measured, at atmospheric pressure, by means of a microcalorimeter, C80 (SETARAM model, Lyon, France). The temperature T was

Results

Experimental molar excess enthalpies HE at 303.15 K are endothermic. They are reported in Table 2. The results have been fitted to the Redlich–Kister type equation:HE=x1(1x1)i1nai(2x11)i1The values of the coefficients ai and the standard deviations δ(HE), given by:(HE)=[i1n(HEHexpE)2(Nn)]1/2were determined by least-squares analysis and are reported in Table 3. N is the number of experimental points and n the number of coefficients ai.

The pure components vapour pressures data used in

Theory

The thermodynamic excess functions of organic liquid mixtures depend on the chemical structures, the sizes and the shapes of the constituent molecules. It has been suggested that additional contributions arise in systems containing molecules of anisotropic shape (orientational order) or of different degrees of internal motion (conformational effects) [16].

In the framework of DISQUAC, mixtures of 2-methylpyrazine with an organic solvent are regarded as possessing four types of surface: (i) type

2-methylpyrazine + n-heptane system

The mixtures with n-alkane imply three types of contacts: (a, b), (b, n) and (a, n).

The interchange parameters for the (a, b) and (b, n) contacts were taken from the literature [20]. Thus, only the DIS and QUAC interaction parameters of the (a, n) contacts must be fitted to VLE and HEm data of the investigated solutions [this work].

2-methylpyrazine + toluene system

In order to reduce the number of interaction parameters, no distinction was made between the aromatic ring in 2-methylpyrazine and toluene.

From structural

Comparison with experiment and discussion

Fig. 1 shows that our experimental values, ​​at 353.15 K, are in good agreement with those of the literature [7].

Fig. 2 shows that HE and GE are positive. TSE values are relatively important for the two binary mixtures containing n-heptane or cyclohexane and GE is temperature dependent. For the third one (with toluene), the molar excess enthalpies HE and the excess Gibbs energy GE are small and the curves are skewed towards the 2-methylpyrazine low concentrations. These smaller excess properties

Conclusions

The new measurements have been made of vapor–liquid equilibria, at different temperature, for systems containing 2-methylpyrazine + n-heptane, + cyclohexane and + toluene. Excess molar enthalpies of the same binary systems were measured by means of a microcalorimeter, at 306.15 K.

The three mixtures have been investigated in terms of the DISQUAC model. Some discrepancies between experimental and calculated values were obtained for the 2-methylpyrazine + toluene system.

The difficulties in representation

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      Comparison of our low-temperature vapour pressures for methyl-pyrazine with this data is hardly possible due to significantly difference in the temperature intervals. For methyl-pyrazine, Sakoguchi et al. [20] reported vapour pressures measured by flow method and Ben-Makhlouf-Hakem et al. [21] reported vapour pressures measured by the static method. Unfortunately, the agreement of the available vapour pressures is poor (see Fig. S1 and Fig. S2), but our data more or less reconcile the previous results.

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