CLASSIFICATION OF ORGANIC COMPOUNDS. 257 



Action of reducing agents. Deoxidizing or reducing agents, 

 especially hydrogen in the status nascendi, act upon organic 

 substances either by direct combination : 



C 2 H 4 + 2H = C 2 H 6 0. 



Ethene Ethyl 



oxide. alcohol. 



or by removing oxygen (and also chlorine or bromine): 

 C 7 H 6 2 + 2H = C 7 H 6 + H 2 0. 



Benzoic Benzoic 



acid. aldehyde. 



In some cases hydrogen replaces oxygen : 



C 6 H 5 N0 2 + 6H = C 6 H 5 NH 2 + 2H 2 O. 



Nitro-benzene. Aniline. 



Classification of organic compounds. There are great difficulties 

 in arranging the immense number of organic substances prop- 

 erly, and in such a manner that natural groups are formed the 

 members of which are similar in composition and possess like 

 properties. 



Various modes of classification have been proposed, some of 

 which, however, are so complicated that the beginner will find 

 it difficult to make use of them. The grouping of organic sub- 

 stances here adopted, while far from being perfect, has the ad- 

 vantages of being simple, easily understood and remembered. 



1. Hydrocarbons. All compounds containing the two ele- 

 ments carbon and hydrogen only. For instance, CH 4 , C 6 H 6 , 

 C 10 H 16 , etc. 



2. Alcohols. These are un saturated hydrocarbons or hydro- 

 carbon residues in combination with hydroxyl, HO. For in- 

 stance, ethyl alcohol, C 2 H' 5 HO, glycerine, C 3 H iii 5 3HO, etc. 



3. Aldehydes. Compounds intermediate between alcohols and 

 acids, or alcohols from which hydrogen has been removed. 

 For instance : 



C 2 H 6 0, C 2 H 4 0, C 2 H 4 2 . 



Ethyl Aldehyde. Acetic acid, 



alcohol. 



4. Organic acids. Unsaturated hydrocarbons in combination 

 with carboxyl, a radical having the composition CO 2 H, or com- 



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