2 : ; -. % BJQLGGJtAL CHEMISTRY 



equivalents and -they unite "'with each other so that the 

 valen\2ies : are-giiU-tiiaH. satisfied. 



The four elements are carbon with four valencies f C 



hydrogen with one (H ), oxygen with two ( O ) and 

 nitrogen with three or five (N== or =NrE). Thus carbon can 

 unite with four hydrogen or two oxygen atoms, but owing to 

 the odd number of valencies associated with nitrogen, carbon 

 and nitrogen cannot form by themselves such simple and 

 even unions. 



Models of these atoms in three planes are extremely useful 

 in understanding the organic compounds, but for purposes 

 of description we must adhere to the plane of the paper and 

 represent the elements as shown above with dashes to represent 

 the valencies. The symbol R is used to represent a molecular 

 group which is of minor importance, and to it are attached 

 the special groups which are of importance in the reaction 

 which is being studied. 



For the present we can confine our attention to three large 

 groups of compounds as these furnish the basis on which 

 we can build up the greater part of Biological Chemistry. 

 As the discussion of the subject requires it we shall introduce 

 additional groups of organic compounds. 



The three classes are carbohydrates, fats and proteins. 



One carbon atom can unite with two oxygen atoms, or 

 with four hydrogen atoms. In the former case we have 

 formed the important gas carbon dioxide (CO 2 ), and in the 



( " " ^ 



latter marsh gas or methane I H C H or CH 4 I . On 



V H 



removing one of these hydrogen atoms the methyl group 

 ( CH 3 ) is left. Two methyl groups unite to form ethane 

 (H 3 C CH 3 or C 2 H 6 ), and on removing one hydrogen atom 

 from ethane the ethyl group ( C 2 H 5 ) is left. By uniting 

 carbon and hydrogen in this way a series of compounds, 

 each containing one CH 2 group more than the preceding 

 member, forms what is called the marsh gas or paraffin 

 series. 



Our first consideration must be the substances formed 

 by oxidation, i.e. the introduction of oxygen into these 

 compounds. 



