1007-37-0Relevant academic research and scientific papers
Kinetic study of the phenolysis of O-methyl and O-phenyl O-2,4-dinitrophenyl thiocarbonates and O-ethyl 2,4-dinitrophenyl dithiocarbonate
Castro, Enrique A.,Arellano, David,Pavez, Paulina,Santos, Jose G.
, p. 6192 - 6196 (2007/10/03)
The reactions of a series of phenols with O-methyl O-2,4-dinitrophenyl thiocarbonate (MDNPTOC), O-phenyl O-2,4-dinitrophenyl thiocarbonate (PDNPTOC), and O-ethyl 2,4-dinitrophenyl dithiocarbonate (EDNPDTC) are studied kinetically in water, at 25.0 °C and an ionic strength of 0.2 M (KCl). All reactions show pseudo-first-order kinetics under an excess of phenol over the substrate, and are first order in phenoxide anion. The reactions of EDNPDTC show a linear Bronsted-type plot of slope β = 0.67, suggesting a concerted mechanism. On the other hand, the phenolyses of MDNPTOC and PDNPTOC exhibit linear Bronsted-type plots of slopes β = 0.27 and 0.28, respectively, consistent with stepwise mechanisms where the formation of an anionic tetrahedral intermediate (T-) is rate determining. By comparison of the kinetics and mechanisms of the reactions under investigation with similar reactions, the following conclusions arise: (i) Substitution of S- by O- in the intermediate T- destabilizes this species. (ii) The change of DNPO in T- to DNPS also destabilizes this intermediate. (iii) Substitution of MeO by PhO as the nonleaving group of the substrate does not affect the kinetics, probably by a compensation of electronic and steric effects. (iv) The change of an amino group in a tetrahedral intermediate to a phenoxy group destabilizes the intermediate.
Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate
Castro,Angel,Arellano,Santos
, p. 6571 - 6575 (2007/10/03)
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 and are first order in phenoxide anion. The reactions of BNPC show a linear Broensted-type plot with slope β=0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Broensted-type plots with slopes β=0.30 and 0.44, respectively, at high 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 BrSnsted plot (pKa0) is 7.1, which corresponds to the pKa 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 ofT-, 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.
