Polar Addition of Hydrogen Halides onto Olefins

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Abstract
Various structures have been proposed for the intermediates of electrophilic additions onto olefins; these include halonium ions, classical carbonium ions, π‐complexes (i.e. nonclassical carbonium ions), and π‐complexes with back‐coordination. It is shown here that it is impossible to use any one of these entities alone to explain all such electrophilic additions; the electrophile itself determines the nature of the transition state formed. Polar addition of hydrogen halides onto olefins appears to proceed via a classical carbonium ion which does not occur as the free ion but as an undissociated ion pair. Various other mechanisms have been excluded by studies reported here of the stereochemical course of such additions.