Transient nitronic acid formation in the protonation of the carbanion of 2,2′,4,4′-tetranitrodiphenylmethane in acidic methanol

Abstract

. Can. J. Chem. 65, 1980Chem. 65, (1987.Rates of deprotonation of 2,2',4,4'-tetranitrodiphenylmethane (1) by a variety of bases B and of protonation of the resulting carbanion (2) by the conjugated acids BH in methanol have been measured at 20°C. The Bronsted aBH coefficients for protonation of 2 by phenol and carboxylic acid buffers are equal to 0.58 and 0.43, respectively, as compared with aBH values of 0.59 and 0.52 res ectively, for the same reactions in 50% H 2 0 -50% DMSO. On the other hand, a comparison of the intrinsic .rate constants k: (in the Marcus sense) in the two solvents reveals that the effect of going from 50% H 2 0 -50% DMSO to methanol is to lower ktaH for the phenol reactions by a factor of 2 but to increase k$'OoH for carboxylic acid reactions by a factor of 5. It is shown that the solvent effects on a B H and k:H can all be understood in the context of the Principle of Imperfect Synchronisation (PIS) recently proposed by Bernasconi. In addition, a fast equilibrium protonation of the carbanion 2 is found to precede its conversion to 1 at low pH, i.e., pH < 5. The resulting protonated species is suggested to be the nitronic acid 2,H arising from fast protonation of a para NOz group of 2. The pK: value associated with the ionization of 2,H is -4.25. Chem. 65, 1980Chem. 65, (1987.Les vitesses de dtprotonation du tetranitro-2,2',4,4' diphCnylmCthane (1) par diverses bases B-ainsi que les vitesses de protonation du carbanion correspondant (2) par les acides conjugues BH ant et6 mesurees a t = 20°C dans le mCthanol. Les Despite the absence of an electron-withdrawing group directly bonded to the exocyclic ionizable carbon, 2,2',4,4'-tetranitrodiphenylmethane 1 has a noteworthy acidity in methanol (1).The p~p O H value for formation of the very stable carbanion 2 is 15.30 at 20°C, i.e., 2 is half-formed in a potassium methoxide solution -0.03 M.We have taken advantage of the stability of 2 to c a n y out a comprehensive kinetic study of the reactions shown in eq. [I] over the pH range of 3-14.86 at 20°C in methanol. k F o H , k p O , and k; are the rate constants for deprotonation of 1 by methanol, methoxide ion, and any buffer base, respectively, while k", k q o H , and k!f;' refer to the rates of reprotonation of the carbanion 2 by the solvated proton, methanol, and any buffer acid, respectively. Our results allow the determination of the intrinsic reactivity of 1 in methanol and reveal the existence of a fast equilibrium between 2 and a new species in the most acidic solutions (pH < 5). This species is postulated as a nitronic acid resulting from the protonation of a NO2 group of 2. The dependence of the protonation rates of 2 upon the acidity and the nature of the protonating agent is also discussed. ResultsRates of proton transfer in methanol were measured using buffer solutions or dilute C H 3 0 K o r benzene sulfonic acid solutions, always under pseudo-first-order conditions. The For personal use only.

Structure and conformation of polynitrodiphenylmethanes and related carbanions

“…Recent measurements of the intrinsic reactivities, in the Marcus sense, of the two carbanions support this latter situation: log ko -1.10 and -0.60 for ionization of (3) and (4), respectively, by carboxylate ions in 50% H20-50% Me2S0. 13 In keeping with the commonly accepted view that the intrinsic reactivity of carbon acids is primarily a measure of the extent of structuralelectronic-solvational reorganization required for carbanion formation,6.1 2,14,15,46 the low k , values found for (3)and (4)indicate that both carbanions have high energy of reorganization. This implies an important delocalization of the negative charge over the two phenyl rings in (3)and (4)and therefore an almost planar arrangement.…”

Transition State Imbalances in the Deprotonation of Picrylacetophenones by Carboxylate and Phenoxide Bases

“…Thus, the appropriate scheme for the conversion of C-3 to 3 at low pH is shown in eq 5 and the measured k obsd values for this process are expected to obey the general equation (6) in buffer solutions (Table S2) and the reduced equation (7) in HCl solutions (Table S6): 11,12 In agreement with eq 7, curvilinear k obsd vs [H + ] plots were obtained from measurements in HCl solutions (not shown, see Table S6, supporting information). Taking into account that the solvent contribution (k …”

The aci-nitro equilibrium of picrylacetone: a kinetic and thermodynamic study in 50∶50 and 30∶70 (v/v) H2O–Me2SO mixtures †

“…4 Although we failed in characterizing C-2H by UV-Visible spectrophotometry, the K a CH , K a NO 2 H and K T values pertaining to Scheme 1 could be derived. 4 So far, aci-nitro equilibria involving nitrotoluene-type carbon acids have been essentially studied photochemically. 5- 7 NMR studies of the deprotonation of picrylketones 3a-d in pure Me 2 SO have revealed that these carbon acids exist essentially in their trinitro forms rather than as the nitronic acids in neutral media and that the exocyclic carbonyl functionality plays a minor role in the stabilization of the conjugate carbanions C-3a-d. 8,9 In other words, the weight of the enolate resonance structure E is negligible as compared with that of the three trinitrocyclohexadienyl structures F-H.…”

“…2, 3 In this regard, a kinetic and thermodynamic study of the reprotonation of the carbanion C-2 in acidic medium in methanol was very revealing, providing evidence for the transient formation of a nitronic acid that we formulated as C-2H. 4 Although we failed in characterizing C-2H by UV-Visible spectrophotometry, the K a CH , K a NO 2 H and K T values pertaining to Scheme 1 could be derived. 4 So far, aci-nitro equilibria involving nitrotoluene-type carbon acids have been essentially studied photochemically.…”

“…Fig 4. Brønsted plots for deprotonation of 3a by phenoxide and carboxylate ions and primary amines in 30% H 2 O-70% Me 2 SO (v/v); T = 25 ЊC, I = 0.5 mol dm Ϫ3 NMe 4 Cl.…”