460 CONSIDERATION OF CARBON COMPOUNDS. 



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

 Benzoic acid. Benzole aldehyde. 



C 7 H 6 O + 2H = C 7 H 8 O. 

 Benzoic aldehyde. Benzylic alcohol. 



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 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 

 k 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 hydrocarbon radicals in combination 

 with hydroxyl, OH. For instance, ethyl alcohol, CjH^OH, glycerin, 

 C 3 H iii 5 (OH) 3 , etc. 



3. Aldehydes. Hydrocarbon radicals 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 H 6 0, CH 3 .COH, C 2 H 4 2 , 



Ethyl alcohol. Aldehyde. Acetic acid. 



4. Organic acids. Hydrocarbon radicals in combination with 

 carboxyl, a radical having the composition CO 2 H, or compounds 

 formed by replacement of hydrogen in hydrocarbons by carboxyl. 

 Instances : Acetic acid, CH 3 CO 2 H ; pyrotartaric acid, C 3 H 6 (CO 2 H) 2 . 



5. Ethers. Compounds formed from alcohols by replacement of 

 the hydrogen of the hydroxyl by other hydrocarbon radicals, or, 

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



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 



