Which of the following is the correct order of decreasing reactivity towards nucleophilic substitution?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → allyl chloride > n-propyl chloride > vinyl chloride
Reactivity towards nucleophilic substitution depends on how easily the C–Cl bond breaks: the type of carbon holding the chlorine and the stability of any carbocation formed decide it.
First – allyl chloride, \( \ce{CH2=CH-CH2-Cl} \): Cl is on an sp3 carbon next to the C=C (an allylic carbon). The allylic carbocation formed is stabilised by resonance, so the C–Cl bond breaks easily and the reactivity is high.
\( \displaystyle \ce{CH2=CH-CH2^+} \longleftrightarrow \ce{{}^{+}CH2-CH=CH2} \)
Second – n-propyl chloride, \( \ce{CH3CH2CH2-Cl} \): an ordinary primary haloalkane. Cl is on an sp3 carbon, so substitution goes at the normal rate, but there is no resonance help as in the allylic case.
Third – vinyl chloride, \( \ce{CH2=CH-Cl} \): Cl is on an sp2 carbon of the C=C (a vinylic halide). The two reasons that make haloarenes unreactive apply here too: the lone pair on Cl is in conjugation with the π electrons of the double bond, which gives the C–Cl bond partial double bond character, and the sp2 carbon, with more s-character, holds the bond pair more tightly. The bond is hard to break, so vinyl chloride is the least reactive.
\( \displaystyle \ce{CH2=CH-Cl} \longleftrightarrow \ce{{}^{-}CH2-CH=Cl^{+}} \)
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “… Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. … For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction.”
(a) Vinylic halides – “These are the compounds in which the halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-carbon double bond (C = C).”
6.7.2 Reactions of Haloarenes – “(i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring … C—Cl bond acquires a partial double bond character due to resonance.”
“(ii) Difference in hybridisation of carbon atom in C—X bond: In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp2-hybridised.”
“The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter. … Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.”