2/o PRACTICAL ORGANIC AND BIO-CHEMISTRY 



TRISUBSTITUTION DERIVATIVES OF BENZENE. 



Trisubstitution derivatives of benzene exist in the three forms : 



Vicinal. Asymmetric. Symmetric. 



and the number of isomers is very large. Mention can only be made 

 of those which occur naturally, or are prepared from natural sources. 

 The trihydric phenols, 



OH OH OH 



OH 



OH 

 Pyrogallol. Hydroxyquinol. Phloroglucinol. 



Pyrogallol, or pyrogallic acid, is obtained by heating gallic acid 

 at 210 until carbon dioxide is no longer evolved: 

 C 6 H 2 (OH) 3 . COOH = C0 3 + C 6 H 3 (OH) 3 . 



It is a colourless crystalline solid, which melts at 1 1 5. It is easily 

 soluble in water, but less in alcohol and ether. It will be noticed 

 that the solubility of phenols in water increases with the number of 

 hydroxyl groups. 



Hydroxyquinol is obtained by fusing quinol with potash. It dis- 

 solves easily in water and melts at 140. 



Phloroglucinol results from the fusion of numerous resins with 

 caustic potash. It is prepared by fusing resorcinol with caustic potash. 



Phloroglucinol is a crystalline solid containing two molecules of 

 water and melting at 218. It is very soluble in CH a 



water and is also soluble in alcohol and ether. In OC f \CO 

 most respects it behaves as a trihydroxyphenol, but HzC I JcH 9 

 it forms an oxime and probably has also a ketonic co 



structure. 



Reactions. 



(1) With ferric chloride, pyrogallol gives a deep-blue coloration; 

 hydroxyquinol gives a greenish-brown colour which changes to blue 

 on adding sodium carbonate and then to red ; phloroglucinol gives 

 a blue' violet coloration. 



(2) In alkaline solution in contact with air, they absorb oxygen and 

 the solution becomes brown. Pyrogallol is consequently used for ab- 

 sorbing oxygen. 



(3) They all reduce Fehling's solution and ammoniacal silver 

 nitrate, 



