Markovnikov Rule

In organic chemistry, alkenes undergo addition reactions in which atoms or groups are added across the double bond. In the case of unsymmetrical alkenes, the addition of reagents like hydrogen halides (HX) can lead to the formation of more than one product. To predict the major product formed in such reactions, Markovnikov’s rule is used.

Mechanism of Markovnikov’s Rule

The addition of hydrogen halides (HX) to alkenes occurs through a two-step electrophilic addition mechanism involving the formation of a carbocation intermediate.

Step 1: Formation of Carbocation (Slow Step)

The double bond in an alkene is rich in electrons and acts as a nucleophile. It attacks the hydrogen (H⁺) of HX. The hydrogen atom adds to one of the carbon atoms of the double bond. This leads to the formation of a carbocation on the other carbon atom.

• Only the pathway that forms the more stable carbocation is favored
• Stability order: Tertiary (3°) > Secondary (2°) > Primary (1°)
• Therefore, hydrogen attaches in such a way that the more stable carbocation is formed.

Step 2: Attack of Nucleophile (Fast Step)

• The halide ion (X⁻) formed from HX attacks the positively charged carbocation.
• This results in the formation of the final product.

Example 1: Addition of HBr to Propene
CH₃–CH=CH₂ + HBr → CH₃–CHBr–CH₃

• Hydrogen attaches to the carbon atom of the double bond that already has more hydrogen atoms, so that the more stable carbocation is formed on the other carbon.
• Bromine attaches to the carbon having fewer hydrogen atoms
• Major product: 2-bromopropane

Example 2: Addition of HCl to Propene
CH₃–CH=CH₂ + HCl → CH₃–CHCl–CH₃

• The hydrogen (H⁺) from HCl attaches to the carbon atom which already has more hydrogen atoms (the terminal carbon, CH₂).
• This leads to the formation of a more stable secondary carbocation at the middle carbon.
• The chloride ion (Cl⁻) attacks the positively charged carbon (carbocation).
• Major product: 2-chloropropane

Anti-Markovnikov Rule (Peroxide Effect)

In certain conditions, the addition of hydrogen bromide (HBr) to an unsymmetrical alkene takes place opposite to Markovnikov’s rule. This is known as the anti-Markovnikov rule. In this case, In presence of peroxides, Br attaches to the carbon with more hydrogen atoms due to free radical mechanism, while hydrogen attaches to the carbon having fewer hydrogen atoms.

• This reaction occurs in the presence of peroxides (ROOR).
• It is also called the peroxide effect or Kharasch Effect.
• The reaction proceeds through a free radical mechanism instead of a carbocation.
• Hence, the direction of addition is reversed.

Example: Addition of HBr to Propene (in presence of peroxide)
CH₃–CH=CH₂ + HBr → CH₃–CH₂–CH₂Br

• Bromine attaches to the carbon having more hydrogen atoms.
• Hydrogen attaches to the carbon having fewer hydrogen atoms.
• Product: 1-bromopropane.

Markovnikov vs Anti-Markovnikov

Related Articles:

• Alkyl Halide
• Chemical Reactions of Haloalkanes and Haloarenes
• Methods of Preparation of Haloalkanes and Haloarenes