Answer: Much faster because the benzylic carbocation is resonance-stabilised.
- A Slower due to aromatic stabilisation of the starting chloride itself
- B Essentially the same rate as observed for n-propyl chloride
- C Only modestly faster, by a small and chemically insignificant margin
- D Much faster because the benzylic carbocation is resonance-stabilised
Correct answer: D. Much faster because the benzylic carbocation is resonance-stabilised
Explanation: The benzylic carbocation is stabilised by resonance with the aromatic ring (the positive charge is delocalised into the ring), dramatically lowering the energy of the transition state and accelerating SN1.
SN2 proceeds in a single step with backside attack and inversion of configuration, while SN1 forms a planar carbocation intermediate first, leading to racemisation.
Concept context
Study carbon-halogen compounds: how they are made, how they react via SN1/SN2 and elimination, their stereochemistry, and their environmental impact.