Kinetics of Reversible Carbon Deprotonation of 2-Nitroethanol and 2-Nitro-1,3-propanediol by Hydroxide Ion, Water, Amines, and Carboxylate Ions. A Normal Broensted .alpha. Despite an Imbalanced Transition State

Abstract

Rates of reversible carbon deprotonation of 2-nitroethanol (2) and 2-nitro-1,3-propanediol (3) by hydroxide ion, water, amines, and carboxylate ions and p values for the ionization at carbon (pA^H) and oxygen (pK^H) and ionization of the aci-forms (pK^0H) were determined in aqueous solution at 25 °C. The pÁ^H values for 2 and 3 are 8.60 and 7.68, respectively, as compared to 10.22 for CH3NO2. The acidifying effect of the CH2OH groups is attributed to a combination of inductive electron withdrawal and hyperconjugative stabilization of the respective nitronate ions, possibly coupled with intramolecular hydrogen bonding stabilization of this ion. The higher acidity of 2-nitroethanol compared to nitromethane is reflected in higher rates of proton transfer from 2-nitroethanol, implying a "normal" Brpnsted a between 0 and 1. This contrasts with the negative a value based on the reaction of OH" with nitromethane, nitroethane, and 2-nitropropane (Kresge, A. J. Can. J. Chem. 1974, 52, 1897). Reasons why a normal a value is observed in the current system are discussed.

QM/MM modelling of ketosteroid isomerase reactivity indicates that active site closure is integral to catalysis

“…). Such carbon acid deprotonation reactions have high activation barriers in solution because of the instability of the (di)enolate intermediate and the change in hybridization of the carbon (from sp 2 to sp 3 ) that is necessary during the reaction . Several other enzymes, such as triosephosphate isomerase and citrate synthase , catalyze similar proton abstractions from carbon atoms, and there is wide interest in knowing how such enzymes catalyse these reactions efficiently.…”

Assessing the Activating Effect of the (η5-C5Me5)Ru+ Group: A Kinetic and NMR Study of the Ionization of the (η6-Phenylnitromethane)- (η5-pentamethylcyclopentadienyl)ruthenium Cation in H2O−Me2SO Mixtures

“…All related Brönsted β B and α BH coefficients, e.g., β RNH2 = 0.54; β RCOO - = 0.38, fall in the range (0.5 ± 0.1) commonly found for ionization of carbon acids, suggesting that proton transfer is about half-complete at the transition states of reactions 1. To be noted, however, is that β RNH2 is somewhat higher than β RCOO -, in constrast with what is generally observed and in disagreement with the idea that this coefficient should be higher for the less thermodynamically favored carboxylate systems . Failure of the reactivity−selectivity principle in accounting for data pertaining to the ionization of carbon acids has often been noted by various authors. − Also noteworthy in Figure is that the point for N -methylaminoethanola secondary aminedoes not strongly deviate from the Brönsted line for primary amines.…”

Similar catalytic behaviour of oximate and phenoxide bases in the ionization of bis(2,4-dinitrophenyl)methane in 50% water– 50% Me2SO. Revisiting the role of solvational imbalances in determining the nucleophilic reactivity of α-effect oximate bases

“…low log k 0 values, are found in ionization reactions of carbon acids provided that the formation of the conjugate carbanions is accom- panied by an extensive structural-electronic and solvational reorganization of the carbon skeleton and that the development of this reorganization occurs behind proton transfer at the transition state. 18,19,23- 31 The data in Table 3 are illustrative in this regard. Thus, the two criteria are not met with carbon acids such as nitriles because the conjugate carbanions, even though not totally devoid of resonance, derive most of their stabilization from the polar effect of the cyano group.…”

“…18,19,36 However, it is with compounds like nitroalkanes that large imbalance effects and large intrinsic barriers are found. 18,23,24,30, 31 Due to the strong capability of a NO 2 group to absorb a negative charge, the conjugate carbonions exist as nitronate ions, implying that a very important rearrangement of the carbon skeleton occurs upon ionization. 18 As we have previously demonstrated, 25a,b,37 there is no doubt that the two criteria required to have low log k 0 values are fulfilled in the case of 1.…”

“…Costly desolvation is also the explanation as to why the points for OH Ϫ and other strong RO Ϫ bases with pK a у 11 display generally large deviations from Brønsted plots defined by less basic catalysts in the ionization of carbon acids, thereby resulting in a substantial curvature of these plots at high pK a . 18-20, [22][23][24][25][26][27][28][29][30][31]33 Based on the above ideas, the finding that ArO Ϫ and oximate bases exhibit an identical Brønsted behaviour suggest that these oxyanionic bases undergo comparable solvation changes along the reaction coordinate of equilibrium (3). To test this assumption, a comparison of the pK a values of oximes and phenols over a whole range of H 2 O-Me 2 SO mixtures is of interest.…”