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Holonium ion intermediates are produced when alkenes react with X2, X being Cl, Br or I. Halonium ions are positively-charged, three-membered ring intermediates. Note that nucleophiles, i.e. X- or water, can only attack from the face of the halonium ion intermediate opposite of where the X atom is (otherwise they would simply bounce off of the X atom), a geometry referred t as anti addition, explaining why trans stereochemistry of addition is always observed in the products of these reactions.