AROMATIC COMPOUNDS 259 



Cinnamic Acid, C 6 H 5 . CH=CH . COOH. 



Cinnamic acid is the chief representative of an aromatic compound con- 

 taining an unsaturated acid as the side chain. It occurs in greatest amount 

 in storax, the resin of Styrax offidnalis. 



It is prepared from storax by warming it with dilute sodium hydroxide ; 

 the filtered alkaline solution is acidified with hydrochloric acid. Cinnamic 

 acid is precipitated and purified by crystallisation from water. 



Cinnamic acid is usually prepared by synthesis by Perkin's reaction : 

 C 6 H B . CHO + H 3 C . COONa = C 6 H 5 . CH= CH . COONa + H 2 O. 



Molecular proportions of benzaldehyde and sodium acetate are heated 

 in an oil-bath under a reflux condenser with 3-4 parts of acetic anhydride 

 for 8-10 hours. The mixture is poured into water and unchanged benzalde- 

 hyde is removed by steam distillation; the residue is treated with caustic 

 soda, filtered from oily impurities and acidified with concentrated hydrochloric 

 acid. Cinnamic acid is precipitated and purified by crystallisation from water. 



Cinnamic acid crystallises in needles and melts at 133. It is soluble 

 with difficulty in cold water, more easily in hot water. It dissolves in alcohol, 

 ether and other organic solvents. As an aromatic compound it forms nitro 

 derivatives. As an unsaturated aliphatic compound cinnamic acid forms 

 addition compounds with bromine and halogen acids. It is converted by 

 reduction into phenylpropionic acid or hydrocinnamic acid. It is readily 

 oxidised in the cold by permanganate, a solution of cinnamic acid in alkali 

 decolorising permanganate immediately. 



Cinnamic Aldehyde, C 6 H 5 . CH=CH . CHO. 



Cinnamic aldehyde is the chief constituent of oil of cinnamon from 

 which it may be obtained by treatment with sodium bisulphite. 



It is a liquid which boUs at 247 and has the peculiar and characteristic 

 odour of cinnamon. It resembles the aliphatic aldehydes in properties. 



Phenylalanine, C 6 H 5 . CH 2 . CH . (NH 2 )COOH. 



Phenylalanine was first isolated from an extract of growing seed- 

 lings and subsequently recognised as a constituent of proteins ; it is 

 the constituent which gives rise to phenylpropionic and phenylacetic 

 acids during putrefaction. It also gives rise to phenylethylamine, 

 and most probably cinnamic acid is also derived from it. In the de- 

 composition either carbon dioxide or ammonia is lost and then the 

 side chain is oxidised : 

 C 8 H 5 . CH 2 . CH a , COOH <- C 6 H B . CH 2 . CH(NH 2 )COOH -> C 6 H 8 . CH 3 . CH a . NH 2 



C 6 H 6 . CH 2 . COOH - C 6 H 6 . COOH. 



Phenylalanine is isolated with some difficulty from the complicated mix- 

 ture of amino acids resulting from proteins; full details are given in "The 

 Chemical Constitution of Proteins," Part I. 



It has also been prepared by synthesis (see " The Chemical Constitution 

 of Proteins," 2nd ed. Part I., or 3rd ed. Part II.). 



Phenylalanine crystallises in glistening platelets which melt at 

 275-280. The natural substance is laevorotatory. It has aromatic 

 properties and the properties of an amino acid. 



