Why is the nucleophilic substitution of OH- with 1-bromopentane slower than with 2-bromo-2-methylbutane?


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> Which statement about the reaction of a hydroxide ion with the organic reagent is correct?
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> **A**. 1-bromopentane predominantly follows an $\mathrm{S\_N1}$ mechanism.
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> **B**. 2-bromo-2-methylbutane predominantly follows an $\mathrm{S\_N2}$ mechanism.
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> **C**. Reaction with 1-bromopentane occurs at a slower rate than with 1-chloropentane.
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> **D**. Reaction with 1-bromopentane occurs at a slower rate than with 2-bromo-2-methylbutane.
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I got the answer correct (**D**), but only by eliminating the other three options. However, why is D true? Does this mean that $\mathrm{S\_N1}$ reactions are always faster than $\mathrm{S\_N2}$ reactions when they are reacting with the same nucleophile?


I'd really appreciate some help.


