Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate.
The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC) are subjected to a kinetic investigation in water, at 25.0 degrees C and ionic strength of 0.2 M (KCl). Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and are first order in phenoxide anion. The reactions of BNPC show a linear Brönsted-type plot with slope beta = 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Brönsted-type plots with slopes beta = 0.30 and 0.44, respectively, at high pK(a), and beta = 1.25 and 1.60, respectively, at low pK(a), consistent with stepwise mechanisms. For the reactions of both thionocarbonates, the pK(a) value at the center of the Brönsted plot (pK(a)(0)) is 7.1, which corresponds to the pK(a) of 4-nitrophenol. This confirms that the phenolyses of the thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate. By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the following conclusions can be drawn: (i) Substitution of S(-) by O(-) in an anionic tetrahedral intermediate (T(-)) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T(-) results in an increase of both the rate constant and equilibrium constant, for the formation of T(-), and also in an enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T(-). (iii) Substitution of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.
Castro
E A
EA
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile. ecastro@puc.cl
Angel
M
M
Arellano
D
D
Santos
J G
JG