Markovnikov rule is also known as Markownikoff’s rule. This rule was formulated by Russian Chemist Vladimir Markovnikov in 1870. This rule is used in organic chemistry to know the outcome of some addition reactions of alkenes.
According to Markovnikov’s Rule when hydrogen halide or protic acid (HX) is added to an asymmetric alkene, the acid hydrogen gets attached to the carbon with more hydrogen substituents and halide group gets attached to the carbon with a greater number of alkyl substituents.
Let’s understand the rule with an example to understand it completely. When propene (alkene) reacts with hydrobromic acid or hydrogen bromide HBr (Protic acid) forms two products 1-bromo propene and 2-bromo propene. According to Markovnikov’s rule major product will be 2-bromo propene. As in 2-bromo propene halide group is attached to that carbon which has greater number of alkyl substituents attached to it and acidic hydrogen is getting attached to that carbon which has a greater number of hydrogen substituents.
We are explaining the mechanism of Markovnikov’s rule for the reaction of propene with hydrobromic acid. The mechanism can be explained by following steps –
Step 1. Protonation or addition of acidic hydrogen ion
Hydrobromic acid (HBr) breaks into H+ and Br-. H+ (Electrophile) gets attached to the carbon which has a greater number of hydrogen substituents. Reaction is shown below-
Step 2. Addition of bromide anion
In this step nucleophile or bromide ion attacks on carbocation and forms major product 2-bromo propene. Bromide ion gets attached to the carbon which has a greater number of alkyl substituents. Reaction is shown below-
Reaction of butene with hydrobromic acid.
Reaction of propene with HI –
Application of Markovnikov’s rule in addition reactions of aromatic alkene-
Reaction of propyne with HBr-
Electrophilic addition of acidic hydrogen ion or proton on alkene results in formation of carbocation. It is a more stable carbocation. As we know the most stable carbocation is the one in which the positive charge is held by the carbon with highest number of alkyl substituents. This is the reason behind why majority of the product contains addition of the halide to the carbon having greater number of alkyl group.
It was mainly discovered for addition of hydrogen halides to alkenes.
Markovnikov’s rule is followed by Unsymmetrical alkenes.
Electrophilic addition reactions of aromatic alkenes and alkynes follow Markovnikov’s rule.
Stable carbocation forms the major product.
Protonation step is the rate determining step.
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