Aniline, basicity from nitrobenzene

Aniline may be separated from nitrobenzene by first acidifying the steam distillate with hydrochloric acid to convert aniline to its water-soluble hydrochloride salt 14. The acidic solution is then extracted with diethyl ether to remove nitrobenzene. The aqueous layer, which contains 14, is made basic, and the aniline that is formed is isolated from the mixture by extraction with diethyl ether. 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples of p-nitrophenol. Usually, the o-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pKg values. The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers of known pH (see for example Table 19, p. 43) from a solution of the substance in a suitable solvent such as ethyl ether. This method is more satisfactory and less laborious the larger the difference between the pK value of the impurity and the desired compound. Heterocyclic nitro compounds require similar treatment to the nitroanilines. Neutral nitro compounds can be steam distilled. 

Devise a chemical procedure based on the basicity of aniline to separate it from any unreacted nitrobenzene. 

Steam distillation (Secs. 2.16 and 4.5) is a good technique for separating volatile organic compounds from nonvolatile organic and inorganic substances. However, the reaction mixture is acidic, and it must be made basic prior to performing the steam distillation so aniline is present as the free base, and the 4-aminophenol is converted to its water-soluble sodium phenoxide salt. The aniline and nitrobenzene are then removed from the reaction mixture by steam distillation. The nonvolatile salt of 4-aminophenol and the nonvolatile benzidine remain in the aqueous phase. 

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