Question

In a nucleophilic substitution reaction:

$R-Br+C{l}^{-}\stackrel{DMF}{\to }R-Cl+B{r}^{-}$

Which one of the following undergoes complete inversion of configuration?

• A. $C_6{H}_{5}CH{C}_6{H}_{5}Br$
• B. $C_6{H}_{5}C{H}_{2}Br$
• C. $C_6{H}_{5}CH\left(C{H}_3\right)Br$
• D. $C_6{H}_{5}CC{H}_3{C}_6{H}_{5}Br$

Answer 1

The correct option is C $C_6{H}_{5}CH\left(C{H}_3\right)Br$

In the compound of option B, bromine is attached to a primary carbon atom. The substitution occurs via $S{N}_2$ mechanism which involves the inversion of configuration.
In the remaining compounds the bromine is attached to a secondary or tertiary carbon atom. In case of secondary carbon atom (options A and C), the substitution occurs via a combination of $S{N}_1$ and $S{N}_2$ mechanisms. Hence, complete inversion of configuration is not possible.
In case of tertiary carbon atom, (option D) the substitution occurs via $S{N}_1$ mechanism which cannot result in complete inversion of configuration.