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For alkyl halides containing a halogen atom X, which sequence represents the decreasing order of reactivity toward an S_N2 reaction?

1. R2CHX > R3CX > RCH2X
2. RCH2X > R3CX > R2CHX
3. RCH2X > R2CHX > R3CX
4. R3CX > R2CHX > RCH2X

Correct answer: RCH2X > R2CHX > R3CX

Solution

SN2 proceeds by backside attack, so steric crowding slows it down. Reactivity therefore decreases from primary to tertiary: RCH2X > R2CHX > R3CX.

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