Which alkyl halide from the following pairs would you expect to react more rapidly by an S_N2 mechanism? Explain your answer.
CH₃CH₂CH₂CH₂Br or CH₃CH₂CHCH₃ or CH₃C–C–Br
CH₃CH₂CH₂CH₂Br reacts more rapidly by S_N2 mechanism.
CH₃CH₂CHCH₃ or H₃C–C–Br
CH₃CH₂CHCH₃ reacts more rapidly by S_N2 mechanism.
CH₃CHCH₂CH₂Br or CH₃CH₂CHCH₂Br
CH₃CHCH₂CH₂Br reacts more rapidly by S_N2 mechanism.
Explanation
The question tests understanding of S_N2 reactivity order based on steric hindrance around the carbon bearing the halogen. The context clearly states that for S_N2 reactions, the order of reactivity is: Primary halide > Secondary halide > Tertiary halide. This is because S_N2 mechanism involves a backside attack by the nucleophile, which becomes increasingly difficult as steric hindrance increases around the reaction center.