Question

Account for the following:

Haloalkanes undergo nucleophilic substitution, whereas haloarenes undergo electrophilic substitution.

Answer 1

1. Nature of the Carbon Atom: In haloalkanes, the carbon attached to the halogen is sp³ hybridized and electron-deficient, making it susceptible to attack by nucleophiles.
2. Type of Reaction Favored: Due to the polar nature of the C–X bond, haloalkanes readily undergo nucleophilic substitution reactions, where a nucleophile replaces the halogen.
3. Haloarenes Have Aromatic Rings: In haloarenes, the halogen is attached to an aromatic ring, which has a stable π-electron system that does not favor nucleophilic attack.
4. Resonance in Haloarenes: The lone pair of electrons on the halogen in haloarenes participates in resonance with the benzene ring, making the C–X bond stronger and less reactive toward nucleophiles.
5. Electron Density in the Ring: The resonance also increases electron density at the ortho and para positions of the ring, making haloarenes reactive toward electrophiles.
6. Electrophilic Substitution in Haloarenes: As a result, haloarenes undergo electrophilic substitution, where an electrophile replaces a hydrogen atom on the ring, especially at ortho and para positions.
7. Lack of Aromaticity Disruption: Electrophilic substitution preserves the aromaticity of the ring, making it a favorable reaction for haloarenes.
8. For haloalkane nucleophilic substitution (S_N2):
   \[\mathrm{CH_3Br}+\mathrm{OH}^-\rightarrow \mathrm{CH_3OH} + \mathrm{Br}^-\]
9. For haloarene electrophilic substitution (chlorination):
   \[\mathrm{C_6H_5Cl}+\mathrm{Cl_2} \xrightarrow{FeCl_3}\mathrm{C_6H_4Cl_2}+\mathrm{HCl}\]