ESTERS 



73 



Ethyl Benzoate. 



20 gm. of benzoic acid are dissolved in 75 c.c. of absolute 

 alcohol and I c.c. of concentrated sulphuric acid is added. The flask 

 containing the mixture is connected to a reflux condenser and gently 

 heated for 1-2 hours over a gauze, a piece of porcelain being added 

 to prevent bumping. The esterification is complete when, on testing 

 by pouring a few drops into water, only oil drops and no crystalline 

 benzoic acid is seen. The whole is then poured into about 400 c.c. 

 of water and the oil is allowed to settle. The water is decanted 

 off and the remainder is shaken up with ether in a separating 

 funnel. The aqueous layer is withdrawn, the ethereal layer shaken 

 with sodium carbonate solution and then with water. It is dried with 

 calcium chloride and the ether distilled off over a water-bath. The 

 ester is distilled over a flame and the fraction boiling from 210-215 

 is collected. Ethyl benzoate boils at 2 1 3. 



Properties. 



Esters are usually liquids having a sweet and fragnant odour ; a few 

 are solid. 



Neutral esters of monobasic, dibasic, etc., acids are insoluble in 

 water, or only slightly soluble, e.g. ethyl formate and acetate. 



Acid esters of dibasic, etc., acids are soluble in water, e.g. ethyl 

 sulphuric acid, ethyl oxalic acid, etc. 



Neutral esters are soluble in alcohol and ether, acid esters may be 

 soluble in alcohol, but are insoluble in ether. 



Esters are comparatively inert substances and are unaffected by cold 

 dilute sodium carbonate, sodium hydroxide, hydrochloric acid, sul- 

 phuric acid, but there are exceptions, e.g. methyl oxalate, which is decom- 

 posed by cold dilute caustic soda. They are acted upon by sodium more- 

 or less readily (cf. ethers). All esters are hydrolysed by boiling with 

 water, acids, or alkalies. The last method of hydrolysis is knowri 

 as saponification. They are thus converted into their constituents, 

 namely acid and alcohol. The recognition of these identifies the ester. 

 Esters which are of frequent occurrence in animals and plants are 

 identified in this way. The hydrolysis is effected by boiling under 

 a reflux condenser with aqueous sodium hydroxide, or 80 per cent, 

 sulphuric acid. If only the acid is to be identified hydrolysis is 

 effected by boiling with alcoholic sodium hydroxide. The alcohol 

 is isolated by distilling the alkaline liquid if the alcohol be volatile, by 

 extracting the alkaline solution with ether if not volatile, and it is identi- 

 fied by the reactions for alcohols. If the alcoholic portion of the ester be 

 a phenol or an aromatic alcohol it does not distil and is not extracted 



