Pyrazine ionization constant

Protonation of pyrazine A-oxides takes place at the unsubstituted ring nitrogen as revealed by examination of their UV spectra and ionization constants in water. The same holds for unsubstituted quinoxaline A-oxide and the 3-amino derivative. Pyrazine and quinoxaline di-A-oxides are protonated at one A-oxide oxygen atom (74KGS1554). 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

Ionization. Ionization constants for pyrazine and several C-methylated derivatives have been redetermined for possible correlation with the polarographic half-wave potentials of the same compounds and their 1-alkyl iodides.1373... 

Ionization constants. Known pKt values for pyrazine and six methylated derivatives showed good correlation with newly calculated electron densities on the nitrogen atoms 1052 such a correlation was also observed for 1,4-di-methylpiperazine and a series of (distantly) related m- and p-bis(dimethy-laminomethyl)benzene derivatives.1039... 

Ionization constants of pyrazine compounds are summarized by Barlin (B-82MI 603-0l>. The basicity constants of pyrazine and its methyl derivatives in water have also been measured, and show a linear inverse relationship with polarographic half-wave potentials <85JHC1143>. 

Second-order rate constants have been determined for the alkaline hydrolysis of 2-methoxycarbonylpyrazine in methanol-water (80%w/w) at 10° as 5.2x10 l/mol sec (1263) and for 2-ethoxycarbonylpyrazine at 30° in 50 and 60% dimethyl sulfoxide-water and 60, 70, and 80% ethanol-water as 95.9, 199, 12.4, 10.2, and 8.86x10 1/mol sec (1359). The failure of 2,5-dimethoxycarbonyl-pyrazine to undergo the Schmidt reaction has been accounted for in terms of its normal ionization, and I values in 100% sulfuric acid gave an average of 2.55 (1176). 

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