IX, A, 2 Pratt and Brill: Phthalide Compounds 121 



Phthalimide itself shows a tendency toward similar changes 

 as evidenced by the marked widening of its absorption band 

 upon salt formation. The attraction between oxygen and metal 

 must be considerably less than that between sulphur and metal ; 

 consequently, the compound with two carbonyl groups undergoes 

 less rearrangement of its partial valencies upon the introduction 

 of a metal atom into the molecule. 



The changes characteristic of salt formation in the case of 

 dithiophthalimide are thus diametrically opposite to correspond- 

 ing changes with phthaloxime. The former compound is red 

 with a band in the visible spectrum, while its alkali salts are 

 reddish yellow and produce no color band. The latter is color- 

 less, and gives no selective absorption in the color region, while 

 its salts are red and show a strong color band. This is the 

 most striking example thus far encountered in confirmation of 

 the theory explaining color formation as dependent upon shifting 

 of partial valency equilibrium when phthaloxime is converted 

 into its salts. It also contributes valid evidence in favor of the 

 unsymmetrical structure of phthaloxime, since dithiophthalimide 

 is undoubtedly symmetrical and would necessarily show an 

 analogous absorption spectrum if it possessed a structure similar 

 to that of the oxime. 



The spectrum given by pyridine solutions of dithiophthalimide 

 is also of special interest. It contains a well-marked color band 

 similar to that given by alcohol solutions but appearing at less 

 concentration, and the band characteristic of alkali salts heading 



at - = 2600 is represented by a step-off (fig. 2) . The results of 

 A 



pyridine on dithiophthalimide are thus the reverse of those 

 characteristic for phthaloxime in this solvent.^ The latter com- 

 pound in pyridine solution gives no color band, because this base 

 is not sufficiently active to cause the partial valency equilibrium 

 necessary for its genesis- In the case of dithiophthalimide, also 

 in pyridine solution, the color band does not disappear from the 

 spectrum as in the alkali salts, because again the base does not 

 possess enough residual affinity to upset the conjugation already 

 taking place between the two thionyl groups. This reversal of 

 the effects caused by pyridine is what might be anticipated if 

 the correctness of our views regarding partial valency equili- 

 brium be granted, but is otherwise difficult to explain. 



The presence of thionyl replacing carbonyl cannot be respon- 

 sible for the reversal of the effect produced by alkali. Thio- 



° Pratt and Gibbs, loc. cit. 



126870 2 



