four atoms of hydrogen which has a valency of one. The simplest 

 and best way of expressing these facts by a structural formula is 

 shown in the figure: 



c c 



I D 



c c 



The double bonds in this ring cannot be considered as stationary; 

 for, if they were, a compound in which two adjacent hydrogen 

 atoms had been replaced by other elements or radicals should 

 occur in two different forms according to whether there were a 

 single or a double bond between the two carbon atoms to which the 

 substituting elements or groups were attached — which never proves 

 to be the case. Hence the double bonds must be considered as 

 mobile, each pair continually oscillating back and forth between 

 the carbon atom bearing it and the two adjoining carbon atoms. 

 In practice the formula for benzene is abbreviated to a simple 

 hexagon : 



in which each corner represents a carbon atom. If no chemical 

 symbol is placed outside the ring at any corner, it is understood 

 that an atom of hydrogen is attached at that point. This configura- 

 tion is spoken of as the "benzene ring." When the symbol of some 

 element or radical is written at a corner, it means that the hydrogen 

 atom at that point has been replaced by the element or radical to 

 which the symbol refers. 



When two hydrogen atoms are replaced there are only three 

 possible positions in the molecule which the replacing groups, or 

 substituents, can take, as shown by the following figures, using 

 chlorine as the substituent: 



CI CI 



Cf 



CI 



In the first formula, the substituents are said to be in the "ortho" 

 position to each other; in the second they are in the "meta" posi- 

 tion, and in the third in the "para" position. These three com- 

 pounds are called respectively: ortho-dichlorobenzene, meta- 

 dichlorobenzene, and para-dichlorobenzene. The three prefixes are 

 commonly shortened to the respective initials: "o-" "m-," and 



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