140 Biological Chemistry. 



to determine whether a given compound, C 6 H 4 (X) 2 , is an 

 ortho-, meta- t or >ara-derivative ? 



It will be seen in the sequel that it is possible to pass 

 from one benzene derivative to another by replacement of 

 the substituting groups as, for example, from C 6 H 5 (NO 2 ) to 

 C 6 H 5 (NH 2 ), to C 6 H 5 (OH), or C 6 H 6 . It is, therefore, very 

 often possible to obtain one benzene derivative of which 

 the constitution is known from another of which the con- 

 stitution is not known, simply by the replacement of one 

 substituting group by another. If this method is used, 

 however, it is still necessary to fix the constitution of 

 certain derivatives by an independent method, for it is 

 obviously of no use to convert a derivative of the type 

 C 6 H 4 X 2 into C 6 H 4 XY or C 6 H 4 Y 2 , unless the constitution 

 of the two latter derivatives can be fixed. 



Certain benzene derivatives can be employed for this 

 purpose, the most important of which are the phthalic 

 acids that is, benzene derivatives in which two hydrogen 

 atoms are replaced by carboxyl groups. The three possible 

 formulae of these are 



COOH COOH COOH 



COOH 



COOH 

 ORTHO META PARA 



Of these only one forms an anhydride 



CO 



Now acid anhydrides are known to be readily formed from 

 dibasic acids only when the carboxyl groups are near to 

 one another. This happens only in the or^/to-derivative. 



