The compound from which chlorobenzene cannot be prepared easily is
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The reactivity of the C-X bond (where X is a heteroatom like O, N, or a halogen) attached to a benzene ring is very different from its aliphatic counterpart. Resonance makes the bond stronger and less susceptible to nucleophilic substitution. This is why phenols are not easily converted to haloarenes.
Step 1: Understanding the Concept:
Aryl halides (like chlorobenzene) require specific synthetic pathways due to the unreactivity of aromatic rings toward standard nucleophilic substitution. We must evaluate standard industrial and laboratory syntheses. Step 2: Key Formula or Approach:
Review the reactivity of the functional groups in each option towards halogenation or substitution reactions to yield chlorobenzene. Step 3: Detailed Explanation:
(A) Aniline: Can be easily converted via diazotization ($NaNO_2/HCl$) to benzene diazonium chloride, which then yields chlorobenzene via the Sandmeyer reaction.
(B) Benzene: Direct electrophilic aromatic halogenation with $Cl_{2}$ in the presence of an anhydrous Lewis acid catalyst (like $FeCl_{3}$ or $AlCl_{3}$) easily produces chlorobenzene.
(C) Phenol: Replacing the strong $-OH$ group on an aromatic ring directly with a halogen is exceedingly difficult. While alcohols easily react with $PCl_{5}$ or $SOCl_{2}$ to give alkyl halides, phenols do not. The $C-O$ bond has strong partial double bond character due to resonance. Reactions with $PCl_5$ give very poor yields and mostly result in triphenyl phosphate esters.
(D) Benzene diazonium chloride: Reacts flawlessly with $CuCl/HCl$ (Sandmeyer reaction) or Cu powder/$HCl$ (Gattermann reaction) to give high yields of chlorobenzene. Step 4: Final Answer:
Phenol is the compound from which chlorobenzene cannot be prepared easily.