Solvents Used for Recrystallisation

Many solids may be purified by recrystallisation by dissolving the snbstance in a minimum quantity of hot solvent, filtering the solution, and then cooling the solntion so that crystals of the desired snbstance form while the impurities remain in solution. A list of solvents commonly used for recrystallisation is given in Table 16.2. 

In order to be useful, a solvent shonld dissolve much of the solid substance at higher temperatures and very little of it at lower temperatnres. It should not react with the compound. Solvents with a high boiling point should be avoided if possible. Impurities do not have to be more soluble in the cold solvent than the substance being pnrified. Since the impurities are present at a lower concentration, they will frequently remain in solntion even though less soluble. 

In general, polar componnds (e.g., alcohols, thiols, amines, carboxylic acids, amides) tend to dissolve in (polar) protic solvents (e.g., water, alcohols). Nonpolar compounds tend to dissolve in (nonpolar) aprotic solvents (e.g., benzene, petrol, hexane). 

Often it is possible to use a mixture of miscible solvents where the substance to be recrystallised is soluble in one of the solvents but relatively insoluble in the other. The solute can be dissolved hot in a suitable solvent mixture, which is then allowed to cool. Alternatively, the solute can be dissolved in the solvent in which it is more soluble either at elevated or at room temperature the other solvent is then added until crystallisation just begins, and the resulting mixture is cooled to further induce recrystallisation. 

Solvents Commonly Used for Recrystallisation (solvents listed in approximate order of decreasing polarity) 

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After the main filtrate has been removed, the crystals should be washed in order to remove the mother liquor which, on drying, would contaminate the crystals. The wash liquid will normally be the same solvent as was used for recrystallisation, and must be used in the smallest possible... 

Solvents commonly used for recrystallisation, and their boiling points, are given in Table 7. 

Mixed Solvents. Where a substance is too soluble in one solvent and too insoluble in another, for either to be used for recrystallisation, it is often possible (provided they are miscible) to use them as a mixed solvent. (In general, however, it is preferable to use a single solvent if this is practicable.) Table 8 contains many of the common pairs of miscible solvents. 

If a non-volatile solvent, such as glacial acetic acid, xylene, high-boiling petrol ether, or nitrobenzene, has been used for recrystallisation, it is washed out before drying by means of one which is more easily removed, e.g. ether, benzene, petrol. In general a substance which is sparingly soluble in glacial acetic acid or nitrobenzene is so... 

The extraction apparatus is used chiefly for dissolving out sparingly soluble constituents from mixtures, and for isolating natural products from (dry) vegetable or animal material. Occasionally it is very useful for recrystallising sparingly soluble substances from the extraction thimble by means of an appropriate solvent (especially ether). As a rule, crystals of the dissolved material separate already from the solution in the flask during the process of extraction. This solution soon becomes supersaturated even while hot. 

After the main filtrate has been removed for such treatment, the crystals on the filter pad should be washed to remove remaining traces of mother-liquor which, on drying, would contaminate the crystals. The wash liquid will normally be the same solvent or solvent mixture used for recrystallisation and must be used in the smallest amount compatible with efficient washing, in order to prevent appreciable loss of the solid. With the suction discontinued the crystals are treated with a small volume of the chilled solvent and cautiously stirred with a spatula or with a flattened glass rod (without loosening the filter paper) so that the solvent wets all the crystals. The suction is then applied again, and the crys-... 

Carbon disulphide should never be used if any alternative solvent is available, as it has a dangerously low flash-point, and its vapours form exceedingly explosive mixtures with air. Ether as a solvent for recrystallisation is much safer than carbon disulphide, but again should be avoided whenever possible, partly on account of the danger of fires, and partly because the filtered solution tends to creep up the walls of the containing vessel and there deposit solid matter by complete evaporation instead of preferential crystallisation. 

When the correct solvent for recrystallisation is not known a procedure similar to that given on pp. 15-16 should be followed, but on the semi-micro scale not more than 10 mg. of the solid should be placed in the tapered-end test-tube (Fig. 29(B)) and about o i ml. of the solvent should be added from the calibrated dropping-pipette (Fig. 30(B)). If the compound dissolves readily in the cold, the solvent is unsuitable, but the solution should not be discarded. [In this case recourse should be had to the use of mixed solvents (p. 18). For example if the substance is very soluble in ethanol, water should be added from a calibrated pipette with shaking to determine whether crystallisation will now take place, indicated by a cloudiness or by the separation of solid.]... 

Drying of crystals. Whenever possible crystals should be dried in a smdl vacuum desiccator containing a suitable desiccant. The latter depends upon the nature of the solvent used c/. p. 19). However for most purposes anhydrous calcium chloride is satisfactory. If a hydrocarbon has been used in the recrystallisation, a few thin fresh shavings of paraffin wax are efficacious. 

The inflammable solvents most frequently used for reaction media, extraction or recrystallisation are diethyl ether, petroleum ether (b.p. 40-60° and higher ranges), carbon disulphide, acetone, methyl and ethyl alcohols, di-Mo-propyl ether, benzene, and toluene. Special precautions must be taken in handling these (and other equivalent) solvents if the danger of Are is to be more or less completely eliminated. It is advisable to have, if possible, a special bench in the laboratory devoted entirely to the recovery or distillation of these solvents no flames are permitted on this bench. 

Choice of solvent for recrystallisation. Obtain small samples (about 0 5 g.) of the following compounds from the storeroom (i) salicylic acid, (Li) acetanilide, (iii) m-dinitrobenzene, (iv) naphthalene, and (v) p-toluene-sulphonamide. Use the following solvents distilled water, methylated spirit, rectified spirit, acetone, benzene and glacial acetic acid. 

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