10 UNITY AND DIVERSITY IN BIOCHEMISTRY 



The lower members are very soluble in water and dissociate weakly ; their 

 solubility decreases as the number of carbon atoms increases. 



Except for formic acid, propionic acid and valeric acid, the saturated 

 fatty acids generally found in nature are those with an even number of 

 carbon atoms from 2 to 38. Table III shows their scientific and common 

 names, their empirical formulae and molecular weights. 



{b) Unsaturated Fatty Acids 



The unbranched saturated fatty acids form a perfect homologous series 

 which is represented in nature by the members having an even number of 

 carbon atoms between 2 and 38 (save for formic, propionic and valeric 

 acids) but this is not the case with the unsaturated acids. These acids are 

 made complex by the presence of one or more double bonds which 

 in turn can undergo several types of reaction such as hydrogenation, 

 halogenation, etc. 



1. One Double Botid 



In the same way as the hydrocarbons these acids are named according 

 to the number of carbon atoms they possess, the carboxyl carbon being 

 numbered 1. The presence of a double bond is indicated by the suffix -en, 

 and its position by a numbered prefix, the two numbers indicating which 

 two carbon atoms are joined by the double bond. Thus the acid C18H34O2, 

 commonly known as oleic acid, bears the scientific designation 9 : 10- 

 octadecenoic acid, sometimes abbreviated to 9, octadecenoic acid. It is the 

 most widely distributed in nature, of the monoethenoic acids, but there are 

 several others, all with an even number of carbon atoms. The presence of the 

 double bond admits the existence of a cis and of a trans form. Since nature 

 only contains the cis forms of the aliphatic acids, in the field of biochemistry 

 we may consider the prefix cis as understood, and therefore omit it. 



2. Tzco or More Double Bonds 



Fatty acids having two, three, four, five or six double bonds are not 

 unusual in nature. Two of these are particularly widely distributed, they 

 are, linoleic acid, CigHgjOa, or 9:10:12:13-octadecadienoic acid, and 

 linolenic acid, CigHgoOg or 9:10:12:13:15:16-octadecatrienoic acid. 



(c) Branched Chaiji Fatty Acids 



Certain fatty acids do not have a linear chain, for example, isovaleric acid, 

 3-methyl-butanoic acid CH3.CH(CH3).CH2COOH. This is the only 

 naturally occurring aliphatic acid (besides formic acid, propionic acid and 

 valeric acid) having an odd number of carbon atoms. Nevertheless, the 

 main chain in it has an even number. In the unsaturated series examples of 

 naturally occurring branched chain acids are more common. 



