JEE Chemistry 2023 Question with Solution

Given: We must choose the halogen most reactive towards the $S_N1$ reaction in compounds A, B, C, and D.

Find: The correct option listing the most reactive halogen position in each compound.

For an $S_N1$ reaction, the rate depends on the stability of the carbocation formed after the leaving group departs. Better leaving groups and more stable carbocations favor the reaction.

The extracted explanation states:

• Compound A forms a benzyl carbocation, which is highly stable due to resonance.
• Compound B forms a primary carbocation, but reacts due to iodine's leaving group strength.
• Compound C forms a tertiary carbocation, which is very stable and reactive.
• Compound D forms a primary carbocation, less stable but reacts due to bromine's moderate leaving group strength.

Thus, the most reactive halogens are:

• A: Br(a)
• B: I(a)
• C: Br(b)
• D: Br(a)

Therefore, the correct option is A.

Given: Reactivity towards $S_N1$ depends mainly on carbocation stability and leaving group ability.

Find: Which labeled halogen leaves most readily in each structure.

Use the principle:

Also, among halides, iodide is generally a better leaving group than bromide.

Applying the extracted solution statements:

1. A gives a benzylic carbocation, stabilized by resonance, so Br(a) is preferred.
2. B is influenced by the stronger leaving ability of iodine, so I(a) is preferred.
3. C gives a tertiary carbocation, so the halogen at Br(b) is most reactive.
4. D reacts through the position indicated as Br(a) according to the extracted explanation.

Hence the sequence matches: A-Br(a); B-I(a); C-Br(b); D-Br(a)

Therefore, the correct option is A.