Why are aryl halides less reactive towards nucleophilic substitution reactions than alkyl halides? How can we enhance the reactivity of aryl halides?

Aryl Halides vs. Alkyl Halides in Nucleophilic Substitution Reactions

Why Aryl Halides Are Less Reactive

1. Resonance Stabilization

The lone pair on the halogen participates in resonance with the aromatic ring, strengthening the C-X bond and making it harder to break.

2. sp² Hybridization

The carbon in aryl halides is sp² hybridized (vs. sp³ in alkyl halides), making it more electronegative and less electrophilic.

3. Steric Hindrance

The planar structure of aromatic rings makes the C-X bond less accessible to nucleophiles compared to tetrahedral alkyl halides.

4. Poor Leaving Group Stabilization

Halide ions are less stabilized when leaving from aryl halides compared to alkyl halides.

Enhancing Aryl Halide Reactivity

1. Electron-Withdrawing Groups (EWGs)

NO₂, CN, or COOH groups at ortho/para positions stabilize the negative charge in the Meisenheimer complex.

Example: 2,4-Dinitrochlorobenzene reacts readily with NaOH

2. Strong Nucleophiles & High Temperatures

Benzyne mechanism requires strong bases (NaNH₂) and heat.

3. Transition Metal Catalysis

Ullmann Reaction:

Aryl halide + Amine → Diaryl amine (Cu catalyst)

Buchwald-Hartwig Coupling:

Aryl halide + Amine → Aryl amine (Pd catalyst)

4. Nucleophilic Aromatic Substitution (S_NAr)

Requires:

• EWGs at ortho/para positions
• Good leaving group (F, Cl)

Example: p-Nitrofluorobenzene + NaOH → p-Nitrophenol

Summary

Aryl halides are less reactive due to resonance and sp² hybridization, but reactivity can be enhanced using EWGs, strong nucleophiles, high temperatures, or transition metal catalysis.