CC BOND FORMATION BY ADDITION OF CARBENIUM IONS TO ALKENES - KINETICS AND MECHANISM

The addition of carbenium ions to CC double bonds, a key step in many syntheses in organic and macromolecular chemistry, is analyzed using the Lewis acid promoted reactions of alkyl chlorides with alkenes as an example. Stereochemical and kinetic experiments suggest that the transition state is slightly bridged and product-like. Rearrangements of the carbenium ions that result from the electrophilic attack can be minimized by adding salts with nucleophilic counter ions. The thermodynamics of the addition reactions are analyzed, and the conditions necessary in order to observe the back reaction (i.e. the Grob fragmentation) are discussed. Multiparameter equations that predict rate constants are derived from kinetic studies on the reactivities of carbenium ions and alkenes. Reactivity-selectivity relationships over a reactivity range that covers eight orders of magnitude show that the structure of the transition state is only changed by variation of substituents in the immediate vicinity of the reaction center.