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Concept
Nucleophilic aromatic substitution
Summary
A nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring. Aromatic rings are usually nucleophilic, but some aromatic compounds do undergo nucleophilic substitution. Just as normally nucleophilic alkenes can be made to undergo conjugate substitution if they carry electron-withdrawing substituents, so normally nucleophilic aromatic rings also become electrophilic if they have the right substituents.This reaction differs from a common SN2 reaction, because it happens at a trigonal carbon atom (sp2 hybridization). The mechanism of SN2 reaction does not occur due to steric hindrance of the benzene ring. In order to attack the C atom, the nucleophile must approach in line with the C-LG (leaving group) bond from the back, where the benzene ring lies. It follows the general rule for which SN2 reactions occur only at a tetrahedral carbon atom.
The SN1 mechanism is possible but very unfavourable unless the leaving group is an exceptionally good one. It would involve the unaided loss of the leaving group and the formation of an aryl cation. In the SN1 reactions all the cations employed as intermediates were planar with an empty p orbital. This cation is planar but the p orbital is full (it is part of the aromatic ring) and the empty orbital is an sp2 orbital outside the ring.
There are six different mechanism by which aromatic rings undergo nucleophilic substitution.
There are 6 nucleophilic substitution mechanisms encountered with aromatic systems:
The SNAr (addition-elimination) mechanism
The aromatic SN1 mechanism encountered with diazonium salts
The benzyne mechanism (E1cb-AdN)
The free radical SRN1 mechanism
ANRORC mechanism
Vicarious nucleophilic substitution.
The SNAr mechanism is the most important of these. Electron withdrawing groups activate the ring towards nucleophilic attack. For example if there are nitro functional groups positioned ortho or para to the halide leaving group, the SNAr mechanism is favored.
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