**p-." When the compound is complex it is customary to number 

 the corners of the ring thus : 



In naming a compound in this way, the number of the corner to 

 which a group is attached is given immediately before the name of 

 the group. Thus, the three compounds shown above may be 

 called respectively: 1, 2-dichlorobenzene, 1, 3-dichlorobenzene and 

 1, 4-dichlorobenzene. 



There is another type of substitution in the benzene ring which 

 is very important in dye chemistry. Two atoms or groups having 

 two valency bonds instead of one may also replace two hydrogen 

 atoms, provided the replacement takes place simultaneously and 

 the hydrogen atoms replaced are situated either in the ortho or in 

 the para position to each other. Thus two oxygen atoms (which 

 are bivalent) may replace two hydrogen atoms (which are mono- 

 valent) forming the compound known as quinone CeHA, the 

 formula for which is 



or as commonly written 







M 



C C 



U I 



c C 







In printed formulae, such as those that follow in this book, the 

 quinone ring is often abreviated still further by omitting the double 

 bonds within the ring. The substituent atoms or groups may or 

 may not be alike, so long as both have two valency bonds entering 

 into the combination. This type of substitution involves a rear- 

 ranging of the double valency bonds in the benzene ring; and in 

 compounds of this type, called quinoid compounds, the double 

 bonds are supposed to be fixed, not mobile as in benzene. This 

 change of the valency bonds takes place very readily in many 

 dyes, and certain peculiarities of their behavior are explained by 

 it; (see for example p. 84). 



Three mono-substitution products of benzene are of importance 

 in considering the structure of dyes, namely; toluene or methyl- 



13 



