Which SN2 reaction in the following pair is faster: (A) with or (B) with ?
A
Reaction (B) is faster because the substrate is more substituted.
B
Both reactions proceed at the same rate.
C
Neither reaction occurs via the SN2 mechanism.
D
Reaction (A) is faster because the substrate is less sterically hindered.
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1
Identify the substrates involved in each reaction: (A) is 1-bromopropane (a primary alkyl halide) and (B) is 2-bromopropane (a secondary alkyl halide).
Recall that SN2 reactions proceed via a backside attack mechanism, which is highly sensitive to steric hindrance around the electrophilic carbon.
Understand that primary alkyl halides (like in reaction A) have less steric hindrance compared to secondary alkyl halides (like in reaction B), making nucleophilic attack easier and faster.
Recognize that the nucleophile in both reactions is hydroxide ion (OH⁻), which is a strong nucleophile favoring SN2 mechanisms.
Conclude that reaction (A) will be faster because the less substituted (primary) alkyl halide offers less steric hindrance, facilitating a faster SN2 reaction.