Which alkyl halide from the following pairs would you expect to react more rapidly by an S_N2 mechanism? Explain your answer.

2-bromo-2-methylpropane is tertiary alkyl halide whereas 2-bromobutane is secondary alkyl halide also, 2-bromo-2-methylpropane has number of substituents are present. Hence, 2- bromobutane reacts more rapidly than 2-bromo-2-methylpropane by an S_N2 mechanism.