AROMATIC COMPOUNDS 255 



(5) It gives a hydrazone with phenylhydrazine. 



(6) It combines with sodium bisulphite. 



(7) It combines with hydrogen cyanide. 



In the following reactions benzaldehyde and other aromatic alde- 

 hydes, which have the aldehyde group attached to the benzene nucleus, 

 differ from aliphatic aldehydes ; 



(1) It does not reduce Fehling's solution or ammoniacal silver 

 solutions. 



(2) It does not polymerise. 



(3) It gives a mixture of alcohol and acid on treating with potash. 



(4) It is converted into benzoin on shaking with an alcoholic solu- 

 tion of potassium cyanide : 



C 6 H 5 . CHO + C 6 H 5 . CHO = C 6 H e . CO . CHOH . C 6 H 8 . 



Benzoin is a complex ketonic alcohol formed by the condensation of 

 two molecules of benzaldehyde. 



(5) It condenses with acetone and aniline on shaking up the two sub- 

 stances with a few drops of caustic soda : 



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



As an aromatic compound benzaldehyde forms nitro, sulphonic acid 

 derivatives, etc. 



If nitrobenzaldehyde in acetone solution be mixed with a few drops 

 of dilute caustic soda, a precipitate of indigo-blue (p. 343) slowly forms : 



/N0 2 / NH \ / NH \ 



H 4 ( + 2 CH 3 . CO . CH 3 = C B H 4 ( >C = C< )C 6 H 4 + 2 H 2 O + 2 CH 3 . COOH 



\CHO \CO/ 



Benzoic Acid, C 6 H 5 . COOH. 



Benzoic acid occurs in gum benzoin and other resins such as 

 balsam of Peru. In gum benzoin it is present chiefly as the ester, 

 benzyl benzoate. 



Preparation. 



(1) Benzoic acid is readily obtained by subliming gum benzoin. 

 Gum benzoin is heated on an iron tray or porcelain basin, the tray 

 being covered with a cone of filter paper or a funnel. The resin 

 melts and the benzoic acid which volatilises condenses on the cone. 

 It is recrystallised from water. 



(2) Benzoic acid is made commercially by oxidising benzyl chloride 

 with 60 per cent, nitric acid. 



(3) It is also prepared by heating the calcium salt of phthalic 

 acid. 



(4) It can be prepared by the hydrolysis of the nitrile, phenyl 

 cyanide. 



(5) As previously mentioned, it results from the oxidation of 

 aromatic compounds possessing a side chain. 



