Which among the following compounds will undergo fastest SN2 reaction.
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Answer
(3) [figure]
Solution
SN2 needs backside attack, so the least hindered C–Br reacts fastest: 1° > 2° > 3°.
(1): Br is on a 2° carbon, so (1) is a 2° bromide
(2): Br is on a 3° carbon, so (2) is a 3° bromide
(3): Br is on a CH2 (1° carbon), so (3) is a 1° bromide, the least hindered
(4): Br is on a 3° ring carbon, so (4) is a 3° bromide
Hence (3) reacts fastest.
Answer: (3)