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JEE Main 2018 · Chemistry · Alcohols, Phenols, Ethers and Haloalkanes · easy

The correct order of reactivity of the following bromides towards SN2 reaction with I^- in acetone is

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The correct order of reactivity of the following bromides towards SN2\mathrm{S_N2} reaction with I−\mathrm{I^-} in acetone is
  1. A. CH3Br>CH3CH2CH2Br>(CH3)2CHBr>(CH3)3CCH2Br\mathrm{CH_3Br>CH_3CH_2CH_2Br>(CH_3)_2CHBr>(CH_3)_3CCH_2Br}
  2. B. CH3Br>(CH3)2CHBr>CH3CH2CH2Br>(CH3)3CCH2Br\mathrm{CH_3Br>(CH_3)_2CHBr>CH_3CH_2CH_2Br>(CH_3)_3CCH_2Br}
  3. C. CH3CH2CH2Br>CH3Br>(CH3)3CCH2Br>(CH3)2CHBr\mathrm{CH_3CH_2CH_2Br>CH_3Br>(CH_3)_3CCH_2Br>(CH_3)_2CHBr}
  4. D. (CH3)3CCH2Br>(CH3)2CHBr>CH3CH2CH2Br>CH3Br\mathrm{(CH_3)_3CCH_2Br>(CH_3)_2CHBr>CH_3CH_2CH_2Br>CH_3Br}
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Answer: (A) CH3Br>CH3CH2CH2Br>(CH3)2CHBr>(CH3)3CCH2Br\mathrm{CH_3Br>CH_3CH_2CH_2Br>(CH_3)_2CHBr>(CH_3)_3CCH_2Br}

SN2\mathrm{S_N2} is governed by steric hindrance at the carbon attacked from the back. Methyl bromide is least hindered and fastest. The primary propyl bromide is next; the secondary isopropyl bromide has two alkyl groups on the reacting carbon and is slower. Neopentyl bromide is primary but the bulky t-Bu\mathrm{t\text{-}Bu} group on the adjacent carbon blocks the backside approach almost completely, making it the slowest (about 10510^5 times slower than propyl bromide).
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