aromatic nucleophilic substitution reaction
Post on 13-Jul-2015
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Aromatic NucleophilicSubstitution Reaction
• SNAr Mechanism
• Benzyne 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
• It is an addition-elimination mechanism.
• Involves the formation of a carbanion with delocalized electrons, called as Meisenheimerintermediate.
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.
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:
• Organic Chemistry by Solomon & Fryhle 10th