Choose the halogen which is most reactive towards SN1 reaction in the given compounds (A, B, C & D):

Step 1: Understand $S_N1$ reactivity

$S_N1$ reactivity depends on carbocation stability:

Stability order:

• Benzylic > Allylic > 3° > 2° > 1° > Methyl
• Resonance stabilization is key

Step 2: Analyze each position

Position A: Br on benzene ring (aryl bromide)

• Aryl halides do not undergo $S_N1$
• Aryl carbocation extremely unstable
• Very low reactivity ✗

Position B: I on allylic position

• Allylic carbocation (resonance stabilized)
• I is best leaving group
• High reactivity ✓✓

Position C: Br on bridgehead (bicyclic system)

• Bridgehead carbocations are very unstable (Bredt's rule)
• Cannot adopt planar geometry
• Very low reactivity ✗

Position D: Br on benzylic position

• Benzylic carbocation (resonance with benzene)
• High reactivity ✓

Step 3: Rank reactivity

Most reactive for $S_N1$:

B (allylic I) ≈ D (benzylic Br) >> A (aryl Br) ≈ C (bridgehead Br)

Between B and D:

• Both have resonance stabilization
• I is better leaving group than Br
• B is most reactive

Answer: (b)

Key Points:

• Allylic/benzylic positions: Highly reactive in $S_N1$ (resonance)
• Aryl halides: Do not undergo $S_N1$ (unstable carbocation)
• Bridgehead halides: Do not undergo $S_N1$ (Bredt's rule)
• Leaving group ability: I > Br > Cl > F
• Resonance stabilization: Delocalizes positive charge over multiple atoms