| Abstract
| - 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 °C and ionic strength of 0.2 M (KCl).Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics andare first order in phenoxide anion. The reactions of BNPC show a linear Brönsted-type plot withslope β = 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOCand MNPTOC show biphasic Brönsted-type plots with slopes β = 0.30 and 0.44, respectively, athigh pKa, and β = 1.25 and 1.60, respectively, at low pKa, consistent with stepwise mechanisms.For the reactions of both thionocarbonates, the pKa value at the center of the Brönsted plot (pKa0)is 7.1, which corresponds to the pKa of 4-nitrophenol. This confirms that the phenolyses of thethionocarbonates 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, thefollowing conclusions can be drawn: (i) Substitution of S- by O- in an anionic tetrahedralintermediate (T-) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T- results in anincrease of both the rate constant and equilibrium constant, for the formation of T-, and also in anenlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T-. (iii) Substitutionof an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclicamines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.
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