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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. 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 : * the free radical SRN1 mechanism * the ANRORC mechanism * Vicarious nucleophilic substitution. 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. == SNAr reaction mechanism == Aryl halides cannot undergo SN2 reaction.The C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is not possible.〔''Organic Chemistry'' J. Clayden , Oxford University Press〕 SN1 reaction is possible but very unfavourable. It would involve the unaided loss of the leaving group and the formation of an aryl cation.〔 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 an activator toward nucleophilic substitution, and a meta director, it allows the benzene carbon to which it is 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 that favors the 2,4-dinitrophenol. 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 again. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nucleophilic aromatic substitution」の詳細全文を読む スポンサード リンク
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