viii, a, 6 Pratt: Phthalides and Related Compounds 425 



oxygen and sulphur makes it possible for sulphuric acid to sup- 

 ply considerable energy to the side chain of compounds such as 

 phthalic anhydride and the phthalids. No such affinity exists 

 between the latent valencies of nitrogen and sulphur, as was 

 pointed out by Thiele 26 in explaining the addition of sulphurous 

 acid to quinone diimide where the acid residue attaches itself, 

 not to the nitrogen, but to the carbon. The condition of all these 

 compounds in concentrated sulphuric acid is different from true 

 chemical combination only in degree. The attraction between 

 the acid and the molecule is insufficient to produce a stable ad- 

 dition product capable of isolation, but the first phase of such 

 combination is nevertheless reached. The tendency is most 

 marked in the molecules here considered that still retain either 

 the two original carbonyl groups or equivalent doubly bound 

 atoms possessing attraction for the latent valency of sulphur, 

 and least in those such as phthaloxime where not only has one 

 carbonyl oxygen been removed, but has been replaced by a 

 nitrogen atom. The action of sulphuric acid becomes almost neg- 

 ligible in this case, and the absorption spectra in acid or alcohol 

 are very similar. 



Phthalanil retains the carbonyl groups in addition to having 

 nitrogen in the lactone ring, and consequently shows a marked 

 shift in the position of its absorption band by sulphuric acid. 

 No new band is developed in the benzene region, however, as 

 the amount of energy supplied by the acid is not sufficient, in the 

 presence of the opposing nitrogen, to separate the two influences 

 into their components. 



When one carbonyl oxygen of phthalanil is replaced by two 

 phenyl groups, the effect is further reduced. The band in the 

 ultra-violet is not shifted by sulphuric acid in this case, and so 

 little energy is supplied to the molecule that only slight conjuga- 

 tion takes place between the remaining carbonyl and the phenyl 

 groups. The color band must head at the same place as it is 

 caused by the same mutual influence, but it now appears at log. 

 3.3 instead of log. 1.5 and is greatly reduced in persistence. 



SUMMARY 



1. The absorption spectra of various derivatives of phthalic 

 acid representing the phthalide and lactone type have been 

 studied in ordinary solvents and in concentrated sulphuric acid. 



"Ann. d. Chem. (Liebig) (1899), 306, 130. 



123668 4 



