vni, a, 6 Pratt: Phthalides and Related Compounds 421 



phthalic anhydride giving phthalide effects no analogous change, 

 but merely lessens the activity to a slight degree. The opinion 

 seems justified that some interaction does take place between 

 the two carbonyl groups of the lactone ring, but that it is 

 subordinate to other influences which may be attributed to 

 internal activity of the carbonyl group. 



The solutions in concentrated sulphuric acid present very 

 marked contrasts to those where alcohol or glacial acetic acid 

 is employed as solvent. In many cases an entirely new band 

 makes its appearance, and the existing spectrum is further al- 

 tered by a shift of the original band. Alcohol possesses a low 

 degree of residual affinity, while sulphuric acid shows this prop- 

 erty in a marked degree. The effect of the latter solvent is to in- 

 crease the energy of the solute, thus causing the absorption band 

 to shift toward the longer wave lengths. The only exception 

 noted was with maleic anhydride, in which case the sulphuric 

 acid solution showed less-marked selective absorption than the 

 acetic acid. Here the increased activity was manifested by the 

 band appearing at much lower concentration than in glacial 

 acetic acid solution. 



The spectra of phthalic acid and anhydride in sulphuric acid 

 are especially interesting. Both substances give the same curve 

 with two well-marked bands. It was thought possible that the 

 small amount of acid used might be dehydrated and consequently 

 be entirely converted into phthalic anhydride. This is not the 

 case, as was clearly shown by adding a hundredth molar sul- 

 phuric acid solution drop by drop to ice-cold absolute alcohol, 

 thus giving a thousandth molar concentration. This solution 

 was photographed at once, and the resulting curve coincided with 

 phthalic acid and not with the anhydride. If the latter com- 

 pound had resulted from dehydration by the concentrated sul- 

 phuric acid, it would not have been hydrolyzed by cold alcohol 

 and its presence could have been detected on the photographic 

 plate. The cancelling effect of hydroxyl in phthalic acid has 

 been prevented by the high residual affinity of the solvent. 



It is also significant that in sulphuric acid solutions a band 

 frequently makes its appearance in the benzene region. In the 

 case of phthalic anhydride the original band is shifted from 



r = 3440 to 3300 and a benzene band at T = 3800 is produced. 



A A 



The two opposing influences previously merged and resulting in 

 a single band representative of the equilibrium thus produced 

 have now been resolved by increasing the energy of the lactone 



