Markovnikov vs Anti-Markovnikov HBr Addition: Peroxide Effect

Markovnikov vs. Anti-Markovnikov Addition of HBr Learn the difference between Markovnikov and anti-Markovnikov addition of HBr to alkenes, including the peroxide effect and reaction mechanisms.

Concept Overview

This question tests the understanding of electrophilic addition reactions to alkenes, specifically the addition of HBr. It contrasts the regioselectivity dictated by Markovnikov's rule with the opposite regioselectivity observed under the influence of peroxides (anti-Markovnikov addition). The underlying mechanisms, involving carbocation intermediates for Markovnikov addition and free radical intermediates for anti-Markovnikov addition, are crucial for explaining these differences.

Solution:

Step 1: Understanding Markovnikov's Rule

Markovnikov's rule states that in the addition of a protic acid HX to an unsymmetrical alkene, the hydrogen atom (H) adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms. Conversely, the halide (X) adds to the carbon atom with fewer hydrogen atoms. This rule is explained by the formation of the more stable carbocation intermediate.

For the addition of HBr to propene ($CH_3-CH=CH_2$):

The alternative pathway forms a less stable primary carbocation:

The secondary carbocation is more stable due to hyperconjugation and inductive effects, thus favoring the formation of 2-bromopropane.

Step 2: Understanding the Peroxide Effect (Anti-Markovnikov Addition)

In the presence of peroxides (like benzoyl peroxide or $H_2O_2$), the addition of HBr to unsymmetrical alkenes proceeds via a free-radical mechanism, leading to anti-Markovnikov addition. In this case, the hydrogen atom adds to the carbon atom of the double bond that has fewer hydrogen atoms, and the bromine atom adds to the carbon atom with more hydrogen atoms. This is often referred to as the "peroxide effect" or "Kharasch effect."

For the addition of HBr to propene in the presence of peroxides:

Step 3: Mechanism of Anti-Markovnikov Addition (Free Radical)

The anti-Markovnikov addition occurs through a free-radical chain mechanism, which involves three main stages: initiation, propagation, and termination.

• Initiation: Peroxides decompose to form alkoxy radicals. These radicals then abstract a hydrogen atom from HBr to generate a bromine radical.

  $$R-O-O-R \xrightarrow{\Delta} 2RO \cdot$$ $$RO \cdot + HBr \rightarrow ROH + Br \cdot$$

• Propagation: The bromine radical adds to the double bond of the alkene. This addition occurs in a way that forms the more stable free radical intermediate. For propene, addition to the terminal carbon forms a more stable secondary radical.

  $$CH_3-CH=CH_2 + Br \cdot \rightarrow CH_3-\dot{C}H-CH_2Br \quad \text{(more stable secondary radical)}$$

  The alternative addition forms a less stable primary radical:

  $$CH_3-CH=CH_2 + Br \cdot \rightarrow CH_3-CHBr-\dot{C}H_2 \quad \text{(less stable primary radical)}$$

  The more stable secondary radical then abstracts a hydrogen atom from another molecule of HBr, regenerating the bromine radical and continuing the chain.

  $$CH_3-\dot{C}H-CH_2Br + HBr \rightarrow CH_3-CH_2-CH_2Br + Br \cdot$$

• Termination: The chain is terminated by the combination of any two radicals.

  $$Br \cdot + Br \cdot \rightarrow Br_2$$ $$CH_3-\dot{C}H-CH_2Br + Br \cdot \rightarrow CH_3-CHBr-CH_2Br$$ $$2 CH_3-\dot{C}H-CH_2Br \rightarrow CH_3-CH(CH_2Br)-CH(CH_2Br)-CH_3$$

  The product formed in the propagation step is 1-bromopropane, which is the anti-Markovnikov product.

Step 4: Comparison and Key Differences

The key difference lies in the nature of the intermediate formed and the reaction conditions.

• Markovnikov Addition: Occurs via an ionic mechanism, forming a carbocation intermediate. The regioselectivity is governed by the stability of the carbocation (tertiary > secondary > primary). This is the normal reaction of HBr with alkenes in the absence of peroxides.
• Anti-Markovnikov Addition: Occurs via a free-radical mechanism, forming a free radical intermediate. The regioselectivity is governed by the stability of the free radical (tertiary > secondary > primary). This occurs specifically with HBr in the presence of peroxides. Other hydrogen halides (HCl, HI) do not show this peroxide effect because the radicals formed from them are not stable enough to abstract hydrogen from HBr.

Key Takeaways:

• Markovnikov's rule predicts the major product of electrophilic addition of HX to unsymmetrical alkenes based on carbocation stability.
• The peroxide effect reverses the regioselectivity of HBr addition to alkenes, leading to anti-Markovnikov products.
• Markovnikov addition proceeds via an ionic mechanism involving carbocations.
• Anti-Markovnikov addition (peroxide effect) proceeds via a free-radical mechanism involving radical intermediates.

Answer: The addition of HBr to an unsymmetrical alkene follows Markovnikov's rule (forming the more substituted halide) in the absence of peroxides, via a carbocation intermediate. In the presence of peroxides, it follows anti-Markovnikov's rule (forming the less substituted halide), via a free-radical mechanism.