Recrystallisation mixed solvents

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.]... 

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

The experimental details for recrystallisation from mixed solvents (or solvent pairs) will be evident from the account already given the best proportions of the two solvents are determined by preliminary small-scale experiments. 

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. 

The technique of recrystallisation from mixed solvents is as follows ... 

Purify the product by flash chromatography on silica gel using a mixed solvent system of 30% ethyl acetate/petroleum ether (40-60). The product is the first major yellow/orange band to be eluted and has an Rf of approximately 0.38. Combine those fractions containing only the desired product and evaporate the solvent to yield 1.16 g of an oil that crystallises on standing. Recrystallise from ethanol to yield pure (S)-A/,A/-dimethyl-1-[(ff)-1, 2-bis(diphenylphosphino)ferrocenyl]ethylamine 23 displaying the appropriate spectroscopic and rotational data. Literature m.p. = 139-140°C. ... 

Beckmann rearrangement of benzophenone oxime to benz-anilide. Dissolve 2 g. of benzophenone oxime in 20 ml. of anhydrous ether in a small conical flask and add 3 g. of powdered phosphorus pentachloride (or 3 ml. of pure tbionyl chloride). Distil off the solvent and other volatile products on a water bath CAUTION ether), add 25 ml. of water, boil for several minutes and break up any lumps which may be formed. Decant the supernatant liquid, and recrystallise, in the same vessel, from boiling alcohol. The product is benzanilide, m.p. 163° confirm this by a mixed m.p. determination with an authentic specimen. 

Free cydohexene from peroxides by treating it with a saturated solution of sodium bisulphite, separate, dry and distil collect the fraction, b.p. 81-83°. Mix 8 -2 g. of cycZohexene with 55 ml. of the reagent, add a solution of 15 mg. of osmium tetroxide in anhydrous butyl alcohol and cool the mixture to 0°. Allow to stand overnight, by which time the initial orange colouration will have disappeared. Remove the solvent and unused cydohexene by distillation at atmospheric pressure and fractionate the residue under reduced pressure. Collect the fraction of b.p. 120-140°/15 mm. this solidifies almost immediately. Recrystallise from ethyl acetate The yield of pure cis-l 2 cydohexanediol, m.p. 96°, is 5 0 g. 

Schemes for recrystallisation from melts are similar to those for solutions, although a solvent is not normally added. Usually, simple sequences of heating (melting) and cooling (partial crystallisation) are followed by separation of the purified crystals from the residual melt. Selected melt fractions may be mixed at intervals according to the type of scheme employed, and fresh feed-stock may be added at different stages if necessary. As Bailey1114 reports, several such schemes have been proposed for purification of fats and waxes.

The pure recrystallised solid product collects on the filter paper. The filtrate contains a solution of soluble impurities. The solid product should now be washed with a small volume of cold solvent. This removes any soluble impurities that are mixed in with the pure solid. 

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