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Pictures of the Day CH320M/CH328M Fall 2010

Halonium Ion - Leads to anti Geometry of Addition so trans products.
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.
Substituted Halonium Ions - Leads to Markovnikov Addition
Substituted halonium ions have a greater amount of positive charge (more blue color) on the carbon atom with the larger number of alkyl substituents. This means that nucleophiles will attack at this position, explaining the regiochemistry of the halohydrin reaction (the OH group ends up on the more substituted carbon atom). The easiest way to understand this is to think about the halonium ion as a resonance hybrid as shown. Since the holonium ion requires attack from the side opposite the X atom, anti addition stereochemistry is always observed, leading to trans products.