Question

Account for the following: 

Haloalkanes are more reactive than haloarenes.

Answer 1

1. Bond Type Difference: In haloalkanes, the carbon–halogen (C–X) bond is a pure single bond, while in haloarenes, the C–X bond has partial double bond character due to resonance with the aromatic ring.
2. Resonance in Haloarenes: In haloarenes (like chlorobenzene), the lone pair of electrons on the halogen overlaps with the π-electron cloud of the benzene ring, making the C–X bond stronger and harder to break.
3. Bond Strength: The C–X bond in haloarenes is stronger than in haloalkanes because of this partial double bond character, so it is less reactive in nucleophilic substitution. 
4. Aromatic Stability: In haloarenes, breaking the C–X bond would disrupt the aromaticity of the ring, which is energetically unfavorable and reduces reactivity.
5. Hybridization of Carbon: The carbon attached to the halogen in haloalkanes is sp³ hybridized, which is less electronegative and more open to nucleophilic attack. In haloarenes, it is sp² hybridized, holding electrons more tightly and resisting nucleophiles.
6. Nucleophilic Substitution: Haloalkanes undergo S_N1 or S_N2 reactions easily due to weaker C–X bonds. Haloarenes do not easily undergo nucleophilic substitution under normal conditions.
   For haloalkanes:
   \[\mathrm{CH_3CH_2Br} + \mathrm{OH^-} \longrightarrow \mathrm{CH_3CH_2OH} + \mathrm{Br^-}\]
   For haloarenes (no reaction generally):
   \[\mathrm{C_6H_5Br} + \mathrm{OH^-} \xrightarrow[\text{difficult}]{\text{heated, catalyst}} \mathrm{C_6H_5OH} + \mathrm{Br^-}\]