Chapter 4: Problem 195

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.

Short Answer

Expert verified

Answer: Chlorobenzene is less reactive than benzyl chloride towards nucleophilic substitution due to the resonance and partial double bond character between the C-Cl bond in chlorobenzene, making the bond stronger and less prone to nucleophilic substitution.

Step by step solution

Chlorobenzene has the formula \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\), where chlorine is directly bonded to the benzene ring. Benzyl chloride has the formula \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}\), which has chlorine attached to a \(\mathrm{CH}_{2}\) group that is linked to the benzene ring. In nucleophilic substitution reactions, the chlorine atom in both molecules can be replaced by a nucleophile. #Step 2: Analyze statement (a)#

Statement (a) mentions the hybridization of carbon containing the chlorine in both molecules. The carbon of \(\mathrm{C}-\mathrm{Cl}\) bond in \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}\) (chlorobenzene) is indeed sp \(^{2}\) hybridized, while in \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}\)'s (benzyl chloride), the chlorine-containing carbon is sp \(^{3}\) hybridized. However, this fact doesn't directly impact the reactivity towards nucleophilic substitution of these compounds. So, statement (a) is not the correct reason. #Step 3: Analyze statement (b)#

Statement (b) talks about the polarity of the \(\mathrm{C}-\mathrm{Cl}\) bond in both molecules. A higher polarity means a more polarized distribution of electrons, which may increase reactivity towards nucleophilic substitution. While it may affect the reactivity, it's not the primary reason behind the difference observed between chlorobenzene and benzyl chloride. Therefore, statement (b) is not correct either. #Step 4: Analyze statement (c)#

Statement (c) involves the resonance and partial double bond character between the \(\mathrm{C}-\mathrm{Cl}\) bond in chlorobenzene. In chlorobenzene, the lone pair electrons on the chlorine atom participate in resonance with the delocalized electrons in the benzene ring which gives the C-Cl bond partial double bond character. As a result, the bond becomes shorter and stronger, making it less prone to nucleophilic substitution. So, statement (c) is the correct reason why chlorobenzene is less reactive than benzyl chloride towards nucleophilic substitution. #Step 5: Analyze statement (d)#

Statement (d) relates to the deactivating property of chlorine in chlorobenzene due to its high electronegativity. Although chlorine is electronegative and can deactivate the benzene ring, it doesn't directly impact the reactivity of chlorobenzene towards nucleophilic substitution. So, statement (d) is not the correct reason. #Step 6: Provide the correct answer#

After analyzing all statements (a), (b), (c), and (d), the correct reason why chlorobenzene is less reactive than benzyl chloride towards nucleophilic substitution is statement (c).