In which of the following reactions Markovnikov's rule is not observed ?

Options

A. `CH_(3)CH=CH_(2) + HCl rarr `
B. `CH_(3)CH=CH_(2) + HBr overset("organic peroxide, hv")to`
C. `CH_(3)CH=CH_(2) + HBr overset(H_(2)O_(2),hv)`
D. Both (2) and (3)

Answer

To determine in which of the given reactions Markovnikov's rule is not observed, we need to analyze each reaction step by step.

### Step 1: Understand Markovnikov's Rule
Markovnikov's rule states that in the addition of HX (where X is a halogen) to an asymmetric alkene, the hydrogen atom (H) will attach to the carbon with the greater number of hydrogen atoms, while the halogen (X) will attach to the carbon with fewer hydrogen atoms. This is because the more stable carbocation intermediate is formed.

### Step 2: Analyze the Reactions
1. **First Reaction (HCl with an Asymmetrical Alkene)**:
- The alkene has a double bond between two carbons, one of which has more hydrogen atoms than the other.
- When HCl is added, H+ will attach to the carbon with more hydrogen atoms, and Cl- will attach to the carbon with fewer hydrogen atoms.
- **Conclusion**: Markovnikov's rule is followed.

2. **Second Reaction (HBr with Peroxide)**:
- The presence of peroxide leads to a radical mechanism instead of the ionic mechanism.
- In this case, the bromine radical (Br•) can attach to either carbon, but it tends to attach to the carbon that forms the more stable radical.
- Here, Br ends up on the carbon with more hydrogen atoms, which is contrary to Markovnikov's rule.
- **Conclusion**: Markovnikov's rule is NOT followed.

3. **Third Reaction (Hydrogen Peroxide with an Asymmetrical Alkene)**:
- Similar to the second reaction, the presence of peroxide leads to a radical mechanism.
- The addition of H2O2 also results in the formation of radicals, and the product will not follow Markovnikov's rule.
- **Conclusion**: Markovnikov's rule is NOT followed.

### Step 3: Identify the Correct Answer
Based on the analysis:
- **Reactions where Markovnikov's rule is NOT followed**: Second and Third reactions.
- **Reaction where Markovnikov's rule is followed**: First reaction.

### Final Answer
Markovnikov's rule is not observed in the second and third reactions (HBr in the presence of peroxide and Hydrogen peroxide with an asymmetrical alkene).

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