Markovnikov Rule can be used to describe the outcome of some chemical addition reactions. The Russain chemist Vladimir Vasilyevich Markovnikov first formulated this rule in 1865.
What is Markovnikov Rule?
The statement of this rule can be written as – “When a protic acid (HX) is added to an alkene that is asymmetrical in nature, the acidic hydrogen attaches itself to the carbon having a greater number of hydrogen substituents whereas the halide group attaches itself to the carbon atom which has a greater number of alkyl substituents”.
To simplify the rule, it can also be stated as – “Hydrogen is added to the carbon with the most hydrogens and the halide is added to the carbon with least hydrogens”.
An example of a reaction that observes Markovnikov’s rule is the addition of hydrobromic acid (HBr) to propene, which is shown below.
It can be observed from the reaction illustrated above that the majority of the product formed obeys Markovnikov’s rule, whereas the minority of the product does not.
Let us consider the addition reaction wherein an alkene reacts with water to give rise to an alcohol. This reaction proceeds via the formation of a carbocation. It is observed in this reaction that the hydroxyl group attaches itself to the carbon with more carbon-carbon bonds whereas the hydrogen atom attaches itself to the other carbon in the double bond which has more carbon-hydrogen bonds.
Markovnikov Rule Mechanism
To understand this mechanism, let us consider the same example illustrated earlier, i.e. the addition reaction of hydrobromic acid with propene. The Markovnikov rule Mechanism can be broken down into the following two steps.
The alkene is protonated and it gives rise to the more stable carbocation as shown below.
From the illustration shown above, we can see that there are two types of carbocations that can be formed from the protonation of the alkene, one is a primary carbocation and the other is a secondary carbocation. However, the secondary carbocation is far more stable and therefore, its formation is preferred over the formation of a primary carbocation.
The halide ion nucleophile now attacks the carbocation. This reaction yields the alkyl halide. Since the formation of secondary carbocation is preferred, the major product of this reaction would be 2-bromopropane as illustrated below.
It is important to note that the Markovnikov rule was developed specifically for its application in the addition reaction of hydrogen halides to alkenes. The opposite of ‘Markovnikov’ addition reactions can be described as Anti-Markovnikov based on the regioselectivity of the reaction.