114 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



ritol (CHaOHCHOH-CHOHCHaOH). mien completely esterified, these esters will 

 contain four fatty acids. (4) Esters of polyhydric alcohols having more than four hydroxyl 

 groups. Esters of the hexitols, mannitol, and sorbitol, have come into prominence within 

 recent years, especially since the commercial production of these hexitols has made it 

 possible to supply them at low cost."' The sorbitol esters have found wide application 

 as emulsifying agents. 



(6) Preparation of Esters. There are a number of general methods 



which may be used for the synthesis of the esters. These are Hsted below. 



a'. Reacting Acid and Alcohol in the Presence of a Catalyst: This is 



exemplified in the reaction of ethanol and palmitic acid to form ethyl 



palmitate and water: 







CH3(CH2)i4C-0H + H0CH2CH3^^CH3(CH2),4C-0CH2CH3+H20 



Palmitic Ethyl Ethyl palmitate 



acid alcohol 



The reaction proceeds when the concentrated alcohol and the concen- 

 trated acid are heated together in the presence of an inorganic acid which 

 acts as a catalyst. Anhydrous sulfuric or hydrochloric acid is generally 

 employed in a concentration of 3 to 5%. Hilditch^ has suggested a con- 

 centration of 2% of sulfuric acid and about twice the weight of methyl 

 alcohol as of the fatty acids. This should give a yield of 97-98%. Dean"' 

 has recommended 4% of sulfuric acid ^\^th 2 to 3 times the weight of meth- 

 anol as of the fatty acids. Methyl alcohol saturated with dry hydrochloric 

 acid gas before or after mixing with the acid but before refluxing has been 

 employed by Cruz and West."^ 



The esterification reaction is a reversible one and is a classical example of 

 the law of mass action. If one of the products of reaction is removed or 

 allowed to escape, the same equilibrium is maintained, irrespective of 

 whether one starts with a mixture of alcohol and fatty acid or with one of 

 ester and water. The equilibrium constant, K, for the reversible reaction : 



RCOOH + HOR' , RCOOR' + HOII 



can be calculated from the proportions in gram moles per liter of the re- 

 actants at equilibrium, by the application of the following formula: 



^ ^ ^ [RCOOR^IHOH] 

 k' [RCOOH] [HOR'] 



where k and k' represent the velocity constants for the esterification and 

 hydrolysis reactions, respectively. 



When K is large, the proportion of the ester (RCOOR') to water will be 

 high for the synthetic reaction. Under such conditions the amount of 



'^s H. J. Creighton, Trans. Eledrochem. Soc, 75, 289-307 (1939). 



"7 H. K. Dean, Utilization of Fats, Chem. Pub. Co., Brooklyn, 1938, p. 75. 



»78 A. O. Cruz and A. P. West, Philippiyic J. Sci., 48, 77-88 (1932). 



