Kinetics and mechanism of the pyridinolysis of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates

The Journal of Organic Chemistry
Enrique A CastroJosé G Santos

Abstract

The reactions of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) with a series of 3- and/or 4-substituted pyridines in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the release of 4-nitrophenoxide (400 nm) or 2,4-dinitrophenoxide (360 nm) anions. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [pyridine] are linear and pH-independent, with slope kN. The Brønsted-type plot (log kN vs pKa of pyridinium ions) for the reactions of 1 is linear, with slope beta = 1.1, in contrast to the plot for the reactions of 2, which is biphasic, with slopes beta1 = 0.25 (high pKa) and beta2 = 0.90 (low pKa) and the curvature center at pKa = p = 7.3. The latter Brønsted plot is consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T+/-) on the reaction path, and a change of the rate-determining step, from breakdown to formation of T+/-, as pyridine basicity increases. For the reactions of 1 the beta value indicates that the mechanism is also stepwise with expulsion of the nucleofuge from T+/- as the rate...Continue Reading

References

Oct 30, 1974·Journal of the American Chemical Society·A C Satterthwait, W P Jencks
Jan 10, 1997·The Journal of Organic Chemistry·Enrique A. CastroJosé G. Santos
Apr 18, 1997·The Journal of Organic Chemistry·Enrique A. CastroJimena Téllez
Oct 25, 2001·The Journal of Organic Chemistry·Enrique A. CastroJosé G. Santos
Oct 25, 2001·The Journal of Organic Chemistry·Enrique A. CastroMaría I. Umaña

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Citations

Sep 30, 2006·Chemistry : a European Journal·Frank BrotzelHerbert Mayr
Mar 25, 2005·The Journal of Organic Chemistry·Enrique A CastroJosé G Santos
Apr 23, 2005·The Journal of Organic Chemistry·Enrique A CastroJosé G Santos

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