MODES OF LINKAGE BY COVALENT BONDS 61 



II. ESTER LINKAGE (AND ANHYDRIDE LINKAGE) 



A. Ternary Lipides 



Lipides are the esters which constitute fats. A distinction is made be- 

 tween ternary Hpides, containing only carbon, hydrogen and oxygen, and 

 complex lipides containing in addition phosphorous and nitrogen. These 

 latter compounds are better considered with the other natural phosphate 

 esters. 



The ternary lipides can be divided into several types, among which are : 



(a) The glycerides, esters of glycerol. 



(b) The waxes, ester of higher alcohols. 



(c) The sterides, esters of sterols. 



(a) Glycerides 



The simple glycerides are those in which the three molecules of acid, 

 which take the place of the H atom in each of the OH groups of glycerol, 

 are identical. The general formula of the simple glycerides is therefore as 

 follows : 



CH2— O— OC— R 



1 

 CH — O— OC— R 



I 

 CH2— O— OC— R 



In certain other glycerides, the mixed glycerides, the three molecules of 

 fatty acid are not identical, for example in distearopalmitin : 



CHo- O— OC— CnHa, 



i 

 CH — O— OC— CoH,, 



CH,— O— OC— CH,-. 



Distearopalmitin 



(1 molecule of glycerol + 2 molecules of stearic acid + 1 molecule of 

 palmitic acid) 



{b) Waxes 



These are esters of the higher molecular weight fatty acids and mono- 

 valent higher alcohols. 



Example : cetyl palmitate, the principal constituent of spermaceti 

 (cetyl alcohol = C16H34O). 



