Which is more reactive chlorobenzene and chloroethane in the nucleophilic substitution and why?
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CBSE 12-science - Chemistry
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CBSE 12-science - Chemistry
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Jillian H.
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Chlorobenzene is less reactive than benzyl chloride towards nucleophilic substitution because (a) the carbon of $\mathrm{C}-\mathrm{Cl}$ bond in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$ is sp $^{2}$ hybridized and holds the electron pair more firmly as compared to sp $^{3}$ carbon containing chlorine in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}$. (b) the $\mathrm{C}-\mathrm{Cl}$ bond in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$ is less polar compared to that in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}$. (c) the C-Cl bond in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$ acquires partial double bond character due to resonance involving the lone pair on chlorine atom. (d) the chlorine in $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$ is deactivating due to its high electronegativity.
Text Solution
Solution : In the molecules of benzyl chloride, the electron pairs on the chlorine atom are not in a position to conjugate with the `pi`-electrons of the ring. <br> <img src="//d10lpgp6xz60nq.cloudfront.net/physics_images/MBD_EM_CHE_XII_C10_E03_029_S01.png" width="80%"> <br> Whereas electron pairs on the chlorine atom are in conjugation with the `pi`-electrons of the ring in the molecules of chlorobenzene. Consequently, the C-Cl bond in chlorobenzene acquires some double bond character, which is not present in benzyl chloride. As a result, the reactivity of benzyl chloride is more than the chlorobenzene.