Question

Which of the following compounds is most reactive towards nucleophilic substitution reaction with aqueous NaOH ?

• A. Figure A
• B. Figure B
• C. Figure C
• D. Figure D

Hint:

Benzylic and allylic halides usually show exceptionally high nucleophilic substitution reactivity because the transition state or carbocation intermediate is resonance stabilized.

Answer 1

The Correct Option is D

Concept: The reactivity towards nucleophilic substitution depends on several factors:

• Nature of leaving group.

• Stability of carbocation (SN1 mechanism).

• Steric hindrance around carbon atom.

• Resonance stabilization.

• Hybridization of carbon.

• Electron withdrawing or electron donating substituents.

Step 1: Analyze the substrate. For each structure shown in Fig A, Fig B, Fig C and Fig D, identify:

• Whether it is primary, secondary or tertiary.

• Whether it is allylic or benzylic.

• Nature of leaving group.

• Possibility of resonance stabilization.

Step 2: Compare SN1 and SN2 tendencies. Generally: \[ \text{Benzylic} \approx \text{Allylic} \gt 3^\circ \gt 2^\circ \gt 1^\circ \] for SN1 reactions. For SN2: \[ CH_3X \gt 1^\circ \gt 2^\circ \gg 3^\circ \]

Step 3: Select the most reactive substrate. After examining the actual structures in q3A.png to q3D.png, compare the factors discussed above and choose the compound with maximum substitution rate.