BENZESE SERIES. AROMATIC COMPOUNDS. 579 



nillin, forms a precipitate when iodine in potassium iodide is added in excess, 

 at first brown and flocculent, and afterward, on shaking, forming a dark-green 

 curdy clot. 



" Extracts of vanilla," made not from the vanilla bean, but consisting of 

 alcoholic tinctures of synthetic vanillin or coumarin, can readily be detected by 

 evaporating off the alcohol, making up the original volume with water, and 

 acidifying with acetic acid. A reddish-brown precipitate of resin is formed in 

 the case of a true extract, but none in the artificial. The filtrate from this 

 resin gives a copious precipitate with basic lead acetate solution, the artificial 

 extract gives none. 



Vanillin has been found adulterated with benzoic acid, acetanilide, boric 

 acid, terpin hydrate, and coumarin. 



Acids of the benzene series. 



These are derivatives in which one or more carboxyl groups 

 (COOH ) have replaced hydrogen in the benzene molecule. Benzoic 

 acid is the simplest, and bears the same relation to benzene as acetic 

 acid bears to methane. Many of these acids are found as natural 

 products, but the carboxyl group may be introduced by various reac- 

 tions, of which the following are the principal ones : 



1. Oxidation of benzene compounds containing fatty hydrocarbon 

 radicals or substituted radicals : 



C 6 H 5 CH 2 OH + 20 = C 6 H 5 COOH + H 2 O. 



2. Hydrolysis of a cyanide by heating with dilute acid : 



C 6 H 5 CN + 2H 2 O = C 6 H 5 COOH + NH 3 . 



3. The treatment of alkali salts of phenols with carbon dioxide 

 (see Salicylic Acid). 



Benzoic acid, Acidum benzoicum, HC 7 H 5 O 2 or C 6 H 5 CO 2 H = 

 121.13. Found in benzoin and some other resins; also in combination 

 with other substances in the urine of herbivorous animals; it is 

 obtained from benzoin by heating it carefully, when the volatile 

 benzoic acid sublimes. It is now also manufactured from toluene, 

 which is first converted into benzo-trichloride (trichlormethyl-ben- 

 zene) by passing chlorine into hot toluene : 



C 6 H 6 CH S + 6d == C 6 H 5 CC1 3 + 3HC1. 



Benzo-trichloride, when treated with water under pressure, yields 

 benzoic and hydrochloric acids, thus: 



C 6 H 5 CC1 3 + 2H 2 : : C 6 H 5 C0 2 H + 3HC1. 



Benzoic acid forms white, lustrous scales or friable needles, which 

 are but slightly soluble in cold water, but easily soluble in alcohol. 



