Markovnikov’s Rule: Statement, Mechanism & Examples

Markovnikov’s Rule is a rule that is used to determine the outcome of various chemical addition reactions. Markovnikov’s rule states that,

“During the hydrohalogenation of an asymmetric alkene, the acidic hydrogen atom combines with the doubly bonded carbon atom having the most hydrogen substituents, while the halide group binds to the carbon atom with the most alkyl substituents.”

• This rule was given by a Russian Scientist, Vladimir Markovnikov in 1865. 
• Markovnikov’s Rule is used to predict the active selectivity of alkene and alkyne electrophilic addition processes.
• It is also known as Markownikoff's Rule. 
• The rule is used in the field of organic chemistry to determine the outcome of some additional reactions of alkenes. 

Read More: Electrophilic Substitution Reactions

Key Terms: Markovnikov’s Rule, Acidic, Hydrogen, protic acid, halide group, carbon atom, electrophile, nucleophile, Anti Markovnikov’s rule, Addition reaction

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What is Markovnikov's Rule?

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According to Markovnikov’s Rule, when a protic acid (HX) is introduced to an asymmetric alkene, the acidic hydrogen connects to the carbon atom with the most hydrogen substituents, while the halide group binds to the carbon atom with the most alkyl substituents. 

It can be simply stated as “Hydrogen is added to the carbon with the most hydrogens, and the halide is added to the carbon with the least hydrogens.” 

• Markovnikov’s rule states that in the addition of a reagent in an unsymmetrical alkene, the negative part of the reagent moves towards the carbon with the double bond having the least number of Hydrogen atoms.
• The hydrogen atom links itself to the carbon atom of the double bond that has a greater number of hydrogens.

Let us consider an addition reaction in which an alkene reacts with water to result in alcohol. This reaction moves forward through the formation of a carbocation. The hydroxyl group in this reaction attaches itself to the carbon having more carbon-to-carbon bonds whereas the hydrogen atom attaches itself to another carbon in the double bond, which has more carbon-to-hydrogen bonds.

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Mechanism of Markovnikov's Rule

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The mechanism of Markovnikov's rule for the reaction of propene with hydrobromic acid is broken down into the following steps:

Step 1: Protonation or the addition of an acidic hydrogen ion is the first step.

H⁺ and Br^- are formed when hydrobromic acid (HBr) is broken down. H⁺ (Electrophile) attaches itself to the carbon with the most hydrogen substituents. 

Step 2: Bromide anion addition

Nucleophilic Attack

This phase involves the attack of a nucleophile or bromide ion on the carbocation. As preference is given to the formation of secondary cation, the major product of the reaction is 2-Bromo propene. The bromide ion attaches itself to the carbon with the most alkyl substituents.

Markovnikov Rule Example

Examples of other Markovnikov reactions are as given below – 

• Butene reacts with hydrobromic acid.
• Propene's reaction with HI
• Propyne-HBr reaction

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Explanation behind Markovnikov's Rule

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When the electrophilic addition of acidic hydrogen proton or ion on alkene takes place, a carbocation is formed.

• The most stable carbocation is one in which the positive charge is carried by the carbon with the most alkyl substituents.
• This is why the majority of products comprise the addition of a halide to carbon with fewer hydrogen substituents. 

Facts about Markovnikov’s Rule

Markovnikov’s Rule was mainly discovered for the addition of hydrogen halides to alkenes.

• It is followed by unsymmetrical alkenes. 
• It is also followed by electrophilic addition reactions of aromatic alkenes and alkynes. 
• The major product forms the stable carbocation. 
• The protonation step determines the rate of the reaction.

Read More: Anti-Markovnikov Addition

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Anti Markovnikov Rule

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The anti-Markovnikov rule explains regiochemistry in which the substituent is attached to a less substituted carbon instead of the more substituted carbon.

• Carbon Cations that are typically produced during alkene or alkyne reactions tend to prefer the more substituted carbon, therefore this procedure is extremely rare.
• This is because substituted carbocation allows for greater hyperconjugation and induction, resulting in a more stable carbocation.
• Under Anti-Markovnikov only HBr may undergo radical haloalkane addition, and Hydrogen Peroxide must be present (H₂O₂).
• This method requires hydrogen peroxide since it is the chemical that initiates the chain reaction in the initiation stage.  

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Examples of Markovnikov and Anti-Marknovnikov Addition Reactions

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The examples of Markovnikov and Anti-Markovnikov reactions are as follows – 

Alkene Hydration 

When alkenes are handled with certain aqueous acids (most often sulfuric acid), an electrophilic addition reaction occurs, yielding alcohol as a product.

• Markovnkov’s rule can be used to predict the regioselectivity of such reactions.
• As a result, these reactions are known as Markovnikov reactions. 
• The H⁺ ion functions as an electrophile in the hydration of alkenes, attacking the alkene to produce a carbocation intermediate (the intermediate with greater stability is protonated).
• Water molecules then attack the carbocation nucleophilically, forming an oxonium ion, which is then deprotonated to provide the necessary alcohol product.

Alkene Hydroboration or Oxidation

The end result of treating alkenes with borane (BH₃) in the presence of hydrogen peroxide or sodium hydroxide is alcohol.

• The boron atom functions as an electrophile in this electrophilic addition process.
• As this reaction defies Markovnikov's rule, it is categorized as an anti-Markovnikov reaction.

Also Read:

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Things to Remember

• In Markovnikov’s rule hydrogen is added to the carbon with the most hydrogens, and the halide is added to the carbon with the least hydrogens.
• A carbocation is formed when an acidic hydrogen ion or proton is electrophilically added to an alkene.
• The most stable carbocation is one in which the positive charge is carried by the carbon with the most alkyl substituents.
• The end result of treating alkenes with borane (BH₃) in the presence of hydrogen peroxide or sodium hydroxide is alcohol.
• The Anti-Markovnikov rule explains regiochemistry in which the substituent is attached to a less substituted carbon instead of the more substituted carbon.