Purification of solid organic compounds by recrystallisation

The student should read Sections 11,27-11,31 where the technique of the recrystallLsation of solids and cognate processes is described. To gain experience, he should carry out the following experiments. 

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. 

Place 0-1 g. of the substance in a semicro test-tube (75 x 10 mm. or 100 X 12 mm.) and proceed systematically with the various solvents as detailed in Section 11,27. Finally, summarise your results, and indicate the most suitable solvent or solvents for the recrystallisation of each of the above compounds. 

If a m.p. determination is required soon after recrystallisation, a small quantity may be rapidly dried by pressing it several times upon a pad of several thicknesses of filter or absorbent paper and placing it upon a watch glass in a warm place. A piece of unglazed porous plate may also be used. 

Optional or alternative experiments are the recrystaUisation of 3-0 g. of crude benzoic or salicylic acid from water. 

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Amides are stable compounds. The lower-melting members (such as acetamide) can be readily purified by fractional distillation. Most amides are solids which have low solubilities in water. They can be recrystallised from large quantities of water, ethanol, ethanol/ether, aqueous ethanol, chloroform/toluene, chloroform or acetic acid. The likely impurities are the parent acids or the alkyl esters from which they have been made. The former can be removed by thorough washing with aqueous ammonia followed by recrystallisation, whereas elimination of the latter is by trituration or recrystallisation from an organic solvent. Amides can be freed from solvent or water by drying below their melting points. These purifications can also be used for sulfonamides and acid hydrazides. 

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