Which would undergo `S_(N^(2))` reaction faster in the following pair and why?
`CH_(3)-CH_(2)-Br and CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-Br`.

Explanation

Reactivity in `S_(N)2` reactions depends upon the steric hindrance at the `alpha`-carbon to which the halogen is attached. Since the steric hindrance increases in the order: `1^(@) lt 2^(@) lt 3^(@)` alkyl halides, therefore, reactivity decreases in the reverse order: `1^(@) gt 2^(@) gt 3^(@)` alkyl halides.
In other words, `CH_(3)CH_(2)Br` being a `1^(@)` alkyl bromide undergoes `S_(N)2` reaction faste3r than the `3^(@)` alkyl bromide.
`CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(.^(alpha)C)-Br`