Question

In a $S_{N}2$ substitution reaction of the type,
$R-Br+C{l}^{-}\stackrel{DMF}{\to }R-Cl+B{r}^{-}$
Which one of the following has the highest relative rate?
[1 Mark]

• A. $(C{H}_3{)}_{3}CBr$
• B. $(C{H}_3{)}_{3}C-CH(Br)-C(C{H}_3{)}_3$
• C. $(C{H}_3{)}_{3}C-C{H}_{2}Br$
• D. $C{H}_3-C{H}_{2}Br$

Answer 1

The correct option is D $C{H}_3-C{H}_{2}Br$

$S_{N}2$ mechanism is followed in case of primary and secondary alkyl halides i.e. $S_{N}2$ reaction is favoured by small groups on the carbon atoms attached to halogen. So, reactivity order is:
$C{H}_3–X>R–C{H}_2–X>R_{2}CH–X>R_{3}C–X$. Primary is more reactive than secondary and secondary is more reactive than tertiary alkyl halides.