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Which alkyl halide from the following pair would you expect to react more rapidly by an SN2 mechanism? Explain your answer. CHA3CHA2CHCHA3...|....Br or .....CHA3..|HA3C−C−Br..|....CHA3 - Chemistry

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Question

Which alkyl halide from the following pair would you expect to react more rapidly by an SN2 mechanism? Explain your answer.

\[\begin{array}{cc}\ce{CH3CH2CHCH3}\\\phantom{...}|\\\phantom{....}\ce{Br}\end{array}\] or \[\begin{array}{cc}\phantom{.....}\ce{CH3}\\\phantom{..}|\\\ce{H3C - C - Br}\\\phantom{..}|\\\phantom{....}\ce{CH3}\end{array}\]

Explain

Solution

The SN2 process involves a transition state with both an incoming nucleophile and a leaving group surrounding the carbon atom. Five atoms are simultaneously bonded together. A transition state requires minimal steric hindrance. Hence, 1° alkyl halides are the most reactive to SN2, followed by 2° and 3°.

1° RX > 2° RX > 3° RX

Based on the above order, \[\begin{array}{cc}
\ce{CH3CH2CHCH3}\\
\phantom{...}|\\
\phantom{....}\ce{Br}\
\end{array}\] is more reactive.

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