296 CONSIDERATION OF CARBON COMPOUNDS. 



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



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

 Benzole acid. Benzole aldehyde. 



In some cases hydrogen replaces oxygen : 



C 6 H 5 N0 2 + 6H = C 6 H5NH 2 + 2H 2 O. 

 Nitro-benzene. Aniline. 



Classification of organic compounds. There are great diffi- 

 culties in arranging the immense number of organic substances 

 properly, 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 substances here adopted, 

 while far from being perfect, has the advantages of being simple, 

 easily understood, and remembered. 



1. Hydrocarbons. All compounds containing the two elements 

 carbon and hydrogen only. For instance, CH 4 , C 6 H 6 , C 10 H 16 , etc. 



2. Alcohols. These are unsaturated hydrocarbons or hydrocarbon 

 residues in combination with hydroxyl, OH. For instance, ethyl 

 alcohol, C 2 IT 5 OH, glycerin, C,W li 5 (OH) 3 , etc. 



3. Aldehydes. Unsaturated hydrocarbons in combination with the 

 radical COH ; they are compounds intermediate between alcohols 

 and acids, or alcohols from which hydrogen has been removed. For 

 instance : 



C 2 HeO, C 2 H 4 0, C 2 H 4 2 - 



Ethyl alcohol. Aldehyde. Acetic acid. 



4. Organic acids. Unsaturated hydrocarbons in combination 

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

 pounds formed by replacement of hydrogen in hydrocarbons by 

 carboxyl. Instances : Acetic acid, CH 3 CO 2 H ; pyrotartaric acid 

 C 3 H 6 (C0 2 H) 2 . 



5. Ethers. Compounds formed from alcohols by replacement of 

 the hydrogen of the hydroxyl by other unsaturated hydrocarbons, or, 

 what is the same, by other alcohol radicals. For instance : 



' C 2 H 5 x' C H 3 /> 



Ethyl alcohol. Ethyl ether. Ethyl-methyl ether. 



6. Compound ethers or esters. Formed from alcohols by replace- 

 ment of the hydrogen of the hydroxyl by acid radicals, or from acids 



