This reaction involves the addition of HBr to an alkene, but with a special condition: the presence of a peroxide (ROOR). This is crucial because it changes the mechanism from standard electrophilic addition to free radical addition, following Anti-Markovnikov's Rule.

Step 1: Understanding the Peroxide Effect

Normally, HBr adds to alkenes via electrophilic addition, following Markovnikov's rule (H adds to the less substituted carbon). However, in the presence of peroxides (ROOR), the mechanism shifts to a free radical chain reaction. This is known as the peroxide effect or Kharasch effect.

Step 2: Free Radical Mechanism

The peroxide (ROOR) undergoes homolytic cleavage to form alkoxy radicals (RO•), which then abstract a hydrogen from HBr, generating a bromine radical (Br•).

$ROOR\to 2RO•$
$RO•+HBr\to ROH+Br•$

Step 3: Addition to the Alkene

The bromine radical (Br•) adds to the alkene. The radical adds to the less substituted carbon of the double bond, forming a more stable (secondary) carbon radical. This is the step that determines the Anti-Markovnikov orientation.

$Br•+CH3-CH=CH2\to CH3-\stackrel{•}{C}H-CH2-Br$

Step 4: Chain Propagation

The secondary carbon radical then abstracts a hydrogen from another HBr molecule, forming the final Anti-Markovnikov product (1-bromopropane) and regenerating a bromine radical to continue the chain reaction.

$CH3-\stackrel{•}{C}H-CH2Br+HBr\to CH3-CH2-CH2Br+Br•$

Final Answer

The major product is 1-bromopropane, $CH3CH2CH2Br$. This corresponds to the structure in the first option.

Related Topics & Formulae

Markovnikov's Rule: In the electrophilic addition of HX to an unsymmetrical alkene, the hydrogen atom adds to the carbon with the greater number of hydrogen atoms.

Anti-Markovnikov's Rule (Peroxide Effect): Applies only to HBr (not HCl or HI) in the presence of peroxides. The bromine adds to the less substituted carbon.

Stability of Carbon Radicals: The order of stability is tertiary > secondary > primary > methyl. This stability dictates the regiochemistry in the free radical addition step.