Purification, via derivative

Relatively few derivatives of organic substances are suitable for use as aids to purificatiom This is because of the difficulty in regenerating the starting material. For this reason, we list below the eommon methods of preparation of derivatives that can be used in this way. 

Whether or not any of these derivatives is likely to be satisfactory for the use of aity particular case will depend on the degree of difference in properties, such as solubility, volatility or melting point, between the starting material, its derivative and likely impurities, as well as on the ease with which the substanee can be reeovered. Purification via a derivative is likely to be of most use when the quantity of pure material that is required is not too large. Where large quantities (for example, more than 50g) are available, it is rrsually more economieal to purify the material directly (for example, in distillations and recrystallisations). 

The most generally useful purifications via derivatives are as follows. 

Hydrolysis of the ester is achieved by refluxing in aqueous N or 2N NaOH solution until the insoluble ester dissolves. The solution is then cooled, and the alcohol is extracted into a suitable solvent, e.g. ether, toluene or alcohol-free chloroform. The extract is dried (CaS04, MgS04) and distilled, then fractionally distilled if hquid or reciystalhsed if sohd. (The / -nitrobenzoic acid can be recovered by acidification of the aqueous layer.) In most cases where the alcohol to be pmified can be readily extracted from ethanol, the hydrolysis of the ester is best achieved with N or 2N ethanolic NaOH or 85% aqueous ethanolic N NaOH. The former is prepared by dissolving the necessary alkali in a minimmn volmne of water and diluting with absolute alcohol. The ethanolic solution is refluxed for one to two horns and hydrolysis is complete when an aliquot gives a clear solution on dilution with four or five times its volmne of water. The birlk of the ethanol is stilled off and the residue is extracted as above. Alternatively, use can be made of ester formation with benzoic acid, toluic acid or 3,5-dinitrobenzoic acid, by the above method. 

Other derivatives can be prepared by reaction of the alcohol with an acid anhydride. For example, phthalic or 3-nitrophthalic anhydride (1 mol) and the alcohol (Imol) are refluxed for half to one hour in a non-hydroxylic solvent, e.g. toluene or alcohol-free chloroform, and then cooled. The phthalate ester crystallises out, is precipitated by the addition of low boiling petrolemn ether or is isolated by evaporation of the solvent. It is recrystallised from water, 50% aqueous etlmol, toluene or low boiling petroleum ether. Such an ester has a characteristic melting point and the alcohol can be recovered by acid or alkaline hydrolysis. 

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