418 The Philippine Journal of Science 1913 



It was at first thought that an equilibrium might exist between 

 the partial valencies of the two doubly bound carbonyl groups 

 of phthalic acid and that the resulting influence of this conju- 

 gation on the benzene ring, although modified and lessened by 

 hydroxyl, might still be of sufficient magnitude to result in a 

 characteristic band. In this event, the elimination of hydroxyl 

 upon anhydride formation should increase the activity of the 

 carbonyl groups and cause phthalic anhydride to produce a band 

 emphasizing this condition. Reference to figs. 1 and 2 shows 

 that the result of removing hydroxyl is to shift the band toward 

 the longer wave lengths without altering the persistence and 

 general type. It is not reasonable to suppose that the equi- 

 librium of the carboxyl groups is the same as that of the lactone 

 ring when these compounds are dissolved in solvents such as 

 alcohol or acetic acid. The fact that the absorption spectra 

 show such great similarity indicates a closer relationship between 

 the benzene ring and the band than between the side chain and 

 the band. 



In all of these compounds there exists conjugation between 

 the benzene ring and the substituting groups, and this condition 

 is primarily responsible in every case for the characteristic 

 modification of the benzene spectrum. 



This type of conjugation existing between a side chain and 

 a benzene ring was briefly discussed by Thiele 19 in his original 

 paper on partial valency, and his structural formula for phthalic 

 acid appears to be in accord with the absorption spectrum of 

 this compound. However, the peculiar equilibrium of valencies 

 depends upon two doubly bound oxygen atoms in the carboxyl 

 groups, as indicated in the formula: 



This represents the benzene ring modified by the action of 

 two partial valencies acting with side-chain influences. A similar 

 effect could be ascribed to the lactone ring in phthalic anhydride, 

 and the absorption spectra of these and analogous compounds 

 thus explained. When one double bond is destroyed by substi- 

 tuting two atoms or groups for a carbonyl oxygen, the equilibrium 

 of the resulting molecule could no longer be similarly repre- 



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



