Arrange the following halides in decreasing order of reactivity for Williamson's synthesis `:`
`CH_(3)CH_(2)CH_(2)Br,CH_(3)CH_(2)CH_(2)Cl,`
`H_(2)C=CH-CH_(2)Cl,CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-CH_(2)Br`

Explanation

[Hint]
`H_(2)C=CH-CH(2)ClgtCH_(3)-CH_(2)-CH_(2)Br gt CH_(3)CH_(2)CH_(2)Cl gt CH_(3)-underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-CH_(2)Br`
Allylic compounds will be more reactive than saturative halides. Bromides are more reactive than chlorides because C-Br bond is weeaker than C-Cl bond.
`CH_(3)-underset(CH_(3))underset(|)overset (CH_(3))overset(|)(C)-CH_(2)-Br` is least reactive because of steric hindrance.