Which compound in each of the following pairs will react faster in `SN^2` reaction with OH-?
I) `CH_3Br or CH_3I II) (CH_3)_3"CC"l or CH_3Cl III) CH_2 = CHBr or CH_2 = CHCH_2Br`

Options

A. `CH_3Br, (CH_3)_3"CC"l, CH_2 = CHBr`
B. `CH_3Br, (CH_3 )_3"CC"l, CH_2 = CHCH_2 Br`
C. `CH_3I, CH_3Cl, CH_3-CH_2Br`
D. `CH_3I, CH_3Cl, CH_2= CHCH_2Br`

Answer

D

Explanation

`CH_3` I as I- is a weaker base and a good leaving group, hence more reactive.
`CH_3 Cl` will react faster as the approach of nucleophile to the substitution centre is easier in case of primary halides as opposed to tertiary halides due to steric hindrance. `CH_ 2 = CHCH_2` Br will be less reactive as vinylic halides are unreactive towards `S_N2` reactions. This is because the planar geometry of the alkene does not allow back-side attack, and the electron-rich nature of the alkene does not attract nucleophiles.