450 



D. O. JORDAN 



OH 



Fig. 2. Structure of 2-hydroxy-4,6-dimethylpyrimidine (Pitt^). 



the characteristic absorption spectrum of pyrimidines, purines, and of 

 nucleic acids is a broad band around 2400-2800 A. [Cf , Beaven, Holiday and 

 Johnson, Chapter 14.]. On increasing the pH of the sokition, a shift in Xmax. 

 to longer wavelengths is observed which has been interpreted by Loof- 

 bourow et al.^ and by Stimson and Reuter^-^ as an enolization of the pyrimi- 

 done. However, as Marshall and Walker^" point out, this shift is more prob- 

 ably due to ionization than to enolization. The problem has been consider- 

 ably clarified by the work of Marshall and Walker,'" who have examined 

 the ultraviolet absorption spectra of a number of 2- and 4 (6) -substituted 

 pyrimidines and the corresponding Ni- and Na-methylated derivatives in 

 aqueous solution at various pH values. The ^K'a value of each pyrimidine 

 was determined and the pH of the solution to be measured chosen so as to 

 be pK'o ± 2. In this way only neutral molecules or ions were present in the 

 solution and the spectra were not confused by being those of mixtures of 

 ions and neutral molecules. Marshall and Walker conclude that the shift in 

 Xmax. is due to ionization and that potential 2- and 4(6)-hydroxypyrimidines 

 should be represented in the keto form, i.e., as pyrimidones. A similar con- 



« J . R. Loofbourow, M. M. Stimson, and M. J. Hart, J. Am. Chem. Soc. 65, 148 (1943) . 

 « M. M. Stimson and M. A. Reuter, J. Am. Chem. Soc. 65, 151 (1943). 

 ^ M. M. Stimson and M. A. Reuter, /. Am. Chem. Soc. 67, 847 (1945). 



8 M. M. Stimson and M. A. Reuter, J. Am. Chem. Soc. 67, 2191 (1945). 



9 M. M. Stimson, /. Ajn. Chem. Soc. 71, 1470 (1949). 



'« J. R. Marshall and J. Walker, J. Chem. Soc. 1951, 1004. 



