114 



AARON BENDICH 



(hence only "apparent") since in the spectrophotometric method the ac- 

 tivity coefficients (which probably do not differ greatly from unity, inas- 

 much as the concentrations are between 10"^ and 10~^ molar) are not 

 known and in the titrimetric technique (concentrations from lOr^ to 

 IQ-i molar) the liquid junction potentials are also not known. Nonethe- 

 less, the agreement (Table II) by the two methods is good. 



The pKai values for the parent compounds pyrimidine and purine are a 

 measure of the basicity of ring nitrogen and are considerably lower than 

 those for pyridine (5.23) and imidazole (7.03). (For a discussion of the 

 basicity of nitrogen-containing heterocycles, see Albert et al.^^^) The pK„, 

 value of purine (8.9) concerns the removal of a proton from the imidazole 

 portion of the neutral molecule as does the pK^, (9.80) for adenine. While 



the pKa, for adenine (4.15) and guanine (3.3) refer to the amino groups 



+ 



( — NH3 ;:± — NH2 + H+), it is not yet established which group is involved 

 in the second and third dissociations for guanine. 



As for the derivatives containing potentially tautomeric groups, the 

 question arises whether, for example, it is the carbonyl form (A) 



^ 



N 



H 

 A 



O 



/- 



N 



+ H^ 



A' 



or the enol form (B) 

 C 



\ 



\ 



O N "~ HO N "~ "O N 



+ H+ 



H 



B 



which dissociates in aqueous solution; i.e., does enolization precede ioniza- 

 tion? Since the ionic forms A' and B' are resonance hybrids, they are not 

 capable of independent existence, and it is not to be expected that ultra- 

 violet spectroscopy can furnish a direct answer. Although the hypothetical 

 intermediate or uncharged enol (lactim) form may exist in solution, defini- 

 tive evidence for such existence has not been demonstrated by ultraviolet 

 spectroscopy. This might be due to its presence in too small a quantity, or 

 to the possibility that its spectrum might be the same as that for the anionic 



32^ A. Albert, R. Goldacre, and J. Phillips, J. Chem. Soc. 1948, 2240. 



