Aromatic NucleophilicSubstitution Reaction

Index

• SNAr Mechanism

• Benzyne mechanism

SNAr mechanism

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

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:

In presence of an electron withdrawing group

Mechanism

Features

• It is an addition-elimination mechanism.

• Involves the formation of a carbanion with delocalized electrons, called as Meisenheimerintermediate.

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.

Mechanism

Evidence

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:

NMR

Phenylation

Bibliography

• Organic Chemistry by Solomon & Fryhle 10th

edition (981-987).