448 CONSIDERATION OF CARBON COMPOUNDS. 



C H O.OH 1 is a formula indicating that acetic acid is composed of 

 two univalent radicals which may be taken out of the molecule and 

 replaced by other atoms or groups of atoms. This formula indicates 

 also that acetic acid is analogous to hydroxides, the radical C 2 H 3 O 

 having replaced one atom of hydrogen in H 2 O. 



CH .CO 2 H l is a formula indicating that acetic acid is composed of 

 the two compound radicals, methyl and carboxyl. 



It may be said finally, that quite a number of other rational 

 formulas have been applied, or, at least, have been proposed by 

 different chemists and at different times, to represent the structure of 

 acetic acid, but it should be remembered that these formulas are not 

 intended to represent the actual arrangement of the atoms in space, 

 but only, as it were, their relative mode of combination, showing 

 which atoms are combined directly and which only indirectly, that 

 is, through the medium of others. 



41. CONSTITUTION, DECOMPOSITION, AND CLASSIFICATION 

 OF ORGANIC COMPOUNDS. 



Radicals or residues. The nature of a radical or residue has 

 been stated already in Chapter 8, but the important part played by 

 radicals in organic compounds renders it necessary to consider them 

 more fully. 



In most compounds there is one or several groups of atoms which re- 

 main unchanged in the various reactions to which the compounds may 

 be submitted. The group behaves like a unit or an element, although 

 it cannot exist in the free state. Such groups are called radicals. 



QUESTIONS. What is organic chemistry, according to modern views ? Men- 

 tion the four chief elements entering into organic compounds, and name the 

 elements which may be made to enter into organic compounds by artificial 

 processes. State the reason why the four elements, carbon, hydrogen, oxy- 

 gen, and nitrogen, are better adapted to form a larger number of compounds 

 than most other elements. State the general properties of organic compounds. 

 Why does a qualitative analysis of an organic*com pound, in most cases, 'not 

 disclose its true nature? By what test may the organic nature of a compound 

 be established? By what tests may the^ presence of carbon, hydrogen, and 

 nitrogen be demonstrated in organic compounds? State the methods by which 

 the elements carbon, hydrogen, oxygen, sulphur, and phosphorus are deter- 

 mined quantitatively. By what general method may a formula be deduced 

 from the results of a quantitative analysis ? What is meant by an empirical, 

 molecular, and constitutional formula ; how are they determined, and what is 

 the difference between them ? 



