Measurement and correlation of solubility of 4-chloro-2,5-dimethoxynitrobenzene and 4-chloro-2,5-dimethoxyaniline in methanol, ethanol, xylene and toluene
Introduction
Compared to inorganic dyes, organic dyes have bright colours and powerful tinting strength. They can also play a protective role for products because of their unique structural features. According to their structures, organic dyes can be classified as azo, phthalocyanine, quinolone, etc. [1], [2]. The compound 4-chloro-2,5-dimethoxyaniline (CDMA, CAS No. 6358-64-1) is an important intermediate of azo dyes. It can be prepared for the Permanent Yellow HR (Colour Index No. CI Pigment Yellow 83 (2208)) and the Permanent pink FBB-B (Colour Index No. CI Pigment Red 146 (12485)) by diazotization reaction. Both dyes have a wide range of application in industry and they can be used for the colouration of superior ink, plastic, rubber, etc. with extremely good performance [3].
In the current industry production process of CDMA, hydroquinone is the main starting material, which is alkylated with dimethyl sulfate under alkaline conditions to prepare 1,4-dimethoxybenzene. Then 1,4-dimethoxybenzene is dissolved in 1,2-dichloroethane, reacts with sulfuryl chloride to prepare 2,5-dimethoxy-4-chlorobenzene [4], which is nitrated with dilute nitric acid to obtain 4-chloro-2,5-dimethoxynitrobenzene (CDMB, CAS No. 6940-53-0). Finally CDMB is reduced to obtain CDMA (the process is shown in figure 1) [5], [6].
Although the process of iron powder reduction CDMB is proven in industry, its energy consumption is high and the process is cumbersome. The process causes serious pollution to the environment due to the large amounts of iron sludge produced. With the increasingly stringent environmental requirements, the iron powder reduction process is being phased out gradually, and replaced by the clean, efficient and low-powered hydrogenation process [7], [8]. As can be seen from a previous paper [8], many organic reagents can be used as the solvent for the hydrogenation reaction, such as methanol, ethanol, toluene, xylene, etc. Meanwhile the solubility of CDMA in these solvents affects the separation and purification of the CDMA. However, no report about the solubility of CDMB and CDMA in these solvents could be found.
In this work, the solubility of CDMB and CDMA in methanol, ethanol, toluene and xylene has been measured by a dynamic method over the temperatures range from (278 to 338) K and ambient pressure. The Wilson and the NRTL models, which have been successfully used to describe the (solid + liquid) equilibrium (SLE) [9], [10], [11], [12], [13], [14], were employed to correlate the experimental results. The interaction parameters of the models were obtained. According to the solubility values obtained, the preferred solvent for the hydrogenation reaction of CDMB is suggested.
Section snippets
Experimental materials
The purchased CDMB was kept in the oven at T = 353.15 K to dry to constant weight before using. Since CDMA is easily oxidised in air, the sample of CDMA was dried in vacuum to a constant weight at temperature 353.15 K. The purities of CDMA and CDMB were determined by HPLC analysis. Other chemical reagents were used without further purification. The detailed information of the materials used in the experiment is listed in table 1.
Apparatus and procedure
The solubility was determined by the dynamic method [15]. The
(Solid + liquid) equilibrium equation
According to the thermodynamic description of SLE, the solubility equation is based on the equality of chemical potentials between components in all the coexisting phases. The SLE behaviour can be calculated by the following equation which involves the properties of the pure solute, such as enthalpy of fusion, melting point, etc. [9].
Here x denotes the mole fraction of solute in the liquid phase, γ denotes the activity
Experimental results
The melting points of CDMB and CDMA measured by DSC are Tfus = (413.66 ± 0.11) K and (391.85 ± 0.08) K, respectively. The molar enthalpies of fusion of CDMB and CDMA by DSC are ΔfusH = (28.02 ± 0.45) kJ · mol−1 and (26.68 ± 0.36) kJ · mol−1, respectively. The numbers after symbol ± are the standard uncertainties which are calculated according to three times measurements with a level of confidence of approximately 68%. The melting point of CDMA is in good agreement with the data reported by Boehm Walter T = (392.15
Conclusions
The solubility of CDMB and CDMA in methanol, ethanol, xylene and toluene has been determined by the dynamic method within the temperature range from (278 to 338) K. Solubility of both compounds increases with the temperature in all solvents and the greatest solubility of both systems is obtained in toluene. The solubility results have been correlated by the Wilson and the NRTL models. Both the Wilson and the NRTL models can describe the systems well over the range of temperature and
Acknowledgments
The authors thank the support by the Program of Introducing Talents of Discipline to Universities, China, No. B06006.
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