Arrange the following in decreasing order of reactivity towards `SN^2` reaction (1-Bromopropane,2-Bromo-2-methylpropane ,2-Bromopropane)

Explanation

`S_(N)2` reactions involve the formation of transition state. Higher the steric hindrance, lesser the stability of transition state and lower is their reactivity towards `S_(N)2`. The order of decreasing reactivity of compounds is given as :
`undersetunderset((1^(@)))("1-bromopentane")(CH_(3)CH_(2)CH_(2)CH_(2)CH_(2))Brgtundersetundersetundersetunderset((2^(@))" ")("2-bromopentane ")("Br ")("| ")(CH_(3)CHCH_(2)CH_(2)CH_(3))gtundersetunderset(3^(@))("2-bromo-2-methylbutane")(CH_(3)undersetunderset(CH_(3)" ")("| ")oversetoverset("Br ")("| ")(C CH_(2)CH_(3)))`
This order is due to steric reasons, the ordre of reactivity in `S_(N)2` follows the order. `1^(@)gt2^(@)gt3^(@)`.