1847-96-7Relevant academic research and scientific papers
Concerted mechanisms of the reactions of methyl aryl carbonates with substituted phenoxide ions
Castro,Pavez,Santos
, p. 3129 - 3132 (2007/10/03)
The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 °C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (kobsd) are found throughout. Plots of kobsd vs substituted phenoxide concentration at constant pH are linear, with the slope (kN) independent of pH. The Broensted-type plots (log kN vs pKa of the phenols) are linear with slopes β = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitudes of these Broensted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Broensted plot for a stepwise mechanism should be pKa0 = 7.1 (the pKa of 4-nitrophenol). This curvature does not appear within the pKa range of the substituted phenols studied (5.3-10.3), indicating that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates that would be formed if the reactions were stepwise. The reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate by a phenoxy group destabilizes the intermediate perhaps to the point of nonexistence. The kN values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The kN values found in the present reactions are subjected to a dual regression analysis as a function of the pKa, of both the nucleophile and leaving group, the coefficients being βN = 0.5 and βig = -0.3, respectively. These coefficients are consistent with a concerted mechanism.
