170 CHEMISTRY FOR AGRICULTURAL STUDENTS 



CH3 - CH2OH, can be replaced by methyl to form propyl 

 alcohol, CH3 - CH2 ~ CHoOH. Thus, each member of each 

 group will differ from the preceding member by CHg, and the 

 difference being constant, each group can be represented by a 

 genera] formula. Thus, all the paraffin hydrocarbons are 

 found to have the composition CnH2n+2J ^^^ ^ '^^ corres- 

 ponding alcohols the composition CnHgn+iOH. It has been 

 observed that the physical properties of each group vary with 

 the molecular complexity; the melting points of the hydro- 

 carbons, and the boiling points of the alcohols, rose, and the 

 relative densities of the fats fell, with each addition of CHg to 

 the molecular formula. 



Each class of compounds is characterised by a distinguish- 

 ing group. The — CH3 group of the hydrocarbons becomes 



- CH2(0H) in the alcohols, — CHO in the aldehydes, dnd 



- CO(OH) in the acids. The relation of methane, ethane, 

 propane, and butane, to their corresponding alcohols, alde- 

 hydes, and acids, will be made clear by the following table : — 



It is these characteristic groups that give to each class of 

 organic compounds their distinctive properties. Thus, the 

 group CO(OH) renders a compound an acid, because it is 



