( 2fi7 ) 



already been made in a miinhi'r of cases of nitralion of the sub- 

 stances C„H,X. 



For tlie present we must content onrselves witl» accepting the 

 resnlts of those (piaiititative shulies as facts. Doing this, we may 

 pnt the foIh)wing (piestion: (iiven a benzene derivative O^H^XY, 

 in which a Ihii-d gronj) Z is introduced. If now we know the 

 relative quantities of the isomers of both C„1I^XZ and (!„H^YZ which 

 are formed by tlie introchiction of Z into (VHr,-"*^ '^'J^' '>i*f' ^'«l^^s^ : 

 can we then deduce from tiiis Ihe sjrnctnre and relative (piantities 

 of the isomers C^H^XYZ which are formed by Ihe introduction of 

 Z into C«H,XY? 



Suppose (by way of an exam})le) that we ha\e determined how 

 mucjh para- and ortho-compound is formed in the nitration of chloro- 

 benzene and how much meta- and ortho-benzoic acid in the nitration 

 of benzoic acid; can we then determine beforehand \vliich and how 

 much of the possible nitrochlorobenzoic acids will be formed in the 

 nitration of chlorobenzoic acid? 



(Qualitatively this problem has been studied rather fully, bnl as a 

 rule not very systematically. In a great many cases it has been 

 determined which of the possible isomers C^H^XYZ are formed by 

 the introduction of Z into C^H^XY and one has tried to draw con- 

 clusions therefrom which render it j)0ssible to predict what may be 

 ex])ected in unknown cases. Beilstein has snnnnarised these as 

 follows: "In the introduction of a group Z into a substance ('„II^XY 

 both X and Y exert an iiitlnence but that of one of these gronps 

 is predominant and directs Z." 



Undoubtedly, this rule is correct in a great many cases, ImiI nol 

 in a good many others. For ijistance it cannot be ai)plie(l to the 

 nitration of m-nitroanisol, which 1 have in\"estigaled. In any case it 

 shows that the gronps X and V do nol exert their dii-ecting intluence 

 independently of each oilier bul llial this is uiodilied i>y iheir 

 simultaneous preseiu'e. This has been fully coulirnied by a (pianti- 

 tative investigation in the case of a jiuuibei- of nil rations of Ihe 

 compounds C^H^XY. 



If the groups X aiul V (wei-led a directing influence on a third 

 substituent iiulependently of each other we ought to have the following: 

 If we call the proportion in which the ihvoc isomers are foi-med 

 when Z is introduced into (\5ll.X 



Portho ■ *^lmot:i • I'para 



and that of the three isomers when Z is introduced into C^HjY 



P ortho ' ^1 meta • ^ para 



