Question

Account for the fact that halogen in chlorobenzene is less reactive than in methyl chloride.

Answer 1

1. Reason for lower reactivity of halogen in chlorobenzene (aryl halide) than in methyl chloride (alkyl halide):
   • In chlorobenzene (C₆H₅Cl), the chlorine atom is bonded to an sp² hybridized carbon of the benzene ring. 
   • The lone pair on chlorine participates in resonance with the aromatic ring, delocalizing electron density and increasing the C–Cl bond strength.
   • This resonance effect creates partial double bond character to the C–Cl bond, making it stronger and less reactive towards nucleophilic substitution or hydrolysis.
   • In methyl chloride (CH₃Cl), chlorine is bonded to an sp³ hybridized carbon without resonance stabilization, so the C–Cl bond remains a typical single bond, more easily broken.
     Therefore, chlorine in chlorobenzene is less reactive than in methyl chloride.
2. Structures and IUPAC Names:
   • Methyl chloride (IUPAC: chloromethane)
     Structure: CH₃–Cl
   • Chlorobenzene (IUPAC: chlorobenzene)
     Structure: C₆H₅–Cl (aromatic ring with Cl substituent)
3. Equations showing reactions:
   • Hydrolysis of methyl chloride:
     \[\ce{CH3Cl + H2O → CH3OH + HCl}\]
   • Hydrolysis of chlorobenzene is very slow due to resonance stabilization, typically negligible under normal conditions:
     \[\ce{C6H5Cl + H2O → (no significant reaction)}\]