Protonation and Site of Protonation of Anilines. Hydration and Site of Protonation after Hydration

Article abstract of DOI:10.1021/ja00411a004

The gas-phase basicities (proton affinities) of 15 substitueted anilines were determined by measuring proton-transfer equilibria with a pulsed electron beam high-pressure mass spectrometer.The gas-phase basicities are compared with experimentally measured ionization energies of the nitrogen 1s electron.A straight-line relationship is observed for the substituted anilines that are nitrogen protonated.This line includes aniline. m-NH2-, m-MeO-, m-MeS-, m-OH-, and m-C2H5-aniline were found to have higher gas-phase basicities than those predicted by the linear relationship.These compounds are ring protonated.The energies for proton transfer from ring to nitrogen for these compounds can be estimated from the straight-line correlation.Hydration equilibria BH⁺+OH2=BHOH2⁺ were measured for several protonated anilines BH⁺.The BH⁺ which are normally nitrogen protonated have hydrogen bonding energies to water which increase approximately linearly with the acidity of BH⁺.The ring-protonated BH⁺ do not fit this relationship; they have lower hydration energies.Hydration induces proton transfer from ring to nitrogen for m-MeS, m-OH, and m-C2H5.The hydrated m-MeO and m-NH2 remain ring protonated.The water molecule is expected to hydrogen bond to the neutral NH2 group of these two ions.The two hydrogens of the amino group carry fractional positive charges which make this a relatively strong hydrogen bond.The gas-phase basicities are also compared with the aqueous basicities.An approximate linear dependence is obtained for the nitrogen-protonated ions.The gas-phase substituent effect is attenuated by a factor of 4 in liquid water.