MONOBASIC FATTY ACIDS. 279 



42. MONOBASIC FATTY ACIDS. 



General constitution of organic acids. When hydroxyl, HO, 

 replaces hydrogen in hydrocarbons, alcohols are formed; when 

 the univalent group C0 2 H, known as carboxyl, replaces hydrogen 

 in hydrocarbons, acids are formed. Monatomic, diatomic, and 

 triatomic alcohols are formed by introducing hydroxyl once, 

 twice, or three times respectively into hydrocarbon molecules; 

 monobasic, dibasic, and tribasic acids are formed by substituting 

 one, two, or three hydrogen atoms by carboxyl. For instance : 



Ethane. Propionic acid. Succinic acid. 



The constitution of carboxyl is represented by 0=C O H, 

 which shows that of the four affinities of the carbon atom, two 

 are saturated by an atom of oxygen, one by hydroxyl, whilst 

 one is unprovided for ; any univalent hydrocarbon residue may 

 attach itself to this unprovided affinity when an acid is formed. 

 Acids may, therefore, be looked upon as being composed of 

 hydrocarbon residues and hydroxyl, united by the bivalent 

 radical CO. By replacement of the hydrogen of the hydroxyl 

 (or of the carboxyl, which is the same) by metals the various 

 salts are formed. 



What is termed the acid radical is the group of the total num- 

 ber of atoms present in the molecule, with the exception of the 

 hydroxyl. In acetic acid, C 2 H 4 2 , for instance, the radical is 

 CH 3 CO or C 2 H 3 O, which group of atoms is characteristic of 

 acetic acid and of all acetates, and may often be transferred 

 from one compound into another without decomposition. 



The difference between alcohol radicals and acid radicals may 

 also be stated, by saying that the first contain carbon and hydro- 

 gen only, whilst acid radicals contain carbon, hydrogen, and. 

 oxygen. 



In a similar manner, as there are homologous series of alcohols 

 corresponding to the various series of hydrocarbons, there are 



