Addition of Hydrogen Halides to Alkenes
Regioselectivity of Hydrogen Halide Addition
To understand the regioselectivity of the hydrogen halide (HX) addition to alkenes, it is essential to consider the energy diagram of the reaction shown below. The first step in the reaction is the attack by the electrophile on the double bond which requires the highest activation energy of all steps involved. As a result, this is the slowest and, therefore, the rate-determining step of the electrophilic addition reaction. The formation of the intermediate carbenium ion from the alkene is an endothermic reaction step. According to the Hammond postulate, the structure of the corresponding transition state resembles that of the product, which in this case is the carbenium ion. Therefore, the transition state can be assumed to carry a partial positive charge.
If such HX addition reactions are carried out with asymmetrically substituted alkenes, the electrophile can add to the double bond at either carbon atom of the double bond. In the reaction of propene with HBr, for example, n-propyl bromide as well as isopropyl bromide can be formed. In practice, however, the isopropyl bromide is predominantly formed. To explain the regioselectivity, Markovnikov's rule has to be applied.
In 1870, after observing many electrophilic addition reactions to asymmetrically substituted alkenes the Russian chemist Vladimir Markovnikov formulated the so-called Markovnikov´s rule. This rule describes the regioselectivity of these addition reactions.
- In the HX addition to asymmetrically substituted alkenes, the hydrogen atom is added to the carbon atom with the most hydrogen atoms.
The regioselectivity can be explained on a scientific rather than an empirical basis by looking at the energy of the two different possible intermediate carbenium ions.
In the first reaction step, the energetically favored and more stable secondary carbenium ion is preferably formed. On the other hand, the higher-energy primary carbenium ion is only reachable via the corresponding transition state according to the Hammond postulate. The fact that this transition state is of higher energy than the transition state leading to the secondary carbenium ion forms the basis for Markovnikov´s rule.
- Markovnikov´s rule expressed scientifically states that addition of HX to a double bond proceeds via the more stable carbenium ion.
The greater stability of secondary carbenium ions as compared to primary carbenium ions can be explained by hyperconjugation and the +I effects of alkyl groups. The addition of HX to alkenes is regioselective but not stereoselective because the intermediate carbenium ion shows a trigonal planar shape. The empty p orbital which is oriented orthogonal to the trigonal plane can be accessed with equal probability from both sides of the plane.