Nucleophilic aromatic substitution
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A nucleophilic 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. There are 5 nucleophilic substitution mechanisms encountered with aromatic systems:
- the SNAr (addition-elimination) mechanism
- the aromatic SN1 mechanism encountered with diazonium salts
- the benzyne mechanism
- the free radical SRN1 mechanism
- the ANRORC mechanism.
The most important of these is the SNAr mechanism, where 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.
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[edit] SNAr reaction mechanism
The following is the reaction mechanism of a nucleophilic aromatic substitution of 2,4-dinitrochlorobenzene in a basic aqueous solution.
In this sequence the carbons are numbered clockwise from 1-6 starting with the 1 carbon at 12 o'clock which is bonded to the chloride. Since the nitro group is a deactivator towards nucleophilic substitution, and a meta director, they allow the benzene carbon to which they are bonded to have a negative charge. In the Meisenheimer complex, the nonbonded electrons of the carbanion become bonded to the aromatic pi system which allows the ipso carbon to temporarily bond with the hydroxyl group (-OH). In order to return to a lower energy state, either the hydroxyl group leaves, or the chloride leaves. In solution both processes happen. A small percentage of the intermediate loses the chloride to become the product (2,4-dinitrophenol), while the rest return to the reactant. Since 2,4-dinitrophenol is in a lower energy state it will not return to form the reactant, so after some time has passed, the reaction reaches chemical equilibrium.
The formation of the resonance-stabilized Meisenheimer complex is slow because it is in a higher energy state than the aromatic reactant. The loss of the chloride is fast, because the ring becomes aromatic once again.
[edit] Nucleophilic aromatic substitution reactions
Some typical substitution reactions on arenes are listed below.
- In the Bamberger rearrangement N-phenylhydroxylamines rearrange to 4-aminophenols. The nucleophile is water.
- In the Sandmeyer reaction diazonium salts react with halides.
- The Smiles rearrangement is the intramolecular version of this reaction type.
Nucleophilic aromatic substitution is not limited to arenes though, the reaction takes place even more readily with heteroarenes. Pyridines are especially reactive when substituted in the aromatic ortho position or aromatic para position because then the negative charge is effectively delocalized at the nitrogen position.
In the compound methyl 3-nitropyridine-4-carboxylate the meta nitro group is actually displaced by fluorine with caesium fluoride in DMSO at 120°C. [1]
[edit] See also
- The nucleophilic substitution reactions in organic chemistry are nucleophilic aliphatic substitution including SN1 reactions, SN2 reactions and SNi reactions, nucleophilic aromatic substitution and nucleophilic acyl substitution.
- List of publications in organic chemistry
