254 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Benzaldehyde. C 6 H 5 . CHO. 



Benzaldehyde was originally isolated from bitter almonds and 

 called oil of bitter almonds. The almonds contain the glucoside, amyg- 

 dalin, which is hydrolysed by the enzyme, emulsin, into glucose ben- 

 zaldehyde and prussic acid. It is the aldehyde of benzyl alcohol from 

 which it may be obtained by oxidation with nitric acid. 



It may be obtained by distilling calcium benzoate with calcium 

 formate in the same way as aliphatic aldehydes. 



Preparation. 



(1) Benzaldehyde is prepared from benzal chloride by boiling it 

 with dilute sulphuric acid or lime water under pressure : 



C,.H 6 . CHC1 2 - C 6 H 6 CH(OH) 3 -> C 6 H B . CHO. 



(2) Or it is prepared by boiling benzyl chloride with lead nitrate 

 or copper nitrate. Benzyl alcohol is probably first formed and is oxi- 

 dised to the aldehyde : 



C 6 H 5 . CH 2 C1 - C 6 H 6 CH a OH - C 6 H 5 . CHO. 



Molecular proportions of benzyl chloride (12*6 gm.) and copper nitrate 

 (20 gm.), dissolved in about the same weight of water (25 c.c.), are boiled for 

 6-8 hours under a reflux condenser, whilst a current of carbon dioxide is 

 passed through the mixture to expel oxides of nitrogen and to avoid further 

 oxidation. When the oil contains no chlorine or only traces as shown 

 by testing it, after washing with water, with silver nitrate and nitric acid, the 

 oil is extracted with ether and the ethereal extract is shaken with saturated 

 sodium bisulphite solution. The crystalline bisulphite compound is filtered off 

 and washed with ether. The benzaldehyde is obtained by decomposing it with 

 dilute sulphuric acid, extracting with ether, drying and distilling. 



(3) It is prepared by Friedel and Craft's reaction from benzene, a mixture 

 of carbon monoxide and chlorine being passed into the benzene. Formyl- 

 chloride is apparently formed which reacts as follows : 



C B H 6 + H . CO . Cl = C 6 H 5 . CHO + HC1. 



Properties. 



Benzaldehyde is a colourless liquid with a strong smell of bitter 

 ilmonds. It boils at 179 and has a sp. gr. of 1-05 at 15. It is very 

 slightly soluble in water, but dissolves in alcohol and ether. It is used 

 extensively for flavouring purposes. 



Reactions. 



In most reactions benzaldehyde resembles the aliphatic aldehydes: 



(1) It is easily oxidised; by exposure to air crystals of benzoic 

 acid gradually separate ; 



C 6 H 8 . CHO + O = C 6 H B . COOH. 



(2) On reduction it yields benzyl alcohol (p. 253). 



(3) It yields benzal chloride with phosphorus pentachloride. 



(4) It gives an oxime with hydroxylamine. 



