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 additional reactions of alkenes.
What is Markovnikov’s Rule?
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 the 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-bromopropene and 2-bromopropene.
According to Markovnikov’s rule, the major product will be 2-bromopropene. As in 2-bromopropene the halide group is attached to that carbon which has a 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.
Mechanism Behind Markovnikov’s Rule
We are explaining the mechanism of Markovnikov’s rule for the reaction of propene with hydrobromic acid. The mechanism can be explained by the 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. The 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-
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Application of Markovnikov’s Rule in Other Reactions
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Reaction of butene with hydrobromic acid.
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Reaction of propene with HI.
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Application of Markovnikov’s rule in addition reactions of aromatic alkene.
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Reaction of propyne with HBr.
Reasoning behind Markovnikov’s Rule
Electrophilic addition of acidic hydrogen ion or proton on alkene results in the formation of a 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 the highest number of alkyl substituents. This is the reason why the majority of the product contains an addition of the halide to the carbon having a greater number of alkyl groups.
Facts about Markovnikov’s Rule
It was mainly discovered for the addition of hydrogen halides to alkenes.
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Markovnikov’s rule is followed by unsymmetrical alkenes.
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Electrophilic addition reactions of aromatic alkenes and alkynes follow Markovnikov’s rule.
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Stable carbocation forms the major product.
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The protonation step is the rate-determining step.