Organic Chemistry, "Change" - Part 13: Extension of Markovnikov's Rule

 Greetings,

This post will explain a more general consequence of Markovnikov's Rule, which can be considered an extension of the rule.  Markovnikov's Rule has an assumption that goes with it, that is; Hydrogen adds first to the double bond. But the real significance of Markovnikov's Rule is that whichever element adds first, to the double bond, that element will bond to the carbon atom resulting in the more stable carbocation transition state.

Markovnikov's Rule is useful, as a Rule-of-Thumb, because most addition reactions, of added compounds consisting of hydrogen, will have hydrogen adding first. However, as with many other rules-of-Chemistry, there are exceptions:

• Radical initiated addition of HBr - The bromine radical adds first to the carbon atom bonded to the greater number of hydrogen atoms.  This produces a more stable radical intermediate with a single unpaired electron on the more substituted carbon atom. Hydrogen then adds to the carbon atom bonded to the lower number of hydrogen atoms.
• Hydro-Boration - Borane, BH3, adds to a double bond with the Borine atom bonding to the less substituted carbon atom. A hydrogen atom, from BH3, adds to the more substituted carbon atom, which sets up a H--BH2 concerted intermediate with a partial positive charge on the more substituted carbon. The resulting product is formed by hydrogen adding to the more substituted carbon atom and a BH2 group adding to the less substituted carbon. 

The two examples, presented above, are anti-Markovnikov, because hydrogen adds to the more substituted carbon atom (the carbon bonded to fewer hydrogen atoms before the reaction begins).

That's all for this post. As always, thank you for reading!