Thermodynamic and kinetic acidity properties of nitroalkanes. Correlation of the effects of structure on the ionization constants and the rate constants of neutralization of substituted 1-phenyl-1-nitroethanes

Abstract

Electron-withdrawing substituents in meta and para positions increase whereas electron-donating substituents decrease the dissociation constants and the second-order rate constants for neutralization of l-phenyl-lnitroethanes by hydroxide ion in 50% (vol) dioxane-water. The ionization constants (PKa) and the neutralization rate constants (log kz) correlate with u constants for aqueous and for 50% (vol) dioxane-water systems. There is linear correlation of the rate constants (log k2) for neutralization and the ionization constants (pK.) of the l-phenyl-1-nitroethanes with a Brpnsted coefficient of 1.17-1.20. The Brmsted coefficient of greater than unity stems from the fact that as the electron-withdrawing capacity of the substituent is increased, the relative effects on the rate constants of neutralization of substituted 1-phenyl-1-nitroethanes are greater than on the ionization constants. meta and para electron acceptor groups accelerate and electron donor groups retard reversal of l-phenyl-l-ethanenitronates to 1-phenyl-1-nitroethanes. The Brqnsted coefficient for correlation of the logarithms of the rate constants of hydrolysis of 1-phenyl-1-ethanenitronates and thepK,'s of their 1-phenyl-1-nitroethanes is -0.17 -(-0.20).Electron-withdrawing meta and para substituents in a 1-phenyl-1-ethanenitronate result in electronic absorption at longer wavelengths. The long wavelength ultraviolet maximum of a 1-phenyl-1-ethanenitronate is shifted bathochronically in poor hydrogen bonding and in aprotic polar solvents. The rate of reversal of a l-phenyl-l-ethanenitronate upon introduction of an electron donor meta or para substituent may be retarded because the carbonitronate behaves relatively electron deficiently at CY carbon as a result of increased hydrogen bonding at oxygens of enhanced nucleophilicity and because of the increased work involved in breaking the hydrogen bonds. For salts of simple nitroalkanes there is a shift in the infrared absorption for C-N to longer wavelength as the charge density of the cation is decreased. Replacement of methyl groups for hydrogen in nitronates results in maximum ultraviolet absorption at shorter wavelengths; solvents of diminished hydrogen-bonding ability or of increased polarity lead to absorption at longer wavelengths. The effects of structure and environment on the properties of alkanenitronates are analogous to those for 1-phenyl-1-ethanenitronates. eutralization of acidic mononitro compounds is N subject to general base catalysis and there is Brdnsted correlation between the rates of neutralization of such nitro compounds and the strengths of unhindered bases.2 Previous studies of the rates of neutralization (or ionization) and the dissociation constants of nitroalkanes have been limited. The results are of interest however in that nitro compounds which are weaker (or stronger) may neutralize (or ionize) faster (slower) than their stronger (or weaker) homologs.(1) (a) (d) initial studylb of the equilibrium properties, the rate constants, and Hammett correlation for neutrali...

Identity Proton-Transfer Reactions from C−H, N−H, and O−H Acids. An ab Initio, DFT, and CPCM-B3LYP Aqueous Solvent Model Study

“…However, Yamataka et al also made fairly high level calculations (B3LYP/6-31+G*) of the activation and reaction free energies for the reaction of CN - with three arylnitromethanes, the unsubstituted compound, and the p -methoxy and p -nitro derivatives. The resulting plot of Δ G ⧧ versus Δ G RXN was accurately linear with a slope, α = 0.51, in marked contrast to the anomalous aqueous solution value of 1.3−1.5 (depending on the base used). , …”

“…The Nitroalkane Anomaly. As discussed in the Introduction, there are two classic experimental examples of transition state imbalance, both having to do with deprotonation of nitroalkanes. − The magnitude of the anomaly is less in solvents unable to act as H-bonding donors to the negative oxygens of the nitronate anions . In fact, the order of the acidities of nitromethane, nitroethane, and 2-nitropropane is itself solvent dependent, tending toward, if not reaching, the gas-phase order when the non-H-bonding donor solvent, DMSO, is used in place of water; see Table .…”

“…The resulting plot of ∆G q versus ∆G RXN was accurately linear with a slope, R ) 0.51, in marked contrast to the anomalous aqueous solution value of 1.3-1.5 (depending on the base used). 17,18 We have looked at the issue in two ways. First, we carried out semiempirical AM1 calculations on the reaction of formate anion with 12 meta and para ring-substituted arylnitromethanes, eq 4.…”

“…The acidities of these nitroalkanes increase in the order given, but the rates of deprotonation by OH - /H 2 O follow the opposite order . Another example was the finding that, although Brönsted β-values for the aqueous-phase deprotonation of a given arylnitromethane by Eigen-normal bases are unremarkable (β = d log k B / d p K BH = 0.5−0.6), the Brönsted α values (α = − d log k A / d p K A ) obtained by varying the acidity of the arylnitroalkanes toward a given base are “anomalous”, being greater than 1.0. , Such a result indicates that the effect of the structural change in the C−H acid ( m - and p -substituents were varied) is greater on the proton-transfer transition state than on the fully formed conjugate base product. These results stand in contrast to proton transfer between normal acids and bases which exhibit equal or nearly equal α and β values lying between 0 and 1 …”

Kinetic and Thermodynamic Acidities of Substituted 1-Benzyl-1-methoxy-2-nitroethylenes. Strong Reduction of the Transition State Imbalance Compared to Other Nitroalkanes

“…Proton transfers involving nitroalkanes have generated an unusual amount of interest over many years. − A major reason is that these reactions show some extreme features regarding reactivity and transition state structure. One such feature is that they have the highest intrinsic barriers 17 or lowest intrinsic rate constants 17 for proton transfers in solution. 8a, Another feature, directly related to the first, is that the transition state imbalance which is characteristic of proton transfers from carbon acids activated by π acceptors 18 manifests itself more strongly than for any other type of carbon acids. ,8d, …”