Choose the correct statement regarding the formation of carbocations A and B given.

Step 1: Identify the carbocations

Starting material: $\ce{H2C=CH-CH=CH2}$ (1,3-butadiene)

When HBr adds, H⁺ can attack either double bond:

Carbocation A: $\ce{H3C-CH2-CH=CH2+}$ (positive charge on terminal carbon)

• This is a primary allylic carbocation
• Resonance: $\ce{H3C-CH2-CH=CH2+ <-> H3C-CH2-CH+-CH2}$

Carbocation B: $\ce{H3C-CH2-CH+-CH2}$ (positive charge on internal carbon)

• This is a secondary allylic carbocation
• Resonance: $\ce{H3C-CH2-CH+-CH2 <-> H3C-CH2+=CH-CH3}$

Wait, let me reconsider the structures. Looking at the image:

Carbocation A: $\ce{-> H3C-CH2-CH=CH2+}$ with ⊕ on C-4 (terminal) Carbocation B: $\ce{-> H3C-CH2-CH+-CH2+}$ with ⊕ on C-3 (internal)

Step 2: Compare stability

Carbocation B is more stable because:

1. It's a secondary carbocation (vs primary in A)
2. It has allylic resonance stabilization
3. Resonance delocalizes the positive charge over 2 carbons

Stability order: B > A

Step 3: Compare formation rates

The more stable carbocation forms faster because:

• Lower activation energy (Hammond's postulate)
• Transition state resembles the carbocation
• More stable carbocation → lower energy transition state → faster formation

Carbocation B forms faster than A

Step 4: Conclusion

Carbocation B is:

• More stable (secondary allylic vs primary allylic)
• Formed faster (lower activation energy)

Answer: (b)

Key Principles:

• Carbocation stability: 3° > 2° > 1° > methyl
• Allylic/benzylic carbocations have resonance stabilization
• More stable carbocation → faster formation (Hammond's postulate)
• In conjugated dienes, internal carbocation is preferred