viii, a, 6 Pratt: Phthalides and Related Compounds 423 



instead of by introducing a metallic atom into the ring. The 

 resulting equilibrium may be represented by the formula: 



,Ph 



/ 



C-Ph. 



// \ / 



C 6 H 4 ^ S /; 



OH 







\=0 

 \)H 



A similar structure may be applied to the phthaleins dissolved 

 in sulphuric acid without invoking quinoid rearrangement. 

 Evidently the condition is unstable, and depends entirely upon 

 the free affinity of the solvent to supply energy for its main- 

 tenance. The addition of water destroys this free affinity of the 

 acid, and consequently restores the molecule to its normal color- 

 less configuration. 



It is also possible to supply the necessary energy to accomplish 

 this change in equilibrium by introducing more active centers 

 in the molecule as in the case previously considered. Meyer and 

 Fischer 25 have studied the absorption spectra of fluoran and di- 

 thiofluoran in alcohol and glacial acetic acid. The former com- 

 pound gave a curve closely resembling those here shown of the 

 phthalide type as might be expected from its analogous structure, 

 while the latter compound is deep red and gave selective ab- 

 sorption analogous to phthalophenone in sulphuric acid. These 

 authors dismiss the question of the marked difference in color 

 by attributing it to the stronger chromophoric action of the 

 thionyl groups substituted for carbonyl. This increased chro- 

 mophoric development may be further amplified by considering 

 that the greater free affinity of the sulphur atoms increases 

 the activity of the lactone ring and causes conjugation with 

 the C 6 H 4 residue. The result is then very similar to that 

 postulated for phthalophenone in sulphuric acid solution, and 

 may be represented by an analogous structural formula. 



I have recently succeeded in preparing a thiophthaloxime 

 that further strengthens this view, as it is a brilliant red and 

 gives deeply colored salts. This interesting compound is now 

 being investigated, and will be described further in a subsequent 

 paper. Other similar colored thio compounds might be men- 

 tioned, for example, dithiodiphenyl phthalide. 



* LOC. tit. 



