Concerted Mechanisms of the Reactions of Methyl Aryl Carbonates with Substituted Phenoxide Ions

Abstract

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl methyl carbonates (NPC, DNPC, and TNPC, respectively) with substituted phenoxide ions are subjected to a kinetic study in water at 25.0 degrees C, ionic strength 0.2 M (KCl). Production of the leaving groups (the nitro derivatives) is followed spectrophotometrically. Under excess of the phenoxide ions pseudo-first-order rate coefficients (k(obsd)) are found throughout. Plots of k(obsd) vs substituted phenoxide concentration at constant pH are linear, with the slope (k(N)) independent of pH. The Brönsted-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 magnitudes of these Brönsted slopes are consistent with a concerted mechanism. In the particular case of the phenolysis of NPC the expected hypothetical curvature center of the Brönsted 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 that these reactions are concerted. The phenolysis of DNPC and TNPC should also be concerted in view of the even more unstable tetrahedral intermediates 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 destabilizes the intermediate perhaps to the point of nonexistence. The k(N) values for the title reactions are larger than those for the concerted phenolysis of the corresponding ethyl S-aryl thiolcarbonates. The k(N) values found 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 coefficients being beta(N) = 0.5 and beta(lg) = -0.3, respectively. These coefficients are consistent with a concerted mechanism.

Phenolysis of diaryl thiolcarbonates and thionocarbonates

“…The values of the Brønsted slopes found for the reactions of both thiolcarbonates 1 and 2 are in accordance with those obtained in the concerted phenolysis of the following carbonates: 4-nitrophenyl, 2,4-dinitrophenyl and 2,4,6-trinitrophenyl methyl carbonates (b ¼ 0.67, 0.48 and 0.52, respectively), [9] phenyl, 4-nitrophenyl, 4-chlorophenyl and 4-methylphenyl 4-nitrophenyl carbonates (b ¼ 0.61, 0.66, 0.67 and 0.48, respectively) [10][11][12] and phenyl, 4-chlorophenyl and 4-methylphenyl 2,4-dinitrophenyl carbonates (b ¼ 0.49, 0.32 and 0.41, respectively). [10,12] The slopes of the plots in Fig.…”

“…A comparison of the nucleophilic rate constants (k N ) for the phenolysis of thiolcarbonates 1 and 2 in 44 wt% ethanol-water (this work) with those of the same reactions of phenyl and 4-chlorophenyl 4-nitrophenyl carbonates in water [10,12] shows that the carbonates are more reactive (about 80-100-fold) than the corresponding thiolcarbonates towards a given phenoxide. This is in accordance with the larger k N values found in the following reactions when a benzenethiolate leaving group is substituted by its phenoxide analogue: (i) the phenolysis of alkyl aryl carbonates [9] compared with that of alkyl aryl thiolcarbonates [13] (10-40-fold); (ii) the phenolysis of methyl 2,4-dinitrophenyl thionocarbonate, [14] relative to that of O-ethyl 2,4-dinitrophenyl dithiocarbonate; [14] (iii) the benzenethiolysis of methyl aryl carbonates [19] compared with that of ethyl S-aryl thiolcarbonates [19] (2-5-fold). Also, the reactions of secondary alicyclic amines with methyl 2,4,6-trinitrophenyl carbonate [21] show greater k N values than those with ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate.…”

“…There have been many reports on the kinetics and mechanisms of the phenolysis of esters [1][2][3][4][5][6] and thioesters, [7,8] but less regarding the phenolysis of methyl aryl carbonates, [9] diaryl carbonates, [10][11][12] O-alkyl S-aryl thiolcarbonates [8,13] alkyl aryl thionocarbonates [8,11,14] and diaryl thionocarbonates. [8,11,14] The kinetic studies on the phenolysis of methyl aryl carbonates [9] and O-ethyl S-aryl thiolcarbonates [13] show concerted mechanisms, in contrast to those of methyl aryl thionocarbonates, which show stepwise mechanisms. [11,14] To our knowledge, there are only two works in the literature on the mechanism of the phenolysis of diaryl thionocarbonates (that of bis(4-nitrophenyl) thionocarbonate [11] and phenyl 2,4-dinitrophenyl thionocarbonate [14] ) and there are no reports on the phenolysis of O-aryl S-aryl thiolcarbonates.…”

Effect of substitution of oxygen by sulfur in the nonleaving group of a carbonate: kinetics of the phenolysis and benzenethiolysis of S‐methyl aryl thiocarbonates

“…This is also true when the S and O atoms are in the leaving group: the nucleophilic rate constants (k N ) for the phenolysis of methyl aryl carbonates are larger than those of the corresponding S-aryl thiocarbonates. 3,5 Comparison of the nucleophilic rate constants for the benzenethiolysis of 2 (this study) with those for the corresponding carbonate (4) 4 shows that the reactivities of these compounds toward benzenethiolates are similar.…”

“…Comparison of the nucleophilic rate constants (k N ) between the phenolysis of thiocarbonates 1 and 2 (this work) and the phenolysis of the corresponding carbonates (3 and 4), 5 shows that the latter are more reactive (5 to 10-fold) toward phenoxides than thiocarbonates.…”