Rate constants, pyrazine formation

The formation of pyrazines fit a zero order reaction. Plotting concentrations of pyrazines formed versus time of reaction gave the better fit of the line, usually with a coefficient of determination (r2) of greater than 0.95. For a pseudo first order reaction, a curve rather than a line would be obtained. General least squares analysis of the data was used to compute rate constants (27). Two zero points were used for each regression. Duplicate samples were tested at the early sampling times vs. triplicate samples at later times. Each data point collected was treated separately in the regression analyses. 

Activation energies for the formation of pyrazines were calculated using the Arrhenius relationship, which relates the rate constant, k, to temperature ... 

A further relationship between reaction rate and sty has been preposed by Labuza. A linear relationship between the logarithms of reaction rate constants and ay was found to exist for many foods within a range of ay 0.3 to 0.8. Figure 4 depicts this relationship for the rate of formation of pyrazine and 2-methylpyrazine as a function of sty. Table V lists the regressions determined for the three rate constants which fell within this range. Correlation coefficients (r2) for these regressions were quite high, at 0.99 for both pyrazine and 2-methylpyrazine. 

Figure 4 Natural logarithm of rate constant for the formation of pyrazine and 2-methylpyrazine with time vs. water activity.

With [Co(NH3)5imidazole] reaction with [Fe(CN)50H2], ion pair formation with a stability constant of 670 Af" precedes inner-sphere complexation to give [(NH3)5Co(imid)Fe(CN)s] with a rate constant of 4.9 s at 25 C and 0.1 M ionic strength. Internal electron transfer with a rate of 0.165 s is faster than for corresponding pyrazine and 4,4 -bipy bridged complexes and there is an apparent inverse relationship with distance. This effect is ascribed to solvent polarization. 

Rate constants were reported for the formation of substituted pyrazinium radicals by the reduction of pyrazines with or by oxidation of hydropyrazines with Co(III) complexes and UO. ... 

Hydrogen-deuterium exchange rates of H2 and in some 3-substituted pyrazine 1 -oxides in Na0D-D20 have been correlated with a constants, and the log of the H2 exchange rates have been shown to be rectilinearly related to the pK values of these compounds (745). A considerable exchange-rate enhancement was caused by replacement of the =C4-H function in a pyridine A-oxide by a =N4 with formation of a pyrazine A-oxide (745). 

---