Concerted mechanisms of the reactions of methyl aryl carbonates with substituted phenoxide ions

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phe noxide ions arp subjected to a kinetic study in water at 25.0 degreesC, ion ic strength 0.2 M (KCl). Production of the leaving groups (the nitro deriva tives) is followed spectrophotometrically. Under excess of the phenoxide io ns pseudo-first-order rate coefficients (k(obsd)) are found throughout. Plo ts of k(obsd) vs substituted phenoxide concentration at constant pH are lin ear, with the slope (k(N)) independent of pH. The Bronsted-type plots (log k(N) vs pK(a) of the phenols) are linear with slopes beta = 0.67, 0.48, and 0.52 for the phenolysis of NPC, DNPC, and TNPC, respectively. The magnitud es of these Bronsted slopes are consistent with a concerted mechanism. In t he particular case of the phenolysis of NPC the expected hypothetical curva ture center of the Bronsted plot for a stepwise mechanism should be pK(a)(0 ) = 7.1 (the pK(a) of 4-nitrophenol) This curvature does not appear within the pK(a) range of the substituted phenols studied (5.3-10.3), indicating t hat these reactions are concerted. The phenolysis of DNPC and TNPC should a lso be concerted in view of the even more unstable tetrahedral intermediate s 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 dest abilizes the intermediate perhaps to the point of nonexistence. The k(N) va lues for the title reactions are larger than those for the concerted phenol ysis of the corresponding ethyl S-aryl thiolcarbonates. The k(N) values fou nd in the present reactions are subjected to a dual regression analysis as a function of the pK(a) of both the nucleophile and leaving group, the coef ficients being beta (N) = 0.5 and beta (lg) = -0.3, respectively. These coe fficients are consistent with a concerted mechanism.