Which of the following is correct order of decreasing reactivity towards nucleophilic substitution reaction?

allyl chloride > n-propyl chloride > vinyl chloride

The correct order of decreasing reactivity towards nucleophilic substitution is indeed 2. Allyl chloride > n-propyl chloride > vinyl chloride.

This order is determined by the stability of the carbocation intermediates formed during nucleophilic substitution reactions. Allyl chloride forms a resonance-stabilized allyl cation, which is more stable than the non-stabilized carbocations formed by n-propyl chloride and vinyl chloride. This makes allyl chloride more reactive towards nucleophiles.

n-propyl chloride forms a secondary carbocation, which is more stable than the primary carbocation formed by vinyl chloride. This makes n-propyl chloride more reactive towards nucleophiles than vinyl chloride.

Vinyl chloride forms a primary carbocation, which is the least stable of the three carbocations. This makes vinyl chloride the least reactive towards nucleophiles.

Therefore, the correct order of decreasing reactivity towards nucleophilic substitution is allyl chloride > n-propyl chloride > vinyl chloride.