Equilibrium studies of 5-substituted 4-hydroxy-2-methylpyrimidines. II. Photometric titration in methanol.

Abstract

Photometric titration was performed in MeOH using nitroaniline derivatives as an indicator and fluorosulfuric acid as titrant. 5-Substituted 4-hydroxy-2-methylpyrimidine and its three methyl derivatives were used as samples. Most of the samples could be determined using this method. Some samples, 5-carbethoxy-4-hydroxy-2-methylpyrimidine (1d) and its three methyl derivatives (2d, 3d and 4d) could not be determined successfully because of the appearance of large errors. These results show that the method is undesirable for determination of a very weak base with basicity less than pKa 2. A modification of the method was attempted in order to titrate such weaker base. The dissociation constant (pKBH) of the sample in MeOH was estimated from the slope of the straight line by Type II or the modified Type II plot. The basicity of the samples in MeOH generally increased by 10 to 100 times as compared with that in water. The degrees of the increase of the basicity were different among the compounds. Much larger increase of the basicity than the other compounds (about 100 times) was found for 5-substituted 1, 2-dimethyl-4 (1H)-pyrimidones.