SUMMARY OF KEY CONCEPTS

•	The electrophilic addition of halogens to an alkene proceeds via a three-membered halonium ion intermediates to give the trans addition product.

 

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Halonium ions were first postulated in 1937 by Roberts and Kimball to account for the observed diastereoselectivity in halogenation of alkenes, see: The Halogenation of Ethylenes Irving Roberts, George E. Kimball J. Am. Chem. Soc.; 1937; 59(5); 947-948. The halonium ion postulate is a excellent example of deductive logic: from experimental results the mechanistic details of alkene electrophilic reactions were deduced.

•	Very hindered alkenes form halonium ions that are resistant to nucleophilic attack. Such ions are sufficiently stable to be characterized by x-ray crystallography. Thus, almost 50 years after haloniums ions were first proposed we have structural evidence that confirms their existence.