What does regiochemical control mean
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Regioselectivity in the HX addition
When considering the addition of hydrogen halides (HX) to alkenes, the energetic aspects of the reaction must be taken into account. In order to really understand a reaction, in addition to the reaction mechanism, the energetic course of the reaction along the reaction coordinate must also be considered. The first step, the addition of the electrophile to the double bond, requires the greatest activation energy and is therefore the slowest, i.e. the rate-determining step. The formation of the carbenium ion intermediate from the alkene is an endothermic reaction, so that, according to the Hammond principle, the associated transition state has more of the character of the product, i.e. in this case the carbenium ion. The transition state will therefore already have a partial charge on the carbon atom.
If such HX addition reactions are carried out with unsymmetrical alkenes, two different orientations are possible in the addition. For example, propene with HBr could do both n-Propyl bromide and isopropyl bromide are formed. However, predominantly only isopropyl bromide is formed. How can this regioselectivity be explained?
By observing many such examples, the Russian chemist Vladimir Markovnikov described the orientation of the HX addition known as the Markovnikov rule as early as 1870:
- The H atom in the HX addition to alkenes attaches to the C atom of the double bond, which already bears most of the H atoms.
Consideration of the energy of the two possible intermediate stages helps to explain this regioselectivity.
The more stable carbenium ion dominates in the first step, it has the lower energy, and so the corresponding transition state with the partially positive charge on the secondary carbenium ion is also more stable.
- The Markovnikov rule then states that the addition of HX to a double bond takes place in such a way that the more stable carbenium ion is formed.
The reason for the greater stability of the secondary carbenium ion compared to the primary is the stabilization by hyperconjugation. The addition of HX to alkenes therefore proceeds under strict regiochemical control, but there is no stereochemical control. The intermediate stage of the addition contains a trigonal-planar carbenium ion to which the halogen can attach itself from both sides with the same probability.
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