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Aromatic nucleophilic substitution reaction
This document summarizes the aromatic nucleophilic substitution (SNAr) reaction mechanism. It involves the formation of a carbanion intermediate called the Meisenheimer intermediate through an addition-elimination process. Aryl halides are relatively unreactive toward nucleophilic substitution, but reactivity increases in the presence of electron-withdrawing groups due to stabilization of the carbanion. Under highly forcing conditions, aryl halides can undergo substitution through a benzyne intermediate that has been trapped using Diels-Alder reactions.
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Aromatic nucleophilic substitution reaction
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- 2. Index • SNAr Mechanism •Benzyne mechanism
- 3. SNAr mechanism • Simplearyl halides, are relatively unreactive toward nucleophilic substitution under conditions that would give rapid nucleophilic substitution with alkyl halides.
- 4. Two effects makethe 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 ofan electron withdrawing group
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- 7. Features • It isan 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.
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- 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:
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- 14. Bibliography • Organic Chemistryby Solomon & Fryhle 10th edition (981-987).