Question

Which of the following is least reactive in a nucleophilic substitution reaction?

• A. $(CH_3)_3C-Cl$
• B. $CH_2 =CHCl$
• C. $CH_3CH_2Cl$
• D. $CH_2 =CHCH_2Cl$

Answer 1

Answer: (B)

$CH_2 =CHCl$

Answer 2

Chlorine of vinyl chloride $(CH_2=CHCI)$ is non-reactive (less reactive) towards nucleophile (in nucleophilic substitution reaction) because it shows the following resonating structure due to $+M$ effect of $- Cl$ atom.
In structure II, $Cl$-atom have positive charge and partial double bond character with $C$ of vinyl group,so it is more tightly attracted towards the nucleus and it does not get replaced by nucleophile in $S_N$ reaction.