Reaction LXXII.—Replacement of the Diazonium Group by Hydroxyl

20 gms. (1 mol.) of freshly distilled aniline are dissolved by gentle warming in 150 gms. of 30% sulphuric acid, and the cooled liquid treated with a 20% solution of sodium nitrite until a sample colours starch-iodide paper (see p. 504) some 16 gms. of NaN02 will be required. The whole 

If the residue in the flask after the steam distillation is filtered hot, and cooled, crystals of p-hydroxydiphenyl separate. It is formed by the coupling of a portion of the phenol first formed with undecomposed diazonium compound. 

S04H + H.C6H4OH = C6H5.C8H4OH + H2S04 + N2. 

The yield is low owing to the heating with the mineral acid tending to cause resipificatispi of the phenol. It has been suggested that heating with a weak acid, e.y., boric acid, would prevent this, but no great improvement in the yield is obtained. 

In an exactly similar manner from 20 gms. of o-, m- or p-toluidine, o-, m- or p-cresol respectively is prepared for these it is better to use only 10% sulphuric acid, a quantity being taken equivalent to the 30% acid employed above. The yields are about 50% (12-5 gms.). 

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