AROMATIC COMPOUNDS 267 



Aspirin is acetyl-salicylic acid It is prepared by heating salicylic 

 acid with acetyl chloride or acetic anhydride : 



OH .O.OC.CH, 



C 6 H 4 ( + CH 3 COC1 = HC1 + C,HC 



\COOH X COOH. 



On hydrolysis by acids or alkalies, it yields acetic and salicylic acids. 



Salol is phenyl salicylate. It is prepared by heating a mixture of 

 sodium phenate and sodium salicylate with phosphorus oxychloride : 



/OH ,OH 



2C 6 H 4 <^ + 2C 6 H 5 ONa + POC1 3 = aNaCl -f NaPO, + 2C 6 H 4 <^ 



X COONa X COOC 6 H 5 . 



On hydrolysis, it yields phenol and salicylic acid. 



If the hydrolysis of aspirin and salol be effected with caustic soda 

 the sodium salts are obtained ; on acidifying with sulphuric acid, sali- 

 cylic acid is precipitated and the acetic acid or phenol can be isolated 

 by steam distillation. 



>4>H 



Tyrosine, C G H/ 



X CH 2 .CH(NH 2 ).COOH. 



Tyrosine or p-hydroxyphenylalanine is a constituent of proteins 

 from which it was'first obtained by Liebig in 1846 who fused cheese 

 (TV/DO?) with caustic potash. It has since been isolated from the pro- 

 ducts of hydrolysis of most proteins. It is found in the liver and 

 other organs in certain diseases in considerable quantities ; in minute 

 amounts it is present in all tissues. 



Preparation. 



The best yield of tyrosine is obtained from silk ; silk is hydrolysed by boiling 

 with concentrated hydrochloric acid for 5 or 6 hours, the solution is evaporated 

 to remove hydrochloric acid, the greater part of the remainder is removed as 

 cuprous chloride by adding cuprous oxide, and on neutralising the solution 

 tyrosine separates out. It may also be obtained by hydrolysing silk and 

 other proteins with six times their amount of 30 per cent, sulphuric acid, 

 removing the sulphuric acid with baryta and concentrating the solution. 

 Tyrosine separates out and is recrystallised. 



It is most easy to prepare tyrosine by the tryptic digestion of caseinogen. 

 100-500 gm. of caseinogen are dissolved in 2-10 litres of -4 per cent sodium 

 carbonate, 1-2 gm. of dried pancreas (trypsin) are added and 1-2 per cent, of 

 toluene or chloroform are shaken up with the solution to prevent putrefac- 

 tion. The solution is kept at 35 for 7-10 days. It becomes cloudy with the 

 separation of tyrosine which gradually settles out. The filtrate gives the 

 reactions for tyrosine and on evaporation and on cooling deposits a further 

 quantity. Almost pure tyrosine may be obtained from the first deposit by 

 dissolving it in -iN hydrochloric acid, boiling with charcoal to decolorise it 

 and exactly neutralising the clear solution with ammonia. Pure tyrosine may 

 be obtained from the second deposit by the same treatment repeated two or 

 three times. 



