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The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring.〔Friedel, C.; Crafts, J. M. (1877) "Sur une nouvelle méthode générale de synthèse d’hydrocarbures, d’acétones, etc.," ''Compt. Rend.'', 84: (1392 ) & (1450 ).〕 There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution. The general reaction scheme is shown below. : Several reviews have been written. == Friedel–Crafts alkylation == Friedel–Crafts alkylation involves the alkylation of an aromatic ring with an alkyl halide using a strong Lewis acid catalyst. With anhydrous ferric chloride as a catalyst, the alkyl group attaches at the former site of the chloride ion. The general mechanism is shown below. : This reaction has one big disadvantage, namely that the product is more nucleophilic than the reactant due to the electron donating alkyl-chain. Therefore, another hydrogen is substituted with an alkyl-chain, which leads to overalkylation of the molecule. Also, if the chloride is not on a tertiary carbon or secondary carbon, then the carbocation formed (R+) will undergo a carbocation rearrangement reaction. This reactivity is due to the relative stability of the tertiary and secondary carbocation over the primary carbocations. Steric hindrance can be exploited to limit the number of alkylations, as in the ''t''-butylation of 1,4-dimethoxybenzene. : Alkylations are not limited to alkyl halides: Friedel–Crafts reactions are possible with any carbocationic intermediate such as those derived from alkenes and a protic acid, Lewis acid, enones, and epoxides. An example is the synthesis of neophyl chloride from benzene and methallyl chloride: :H2C=C(CH3)CH2Cl + C6H6 → C6H5C(CH3)2CH2Cl In one study the electrophile is a bromonium ion derived from an alkene and NBS: : In this reaction samarium(III) triflate is believed to activate the NBS halogen donor in halonium ion formation. ==Friedel–Crafts dealkylation== Friedel–Crafts alkylation is a reversible reaction. In a reversed Friedel–Crafts reaction or Friedel–Crafts dealkylation, alkyl groups can be removed in the presence of protons and a Lewis acid. For example, in a multiple addition of ethyl bromide to benzene, ''ortho'' and ''para'' substitution is expected after the first monosubstitution step because an alkyl group is an activating group. However, the actual reaction product is 1,3,5-triethylbenzene with all alkyl groups as a ''meta'' substituent. Thermodynamic reaction control makes sure that thermodynamically favored ''meta'' substitution with steric hindrance minimized takes prevalence over less favorable ''ortho'' and ''para'' substitution by chemical equilibration. The ultimate reaction product is thus the result of a series of alkylations and dealkylations. : 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「Friedel–Crafts reaction」の詳細全文を読む スポンサード リンク
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