260 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Mandelic Acid, C 6 H 5 . CHOH . COOH. 



Mandelic acid is formed when amygdalin is hydrolysed by boiling with 

 acids ; it is present in amygdalin as the nitrile of mandelic acid : 



y O.C 12 H 2a O n OH 



C 6 H 6 . CH( + 2 H 2 = C 6 H 6 . CH< + 2C 6 H 12 O S 



\CN X CN 



,OH 

 C 6 H 5 . CH + 2H 2 O = C 6 H e . CHOH . COOH + NH 3 . 



It is prepared synthetically from benzaldehyde ; the benzaldehyde is 

 converted into the cyanohydrin and this is hydrolysed. 



Mandelic acid is a colourless crystalline solid which melts at 133. It 

 is soluble in water, ether and other organic solvents. The natural acid is 

 optically active and laevorotatory ; the synthetical acid has been separated into 

 its stereoisomers by the usual methods. 



Mandelic acid closely resembles lactic acid in its properties, as it contains 

 the OH group in the side chain. It differs from salicylic acid and other aro- 

 matic acids which contain the OH group attached to the benzene nucleus. 



DISUBSTITUTION PRODUCTS OF BENZENE. 



The disubstitution products of benzene exist in three forms :- 

 x x x 



X 



para 

 P- 



The number of derivatives is very large. They can be divided into 

 those in which the substituting groups are the same and those in which 

 they are different. 



Dimethylbenzenes or Xylenes, QH 4 : 



These three compounds are present in coal tar and are contained in the 

 benzene fraction from which they are prepared by fractional distillation ; 

 m-xylene exists in largest amount. The fraction in which they are present 

 boils at 136-141. Their boiling-points are so close that they cannot be 

 separated by fractional distillation ; their separation depends on the formation 

 of nitro- and sulphonic acid derivatives. 



They closely resemble benzene and toluene and are obtained from these 

 compounds by the same methods as toluene is prepared from benzene. A 

 different isomer is formed under different conditions. 



They yield nitro- and other derivatives, and on oxidaiton are converted 

 into methyl benzoic acids and into phthalic acids. 



