Question

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

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).

Key concepts

Chlorobenzene Reactivity

Chlorobenzene is a compound where a chlorine atom is directly bonded to a benzene ring. This structure influences its chemical reactivity, particularly in nucleophilic substitution reactions. Nucleophilic substitution is a process where a nucleophile, rich in electrons, targets an electrophilic center in a molecule to form a new bond, replacing another group, in this case, a chlorine atom.
Chlorobenzene shows lower reactivity towards these reactions compared to benzyl chloride. This can be partly explained by the partial double-bond character of the C-Cl bond, which arises from resonance.
Additionally, its reactivity is impacted by the deactivation of the benzene ring caused by the high electronegativity of chlorine. These factors make the C-Cl bond stronger and less susceptible to nucleophiles compared to the C-Cl bond in benzyl chloride, where chlorine is attached to a methyl group rather than the aromatic ring.

• Chlorine's partial double-bond character strengthens the C-Cl bond.
• Deactivation from chlorine reduces the benzene ring's reactivity.

Resonance Effect

In organic chemistry, resonance refers to the delocalization of electrons within a molecule that can spread over adjacent atoms, stabilizing the structure.
In the case of chlorobenzene, this resonance effect significantly impacts the C-Cl bond. The lone pair electrons from the chlorine can participate in resonance with the π-electrons of the benzene ring. This participation creates a series of resonance structures where the C-Cl bond exhibits partial double-bond character.
This characteristic of having electrons shared between the carbon and chlorine atoms makes the bond shorter, stronger, and less reactive towards nucleophiles. Essentially, the resonating structures make the removal of the chlorine atom more challenging because the electrons are not localized just between the carbon and the chlorine, but also involve the aromatic system.

• Resonance increases bond strength and stability.
• Partial double-bond character results from electron delocalization.

Hybridization in Organic Chemistry

Hybridization refers to the mixing of atomic orbitals to form new hybrid orbitals that are ideal for bonding interactions. In organic chemistry, different hybridization states can influence molecular structures and reactivity.
In chlorobenzene, the carbon atom bonded to the chlorine is sp² hybridized. This hybridization is typical for atoms in aromatic systems, allowing for the formation of stable structures like benzene rings with delocalized π-electrons. The sp² hybrid orbitals result in a planar, trigonal shape, which contributes to the resonance effect seen in chlorobenzene.
Meanwhile, in benzyl chloride, the carbon attached to the chlorine is sp³ hybridized, which allows for more flexibility and less electron sharing with the ring system. This sp³ hybridization means the geometry is tetrahedral, lessening the extent of resonance and sharing between the carbon and chlorine atoms. Consequently, the bond is generally weaker and more susceptible to attack by nucleophiles.

• sp² hybridization supports aromatic resonance.
• sp³ hybridization allows increased reactivity with nucleophiles.