Reaction Analysis: Electrophilic Addition to Conjugated Diene

The given reaction involves a conjugated diene undergoing electrophilic addition with HBr. The reactant is 1,3-butadiene, which has alternating double bonds. The major product is determined by the stability of the intermediate carbocation formed during the reaction.

Step-by-Step Mechanism:

Step 1: Electrophilic attack by H⁺ on the diene system.

HBr protonates the diene. Two possible carbocations can form:

• Attack on terminal carbon (C1) gives a secondary allylic carbocation.
• Attack on internal carbon (C2) gives a primary carbocation (less stable).

The more stable secondary allylic carbocation is favored:

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

The allylic carbocation has two resonance forms, leading to two possible products:

• Attack on C1 gives 1,2-addition product (3-bromo-1-butene).
• Attack on C3 gives 1,4-addition product (1-bromo-2-butene).

At low temperature (kinetic control), 1,2-addition is favored. At higher temperature (thermodynamic control), the more stable 1,4-addition product (disubstituted alkene) is major.

Since no temperature is specified, we assume standard conditions where the thermodynamic product (1,4-addition) is major.

Final Answer: The major product is 1-bromo-2-butene, which corresponds to the structure:

CH₃-CH=CH-CH₂Br (or (E)-1-bromo-2-butene)

This matches the third option in the list.

Related Topics:

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

Key Formulae and Theory:

1. Markovnikov's Rule: In electrophilic addition to unsymmetrical alkenes, the electrophile adds to the carbon with more hydrogen atoms.

2. Carbocation Stability Order:

3. Allylic Carbocation: Special stability due to resonance delocalization of positive charge.

4. 1,2 vs 1,4 Addition: Conjugated dienes give both products, with 1,4-addition being thermodynamically controlled product.