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Narrative
Four test tubes contain a solution of sodium iodide in acetone. The following isomeric alkyl halides are added to separate tubes: 1-bromobutane, 1-bromo-2-methylpropane, 2-bromobutane, and 2-bromo-2-methylpropane. The solution containing the primary halide, 1-bromobutane, quickly becomes cloudy as sodium bromide precipitates. The more hindered primary halide, 1-bromo-2-methylpropane, reacts after about 5 minutes. The secondary halide, 2-bromobutane, reacts after about 13 minutes.

Discussion
This order of reactivity (primary > secondary halide) is characteristic of an S_N2 reaction.