π‐Hydrogen Bonding Probes Chemical Reactivity: Bromination of a CC Double Bond and Electrophilic Aromatic Benzylation

Abstract

AbstractThe reactivity trends for two basic organic reactions – addition of bromine to a CC double bond in alkenes and electrophilic aromatic benzylation of alkyl‐ and halobenzenes – are rationalized using both experimental and theoretical quantities. Literature kinetic data are employed. The shifts of phenol O–H stretching frequency upon π‐hydrogen bonding (Δν(OH)exp) with the nucleophilic reactants are shown to provide a quantitative measure of the reactivity for the considered reactions. Very good correlations between Δν(OH)exp and theoretically estimated charge density parameters (atomic charges and shifts of molecular electrostatic potential) show that the O–H frequency shifts reflect the variations of nucleophilic properties in the reaction centers upon structural changes. The Δν(OH)exp shifts perform much better in quantifying reactivity trends than usually employed experimental quantities, such as gas‐phase basicities and proton affinities.