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

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Solution:

Chlorine of vinyl chloride is non-reactive (less reactive) towards nucleophile in nucleophilic substitution reaction because it shows the following resonating structure due to effect of atom.
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In structure II, 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 reaction.