456 D. O. JORDAN 



the dimensions of the uracil molecule are similar to those of 2-hydroxy-4,6- 

 dimethylpyrimidine/ gives the low value of 1.57. It would therefore appear 

 that the weakness of the second dissociation cannot be due to the electro- 

 static field effect and must be caused by a real difference in the groups. 

 This conclusion is not entirely unexpected, since NiH lies between two 

 carbonyl groups, whereas NjH is attached to a carbonyl group and a CH 

 group. This view is also supported by the extreme weakness of the second 

 dissociation of cytosine, pK'a^ 12.1, as compared with that of isocytosine 

 (XIII), pK'a^ 9.42, which suggests that the 2-carbonyl group in uracil pro- 

 motes stronger basic properties in the — NH — CO — grouping than does the 

 6-carbonyl group. This conclusion is sinilar to that reached by Ogston^^ in 

 the somewhat analogous case of the xanthines. 



The introduction of the methyl group in the 5-position of uracil in 

 thymine has little effect on the pK'a value. The basic properties of cytosine 

 and isocytosine can be ascribed to the amino group, the pK'a values being 



O 



II 

 C 



/ \ 



N CH 



II II 



C CH 



/ \ / 



H2N N 



H 



XIII 



comparable with those of the aromatic amines; this view has been con- 

 firmed by titration in the presence of formaldehyde.^"* 



The dissociation constants of adenine and guanine are given in Table I. 

 There is considerable evidence that the weakest dissociation of adenine and 



guanine represents the dissociation — NH — to — N — in the imidazole 

 ring. Benzimidazole, which is analogous to purine, has been found by 



Taylor^^ to have a basic association of pK'a^ 5.30 and an acidic dissociation 



+ 



of pK'a2 12.3. These values must represent the dissociation of the — NH= 

 and — NH— groups, respectively. The value of pK'ai 5.30 may be com- 

 pared with that of 7.1 given by Dedichen^" for imidazole, whereas the 

 value of 12.3 is of the same order as those of the weakest dissociations of 

 guanine, 1- and 3-methylxanthine, and of hypoxan thine, all of which are 

 unsubstituted in the imidazole ring. As would be expected, 7- and 9-methyl- 



28 A. G. Ogston, /. Chem. Soc. 1935, 1376. 

 " H. F. W. Taylor, J. Chem. Soc. 1948, 765. 

 »» G. Dedichen, Ber. 39, 1831 (1906). 



