vni, a, 6 Pratt: Phthalides and Related Compounds 419 



sented. Nevertheless, phthalide gives an absorption spectrum 

 closely resembling that of phthalic acid. This serves to em- 

 phasize the fact that absorption spectra are caused by types of 

 force equilibria, each giving characteristic manifestations, but 

 which are often incapable of rational expression by the struc- 

 tural formulas employed at present. Phthalids belong to the 

 same type as phthalic acid and its anhydride, and the change 

 from acid to anhydride involves no fundamental alteration in 

 the arrangement of forces within the molecule. This lends 

 support to the equilibrium between phenolphthalein as a lactone 

 and its acid as advanced by Kober and Marshall. 20 



Maleic anhydride possesses a structure corresponding to the 

 side ring of phthalic anhydride during that benzene phase when 

 the double bond occurs between the corresponding carbon atoms. 



S\_ c //° HC-C^ 



I !!_ c >0 ' II > 

 \/ c \ HC -%) 



Phthalic anhydride. Maleic anhydride. 



Maleic anhydride, however, shows very little selective absorp- 

 tion, and gives no well-defined band, but rather a rapid extension 

 of transmitted light occurring at high concentration. The effect 

 of introducing this group into the benzene ring corresponds very 

 well with what might be expected. The equilibrium within a 

 compound of two well-defined tendencies, one capable of pro- 

 ducing a step-off at about- =3400, the other giving well-marked 



A 



banded absorption between - = 3724 and - = 4200, might well 



A A 



result in selective absorption as shown by phthalic anhydride. 

 The character of the resulting absorption would represent an 

 equilibrium between the ortho disubstituted benzenes with 

 saturated groups as in o-xylene and the anhydride influence 

 shown in maleic anhydride. 



The band is shifted toward the red by the latter influence, 

 and phthalic anhydride, therefore, gives a less refrangible band 

 than o-xylene. The effect is less marked in phthalic acid be- 

 cause the influence of the side chain is reduced by hydroxyl 

 groups that cancel a large part of its residual affinity and leave 

 the band nearer the normal benzene region. The opposite effect 



20 Rep. Eighth Int. Cong. Applied Chem. (1912), 6, 157. 



