Arrange the following compounds in increasing order of their reactivity towards $S_N2$ reaction:
(A) $CH_3CH_2CH_2CH_2Br$
(B) $(CH_3)_2CHCH_2Br$
(C) $CH_3CH_2CH(Br)CH_3$
(D) $(CH_3)_3CBr$
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (D), (C), (B), (A)
The increasing order of reactivity towards SN2 reaction is:
tert-butyl bromide < secondary alkyl bromide < branched primary bromide < straight-chain primary bromide
So, (D) < (C) < (B) < (A)
Key concept of SN2 reaction
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SN2 is a one-step reaction
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Nucleophile attacks from the backside
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Reaction is highly affected by steric hindrance
Less crowding = faster reaction
(D) (CH₃)₃CBr
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Tertiary alkyl halide
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Very bulky
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Backside attack is almost impossible
→ Least reactive
(C) CH₃CH₂CH(Br)CH₃
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Secondary alkyl halide
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Moderate steric hindrance
→ Reactivity increases compared to tertiary
(B) (CH₃)₂CHCH₂Br
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Primary alkyl halide but branched
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Some steric hindrance due to branching
→ More reactive than secondary but less than straight chain
(A) CH₃CH₂CH₂CH₂Br
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Primary alkyl halide
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Least steric hindrance
→ Most reactive in SN2