The peroxide effect occurs by :

Options

A. ionic mechanism
B. homolytic fission of double bond
C. heterolytic fission of double bond
D. free radical mechanism

Answer

To determine how the peroxide effect occurs, we need to analyze the reaction mechanism involved when alkenes react with halo acids in the presence of peroxides. Here’s a step-by-step breakdown of the process:

### Step 1: Understanding the Peroxide Effect
The peroxide effect refers to the phenomenon where the addition of a halo acid (like HBr) to an alkene (for example, propene) in the presence of peroxides (like H2O2) leads to the formation of a different product than would be expected from the Markovnikov's rule.

**Hint:** Recall that the peroxide effect alters the regioselectivity of the addition reaction.

### Step 2: Initiation of the Reaction
When peroxides (H2O2) are present, they can undergo homolytic cleavage under light or heat, generating free radicals. For instance, H2O2 can break down to produce two hydroxyl radicals (•OH).

**Hint:** Remember that homolytic cleavage results in the formation of free radicals.

### Step 3: Formation of Free Radicals
One of the hydroxyl radicals (•OH) can react with HBr, abstracting a hydrogen atom (H•) from it. This results in the formation of water (H2O) and a bromine radical (Br•).

**Hint:** Identify the role of radicals in the reaction mechanism.

### Step 4: Reaction with the Alkene
The bromine radical (Br•) generated in the previous step can now react with the alkene (propene, CH3-CH=CH2). This can lead to the formation of two possible radical intermediates:
1. CH3-CH•-CH2Br (secondary radical)
2. CH3-CHBr-CH2• (primary radical)

**Hint:** Consider the stability of the radicals formed; secondary radicals are generally more stable than primary radicals.

### Step 5: Stability of Radicals
Among the two radicals formed, the secondary radical (CH3-CH•-CH2Br) is more stable due to hyperconjugation and inductive effects. Therefore, it is favored in the reaction.

**Hint:** Stability of radicals plays a crucial role in determining the major product.

### Step 6: Final Product Formation
The stable secondary radical (CH3-CH•-CH2Br) can then abstract a hydrogen atom from another molecule (like HBr) to form the final product, which is CH3-CH2-CH2Br (1-bromopropane).

**Hint:** The final product is determined by the stability of the radical intermediates.

### Conclusion
The peroxide effect occurs via a free radical mechanism, which is characterized by the formation and reaction of free radicals rather than through ionic or heterolytic mechanisms.

**Final Answer:** The correct option is D: free radical mechanism.