Sulphones, purification

The crude product contains isomers other than that required and also nitrated phenolic compounds resulting from side reactions. The usual method of purification is to treat the crude product with sodium sulphite, which converts asymmetric trinitro compounds to sulphonic acid derivatives, and to wash out the resulting soluble products with alkaline water. The purity of the product is determined by the melting point, the minimum value for Grade I TNT commonly being 80-2°C. Unless adequate purity is achieved, slow exudation of impurities can occur during storage and the TNT then becomes insensitive. 

In all the above sequences, single isomers are produced by careful control of the reaction conditions combined with purification by selective isolation at various points in the synthesis. Occasionally two isomers are produced which give dyestuffs that have very similar properties in these cases it is often quite acceptable and economically beneficial not to separate the individual components but to use the total mixture in dye preparation. An example is the mixture of l-naphthylamine-6- and 7-sulphonic acids (4.46 mixed Cleve s acids), which arises by nitration and reduction of naphthalene-2 Sulphonic acid (Scheme 4.30). 

Various liquid chromatographic techniques have been frequently employed for the purification of commercial dyes for theoretical studies or for the exact determination of their toxicity and environmental pollution capacity. Thus, several sulphonated azo dyes were purified by using reversed-phase preparative HPLC. The chemical strctures, colour index names and numbers, and molecular masses of the sulphonated azo dyes included in the experiments are listed in Fig. 3.114. In order to determine the non-sulphonated azo dyes impurities, commercial dye samples were extracted with hexane, chloroform and ethyl acetate. Colourization of the organic phase indicated impurities. TLC carried out on silica and ODS stationary phases was also applied to control impurities. Mobile phases were composed of methanol, chloroform, acetone, ACN, 2-propanol, water and 0.1 M sodium sulphate depending on the type of stationary phase. Two ODS columns were employed for the analytical separation of dyes. The parameters of the columns were 150 X 3.9 mm i.d. particle size 4 /jm and 250 X 4.6 mm i.d. particle size 5 //m. Mobile phases consisted of methanol and 0.05 M aqueous ammonium acetate in various volume ratios. The flow rate was 0.9 ml/min and dyes were detected at 254 nm. Preparative separations were carried out in an ODS column (250 X 21.2 mm i.d.) using a flow rate of 13.5 ml/min. The composition of the mobile phases employed for the analytical and preparative separation of dyes is compiled in Table 3.33. 

It was further found that the methods separate well the dyes and impurities, as demonstrated on the preparative chromatographic profile of Reactive orange 16 in Fig. 3.115. It was concluded from the results that the method can be used in the future for the purification and analysis of a wide variety of sulphonated azo dyes [171],... 

M. Chen, D. Moir, F.M. Benoit and C. Kubwabo, Purification and identification of several sulphonated azo dyes using reversed-phase preparative high-performance liquid chromatography. J. Chromatogr.A, 825 (1998) 37-44. 

Dimethylphenol [108-68-9] M 122.2, m 68 , b 219 . Heated with an equal weight of cone H2SO4 at 103-105° for 2-3h, then diluted with four volumes of water, refluxed for Ih, and either steam distd or extracted repeatedly with ethyl ether after cooling to room temperature. The steam distillate was also extracted and evaporated to dryness. (The purification process depends on the much slower sulphonation of 3,5-dimethylphenol than most of its likely contaminants.) [Kester lEC 24 770 1932]. It can also be crystd from water, hexane or pet ether, and vacuum sublimed. [Bemasconi and Paschalis JACS 108 1986]. 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

Unless the reagent has been recently purchased it may contain substantial amounts of toluene-p-sulphonic acid. The most satisfactory procedure for the purification of the chloride involves dissolving it in the minimum amount of chloroform (about 2.5 ml per g) and diluting with 5 volumes of light petroleum (b.p. 40-60 °C), which precipitates impurities. The filtered solution is treated with decolourising charcoal, filtered and concentrated to small volume when colourless crystals of the pure reagent, m.p. 68 °C, are obtained these should be washed with chilled light petroleum (b.p. 40-60 °C). 

ZnTPP (Hambright, Washington D.C.), duroquinone (DQ) (Aldrich) and methyl viologen (MV+2) (BDH CHEMICALS) were used without further purification. The sodium salt of anthraquinone 2-sulphonate (AQS) (BDH CHEMICALS) was recrystalized from methanol. Benzyldimethyl-n-hexadecylammonium chloride (BHDC) (BDH CHEMICALS) was dried over P205 and stored under vacuum. The solvent, benzene (pro-analysis) (BDH CHEMICALS) was dried and distilled over sodium wire. Stock solutions of reversed micelles were prepared by adding dropwise the calculated amount of water to the appropriate solution of BHDC. The surfactant concentration used throughout the experiments was 0.1 mol dm 3. 

With difficult samples, for which the peaks cannot be resolved by RP-RP-HPLC, an alternative purification method such as preparative cation-exchange HPLC on a sulphonate derivatized column could be used. 

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