146 Biological Chemistry. 



IV. The hydroxyl derivatives of the aromatic sub- 

 stances differ in many respects from the aliphatic hy- 

 droxyl derivatives. Whereas, in the aliphatic derivatives, 

 a halogen is readily replaceable by a hydroxyl group, this 

 is not the case in aromatic derivatives when the halogen 

 is directly united to a carbon of the benzene nucleus. 

 Ethyl bromide, C 2 H 5 Br, for example, readily yields ethyl 

 alcohol, C 2 H 5 -OH. Bromobenzene, C 6 H 5 Br, on the other 

 hand, is only attacked with very great difficulty by 

 alkalis, the bromine appearing to be very strongly com- 

 bined to the carbon atom. To obtain an aromatic hydroxyl 

 derivative in which the hydroxyl is directly combined with 

 a carbon atom of the ring, or a " phenol " as it is called, 

 other methods must be employed. The two principal are 



(1) the treatment of the corresponding amino-derivative 

 with nitrous acid (or through the diazo-derivative, see 

 p. 148) 



C 6 H 6 NH 2 + HONO = C 6 H 5 -OH + N 2 + H 2 



(2) A more general method is that of the fusion of the 

 potassium salt of the sulphonic acid with potassium 

 hydroxide 



C 6 H 5 -S0 3 K + 2KOH = K 2 S0 3 + C 6 H 5 OK + H 2 



The potassium derivative of the phenol is thereby obtained. 

 The phenols are colourless crystalline substances, the lower 

 members of which are soluble in water. The chief of the 

 phenols, C 6 H 5 -OH, is known generally as " carbolic acid." 

 As the name might indicate, it is a substance of acid 

 character, and in this respect the phenols differ markedly 

 from their aliphatic analogues the alcohols. It may be 

 recalled that the alcohols have a hydrogen replaceable by 

 sodium when they are treated with the metal, but that 

 the sodium derivative thus produced is readily decomposed 

 by water, yielding again alcohol and sodium hydroxide. 

 The phenols, on the other hand, dissolve in sodium hy- 



