CHEMICAL PROPERTIES OE EATTY ACIDS AND RELATED COMPOUNDS 1 17 



54.4; elaidic, 54.4; erucic, 51.2; brassidic, 51.8; and A^-^-oleic, 1.3. It is 

 apparent that the cis forms (oleic and erucic) give identical results with 

 those of their trans forms (elaidic and brassidic). 



The extent to which esterification takes place is also influenced by the 

 number of hydroxyl groups in the alcohol. When more than one alcohol 

 group is present, there is the possibility of several types of ester, i.e., one in 

 which the alcohol groups are all esterified, and another in which only 

 partial esterification is obtained. The number of such partially esterified 

 compounds increases progressively Mith the increase in the number of alco- 

 hol groups. Menschutkin^^^ reported that, when the mole ratio of acid to 

 alcohol groups was kept constant, the percentage of the ester at equilib- 

 rium became progressively lower as the number of alcohol groups was in- 

 creased. Thus, the percentage values at equilibrium when acetic acid was 

 allowed to react at 155°C., without an acid catalyst, with ethanol, or with 

 the several polyhydric alcohols, were as follows: ethanol (1 OH group), 

 66.6; ethylene glycol (2), 53.9; glycerol (3), 46.0; erythritol (4), 40.1; 

 and mannitol (6), 26.4. However, when one mole of acid and one mole of 

 alcohol were reacted, the equilibrium values were all practically the same, 

 falling between 62 and 70%. Esterification of the acids with the poly- 

 hydric alcohols is controlled by the same factors as is that with the mono- 

 hydric alcohols. Thus, they are catalyzed by inorganic acids, and esteri- 

 fication is promoted by heat, by the removal of water, and by agitation. 

 The extent of esterification of the polyhydric alcohols is adversely affected 

 by isomerism (branched chains) as it is in the case of the alcohols having a 

 single hydroxyl group. 



On the other hand, the esterification reaction with the polyhydric alco- 

 hols differs from that between the acids and monohydric alcohols by the 

 fact that it is not promoted by increase in the concentration of the alcohol. 

 The net effect of having a greater proportion of polyhydric alcohols to the 

 acids is to increase the number of partially esterified molecules rather than 

 to augment the proportion of completely esterified molecules. This occurs 

 because certain of the hydroxyl groups of the polyhydric alcohols are pref- 

 erentially attacked; with a larger number of molecules available, only 

 the most susceptible hydroxyl radicals react, and a larger proportion of 

 partially esterified molecules is produced. Practical application of this 

 procedure is made in the manufacture of mono- and diglycerides (see Chap- 

 ter HI). 



b'. Alcoholysis as a Method for the Synthesis of Esters: Ester inter- 

 change with the triglycerides by the use of an excess of alcohol is a simple 

 method for preparing esters, since it does not require the intermediate 

 separation of the acids. However, it has the disadvantage that one seldom 

 obtains pure esters but rather mixtures, inasmuch as the natural triglyc- 

 erides are usually mixed triglycerides. 



