Fractional crystallisation distillation

Fractional crystallisation is occasionally employed. The mixture is dissolved in a suitable solvent, the whole frozen, and then allowed to melt slowly in a centrifuge in order that the successive fractions may be removed as they are formed. The various melts are then fractionally distilled. If necessary, the fractional crystallisation may be repeated. 

Commercial benzene may contain thiophene C H S, b.p. 84°, which cannot be separated by distillation or by fractional crystallisation. The presence of thiophene may be detected by shaking 3 ml. of benzene with a solution of 10 mg. of isatin in 10 ml. of concentrated sulphuric acid and allowing the mixture to stand for a short time a bluish-green colouration is produced if thiophene is present. The thiophene may be removed from benzene by any of the following methods —... 

Vinylacetic acid. Place 134 g. (161 ml.) of allyl cyanide (3) and 200 ml. of concentrated hydrochloric acid in a 1-htre round-bottomed flask attached to a reflux condenser. Warm the mixture cautiously with a small flame and shake from time to time. After 7-10 minutes, a vigorous reaction sets in and the mixture refluxes remove the flame and cool the flask, if necessary, in cold water. Ammonium chloride crystallises out. When the reaction subsides, reflux the mixture for 15 minutes. Then add 200 ml. of water, cool and separate the upper layer of acid. Extract the aqueous layer with three 100 ml. portions of ether. Combine the acid and the ether extracts, and remove the ether under atmospheric pressure in a 250 ml. Claisen flask with fractionating side arm (compare Fig. II, 13, 4) continue the heating on a water bath until the temperature of the vapour reaches 70°. Allow the apparatus to cool and distil under diminished pressure (compare Fig. II, 20, 1) , collect the fraction (a) distilling up to 71°/14 mm. and (6) at 72-74°/14 mm. (chiefly at 72 5°/ 14 mm.). A dark residue (about 10 ml.) and some white sohd ( crotonio acid) remains in the flask. Fraction (6) weighs 100 g. and is analytically pure vinylacetic acid. Fraction (a) weighs about 50 g. and separates into two layers remove the water layer, dry with anhydrous sodium sulphate and distil from a 50 ml. Claisen flask with fractionating side arm a further 15 g. of reasonably pure acid, b.p. 69-70°/12 mm., is obtained. 

The mixed monocyclic aromatics are called BTX as an abbreviation for ben2ene, toluene, and xylene (see Btxprocessing). The benzene and toluene are isolated by distillation, and the isomers of the xylene are separated by superfractionation, fractional crystallisation, or adsorption (see Xylenes and ethylbenzene). Bensene is the starting material for styrene (qv), phenol (qv), and a number of fibers and plastics. Toluene (qv) is used to make a number of... 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

Difficult to distil because it is a solid at ambient temperatures. Purified by fractional crystallisation and sublimation. 

Serine.—Its ester is contained in the fractions which distil between ioo° and 130° at O 5 mm. The mixed esters contained in this fraction are treated with a small quantity of water and then with five to six volumes of petroleum ether, which precipitates serine ester as an oil the oil is then shaken up with petroleum ether to remove admixtures as far as possible and is hydrolysed with baryta water. On removal of the baryta it crystallises when the solution is concentrated, and it is purified by treatment with alcohol, which dissolves other substances which are also present, and recrystallisation from water. Its )8-naph-thalene sulpho-derivative. 

Nitro compounds. Aliphatic nitro compounds are acidic. They are freed from alcohols or alkyl halides by standing for a day with concentrated sulphuric acid, then washed with water, dried with magnesium sulphate followed by calcium sulphate and distilled. The principal impurities are isomeric or homologous nitro compounds. In cases where the nitro compound was originally prepared by vapour phase nitration of the aliphatic hydrocarbon, fractional distillation should separate the nitro compound from the corresponding hydrocarbon. Fractional crystallisation is more effective than fractional distillation if the melting point of the compound is not too low. 

Dimethyl-l-propanol (neopentyl alcohol) [75-84-3] M 88.2, m 52°, b 113.1°/760mm. Difficult to distil because it is a solid at ambient temperatures. Purified by fractional crystallisation and sublimation. 

Even chlorine alone will act on sulphuric acid, giving chloro-sulphonie acid,7 as also will hydrogen chloride on sulphur trioxide 8 or on a mixture of sulphuric acid and phosphorus pentoxide.9 Indeed, the interaction of hydrogen chloride and sulphur trioxide in the presence of sulphuric acid provides the most convenient method for preparing chlorosulphonic acid, the product being purified by distillation in an atmosphere of hydrogen chloride, followed by fractional crystallisation 10... 

These preparations should be prefaced by exercises in fractional crystallisation and fractional distillation (pp. 13, 22, 24). 

Before use diiodomethane is fractionally distilled at 5 mmHg, fractionally crystallised by partial freezing and stored over mercury. 

When the separation procedures described in detail above are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Section 2.26) or a mixture of non-volatile solids may frequently be separated by making use of the differences in solubilities in inert solvents. The progress of such separations may be monitored by application of the various chromatographic techniques detailed in Section 2.31, or indeed these techniques may be employed on the preparative scale for effecting the separation itself (e.g. flash chromatography, p. 217). The techniques of counter current distribution, fractional crystallisation or fractional sublimation (Section 2.21) may also be employed where appropriate. 

Phosphonitrilechloride trimer with an impurity of tetramer is a ciystal-line product (the melting point is. 108-114 °C) with a light odour. It has a slight irritating effect on mucuous membranes. Trimer and tetramer can be separated by fractional vacuum distillation, fractional crystallisation or vacuum sublimation. 

Fractional crystallisation, II. Fractional distillation, 23. Friedel-Crafts Reaction, 29a. 

Reflux it with K2CO3, CaH2, CaO or sodium, then fractionally distil. The near-dry alcohol is furthCT dried by refluxing with Mg activated with iodine, as described for ethanol. Further purification is possible using fractional crystallisation and zone refining at<-10" or preparative gas chromatography. Beilstein 1IV 1668.]... 

Chloroacetic acid [79-11-8] M 94.5, m 62.8°, b 189", pK s 2.87. Crystallise the acid from CHCI3, CCI4, benzene or water. Dry it over P2O5 or cone H2SO4 in a vacuum desiccator. Further purification is by distillation from MgS04, and by fractional crystallisation from the melt. Store it under vacurnn or under dry N2. [Bemasconi et al. J Am Chem Soc 107 3621 1985, Beilstein 2 IV 474.]... 

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