1. Aromatic Nucleophilic
Substitution Reaction

2. Index
• SNAr Mechanism
• Benzyne mechanism

3. SNAr mechanism
• Simple aryl halides, are relatively unreactive toward
nucleophilic substitution under conditions that
would give rapid nucleophilic substitution with alkyl
halides.

4. Two effects make the carbon–halogen bonds of aryl
halides shorter and stronger:
• The carbon of halide is sp2 hybridized, and therefore the
electrons of the carbon orbital are closer to the nucleus
than those of an sp3-hybridized carbon.
• Resonance strengthens the carbon–halogen bond by giving
it double-bond character:

5. In presence of an electron
withdrawing group

6. Mechanism

7. Features
• It is an addition-elimination mechanism.
• Involves the formation of a carbanion with
delocalized electrons, called as Meisenheimer
intermediate.

8. Benzyne Mechanism
Although aryl halides such as chlorobenzene and
bromobenzene do not react with most nucleophiles under
ordinary circumstances, they do react under highly forcing
conditions. Chlorobenzene can be converted to phenol by
heating it with aqueous sodium hydroxide in a pressurized
reactor at 350°C.

9. Mechanism

10. Evidence

11. Trapping of Intermediate
Benzyne intermediates have been “trapped”
through the use of Diels–Alder reactions. One
convenient method for generating benzyne is the
diazotization of anthranilic acid (2-aminobenzoic acid)
followed by elimination of CO2 and N2:

12. NMR

13. Phenylation

14. Bibliography
• Organic Chemistry by Solomon & Fryhle 10th
edition (981-987).