KINETICS OF REVERSIBLE CARBON DEPROTONATION OF 2-NITROETHANOL AND 2-NITRO-1,3-PROPANEDIOL BY HYDROXIDE ION, WATER, AMINES, AND CARBOXYLATE IONS - A NORMAL BRONSTED ALPHA DESPITE AN IMBALANCED TRANSITION STATE

摘要

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 pK(a) values for the ionization at carbon (pK(a)(CH)) and oxygen (pK(a)(OH)) and ionization of the aci-forms (pK(a)(NOH)) were determined in aqueous solution at 25 degrees C. The pK(a)(CH) 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'' Bronsted alpha between 0 and 1. This contrasts with the negative alpha value based on the reaction of OH- with nitromethane, nitroethane, and 2-nitropropane (Kresge, A. J. Can. J. Chem. 1973, 52, 1897), Reasons why a normal alpha value is observed in the current system are discussed, [References: 43]