Adjustment of the Proton-Accepting Ability of Π-Bonds and Lone Pairs

A hydroxyl group is placed on a series of systems containing CC double bonds, and quantum chemical calculations consider the preferred target of an incoming1 HCl molecule to form an H-bond. The O lone pairs are favored over the π-system in most instances but can be reversed by the presence of electron-donating substituents. This reversal occurs when NH₂ groups are added to a simple alkene or if NMe₂ substituents are placed on a conjugated butadiene derivative. There are a variety of means of favoring the aromatic ring as a proton-accepting site over the phenol O. Dispersion is the primary factor in determining the preferred electron-donor site in the aromatic systems. The particular site and orientation of attachment of HCl to the π system is controlled by a combination of electrostatic potential and HOMO disposition.