Out `CH_(3)-underset(CH_(3))underset(|)(CH)-CH_(2)-Cl` and `CH_(3)-CH_(2)-underset(CH_(3))underset(|)(CH)-Cl` , which is more reactive towards `S_(N)1` reaction and why?

Explanation

`CH_(3) –CH_(2)-underset(CH_(3))underset(|)CH_(2)-Cl` is more reactive.
The `S_(N)1`reaction proceeds through the formation of carbocation. The compound which forms more stable carbocation will be more reactive.
`underset(" 1-Chloro-2-methylpropane")(CH_(3)-underset(CH_(3))underset(|)CH-CH_(2)-Cl) overset(-Cl,^(-))to underset(1^(@) "Carbocation")(CH_(3)-underset(CH_(3))underset(|)CH-overset(+)CH_(2))`
`underset("2- Chlorobutane")(CH_(3) –CH_(2)-underset(CH_(3))underset(|)CH-Cl) overset(-Cl^(-))to underset(2^(@) "Carbocation")(CH_(3)-CH_(2)-underset(CH_(3))underset(|)overset(+)CH)`
Since `2^(@)` carbocation is more stable than `1^(@)` carbocation, 2-Chlorobutane will be more reactive towards `S_(N)1` reaction.