158 Biological Chemistry. 



and benzaldehyde. The latter can be prepared directly 

 from benzene, but it is best obtained from benzalchloride, 

 C 6 H 6 -CHC1 2 (p. 154), by hydrolysis with dilute sulphuric 

 acid or calcium hydroxide under pressure 



C 6 H 8 .CHC1 2 + 2H 2 = C 6 H 5 CHO + 2HC1 



It is a colourless oil with an almond-like smell and boiling 

 point 179. Its reactions are similar to those of aliphatic 

 aldehydes. It can be reduced, for example, to benzyl 

 alcohol 



C 8 H 5 CHO + H 2 = C 6 H 5 -CH 2 OH 



a substance which, as it contains a hydroxyl group in the 

 " side-chain," behaves like an ordinary alcohol, but not as 

 a phenol. It can be oxidized to benzoic acid 



C 6 H fl CHO + = C 6 H 5 COOH 



another reaction similar to that of a fatty aldehyde. It 

 also yields bisulphite compounds, an oxime and hydrazone 

 (compare p. 82). 



Carboxylic Acids. 



Benzoic Acid, C 6 H 5 -COOH. This occurs in many 

 resins, and is obtained by sublimation of gum benzoin. 

 Its synthesis from benzyl cyanide (or benzonitrile) has 

 been already mentioned. It is soluble in hot water, from 

 which it separates in crystals on cooling. It has melting 

 point 121'5, and boiling point 249, and readily sub- 

 limes. It forms esters, an acid chloride, anhydride, and 

 amide like aliphatic acids. 



The dicarboxylic acids of benzene are known as the 

 phthalic acids. Their constitution has been already 

 discussed. 



