Pictures of the Day CH310M/CH318M Fall 2007

10-22-07
Halonium Ion - Leads to trans Geometry of Addition
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), explaining why trans stereochemistry of addition is always observed.
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.
Mercurinium Ion - Analogous to Halonium Ion
The mercurinium ion is formed when Hg(OAc)2 reacts with an alkene. It is analogous to a halonium ion in that it is a positively-charged (blue color) three-membered ring intermediate. Water, the nucleophile, must attack from the face opposite the Hg atom, so like the halonium ion intermediates, anti addition stereochemistry is observed in this first step. Subsituted mercurinium ions will have a greater positive charge on a carbon atom with more alkyl groups, and the water nucleophile will preferentially attack this more positively-charged atom. Thus, once again, the OH group will end up on the more highly substituted carbon atom leading to Markovnikov regiochemistry. Two important points. The second step that used NaBH4 scrambles the stereochemistry of the product so cis and trans products are observed following the overall reaction. Second, the mercurinium ion does not rearrange, which is an important advantage compared to the H20/H2SO4 reaction in some cases.