IX, A, 2 Pratt and Brill: Phthalide Compounds 119 



much more nearly approaches the visible region, and the 

 spectrum clearly indicates increased activity of the lactone ring. 

 The spectrum of the sulphuric acid solution is particularly 

 significant. The single band of the acetic acid solution has 

 been separated into 3 bands, one nearer the red and two in 



the far ultra-violet. The shift of the band from - = 3340 



1 ^ 



to - = 3130 is slightly greater than in the case of phthalic 



A 



anhydride under similar conditions, and is entirely analogous, 

 although the increase of persistence and the difference in con- 

 centration at which it appears are both decidedly less marked. 



The more refrangible band given by phthalic anhydride in 

 sulphuric acid solution is represented by two smaller bands 



in its thio-derivative. The small band heading at - =3860 



A 



closely approximates the position of that given by phthalic 



anhydride in this solvent at - = 3800, but the latter is of 



A 



considerable width and depth while the former is small and 



1 

 very shallow. The band at =4200 has no counterpart in the 



A 



spectrum of phthalic anhydride. It is probable that the band 

 of this compound represents a combination of the two bands 

 in near-by regions given by thiophthalic anhydride and that 

 the substitution of sulphur for oxygen results in a sufficient 

 increase of activity to permit their separation by sulphuric acid. 

 This does not conflict with the theory advanced to explain 

 the action of sulphuric acid on compounds of this type. The 

 two carbonyl groups are still present in the thio-anhydride, and 

 one would expect greater activity of the lactone ring in ordinary 

 solvents and corresponding changes in sulphuric acid. 



We have not been successful in various attempts to replace 

 carbonyl by thionyl, using methods similar to those employed 

 for thiophthaloxime or dithiophthalimide. Richard Meyer ^ 

 states that he obtained similar negative results when the 

 anhydride was fused with phosphorous pentasulphide. The 

 chemical behavior and absorption spectrum of thiophthalic 

 anhydride leave no doubt but that the sulphur atom replaces 

 the anhydride oxygen, giving a symmetrical molecule. Since 

 thiophthalic anhydride reacts readily with hydroxylamine elim- 



^Ber. d. deutsck. chem. Ges. (1900), 33, 2574. 



