Kinetic Study of the Phenolysis of Bis(4-nitrophenyl) Carbonate, Bis(4-nitrophenyl) Thionocarbonate, and Methyl 4-Nitrophenyl Thionocarbonate

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 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.

Leaving-Group Substituent Controls Reactivity and Reaction Mechanism in Aminolysis of Phenyl Y-Substituted-Phenyl Carbonates

“…Substrates 5a-5k were readily prepared from the reactions of phenyl chloroformate with Y-substituted phenol in the presence of triethylamine in anhydrous ether as reported previously. 19 The crude products were purified by column chromatography and the purity was checked by their melting points and 1 H NMR spectra. Piperidine and other chemicals were of the highest quality available.…”

Kinetic Study on Aminolysis of O‐2‐Pyridyl Thionobenzoate in Acetonitrile: Effect of Changing Electrophilic Center from CO to CS on Reactivity and Reaction Mechanism

“…have reported that the lifetime of T ± is longer for reactions of substrates possessing a CS electrophilic center than for those of substrates bearing a CO electrophilic center . Because the tendency of the CS − moiety in T ± to form a CS bond (and to expel the leaving group) is weaker than that of the CO − moiety due to a weaker π‐bonding energy of the thiocarbonyl group relative to the carbonyl group . Thus, one can suggest that the difference in the lifetime of T ± is responsible for the difference in the reaction mechanism observed for the reactions of the CO and CS compounds, e.g .…”

Kinetics and Mechanism of Alkaline Hydrolysis of Y-Substituted Phenyl Phenyl Carbonates

“…Substrates 2a-j were readily prepared from the reaction of phenyl chloroformate with Y-substituted phenol in the presence of triethylamine in anhydrous ether as reported previously. 19 Their purity was confirmed from melting point and spectral data such as 1 H NMR. DMSO and other chemicals were of the highest quality available.…”