Reaction Analysis: Electrophilic Addition to Conjugated Diene

The given reaction involves a conjugated diene (1,3-butadiene derivative) undergoing electrophilic addition with HBr. This follows the mechanism of 1,2-addition and 1,4-addition due to resonance stabilization of the allylic carbocation intermediate.

Step-by-Step Mechanism:

Step 1: Protonation of the diene by HBr to form a resonance-stabilized allylic carbocation.

The electrophile (H⁺) adds to the terminal carbon, generating a carbocation that is resonance stabilized:

Step 2: Nucleophilic attack by Br⁻ on the carbocation.

At low temperatures (kinetic control), 1,2-addition dominates, giving the less stable but faster-formed product. At higher temperatures (thermodynamic control), 1,4-addition dominates, giving the more stable alkene.

For this reaction (likely under kinetic control), the major product is from 1,2-addition:

Final Answer: The major product is 3-bromo-1-butene, which corresponds to the second option:

Related Topics:

• Electrophilic Addition Reactions
• Conjugated Dienes and Resonance
• Allylic Carbocation Stability
• Kinetic vs Thermodynamic Control
• Markovnikov's Rule

Key Formulae and Theory:

Resonance Energy: Allylic carbocations are stabilized by resonance, lowering the energy of the intermediate and influencing product distribution.

Markovnikov's Rule: In electrophilic addition, the electrophile adds to the less substituted carbon of the alkene, forming the more stable carbocation.

1,2 vs 1,4-Addition: $\text{1,2-product (kinetic):}\mathrm{CH}3-\mathrm{CHBr}-\mathrm{CH}=\mathrm{CH}2$
$\text{1,4-product (thermodynamic):}\mathrm{CH}3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}2\mathrm{Br}$