The Electrical Effect of the Trifluoromethyl Group
The Electrical Effect of the Trifluoromethyl Group
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三氟甲基的电效应
DOI：
--
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发表时间：
1950
期刊：
影响因子：
--
通讯作者：
E. A. Mcelhill
中科院分区：
文献类型：
--
作者：
John D. Roberts;R. Webb;E. A. Mcelhill
在亲电芳香取代反应中，三氟甲基和三氯甲基具有很强的元取向效应，这通常归因于电子吸引效应，而电子吸引效应在取向基的元位置上最不强烈。阳离子电荷在o位和p位的优先感应可能归因于两种相当不同类型的共振形式的贡献。I型是一种类似于烷基的“无键”共振（I型的超共轭共振可能对a更有效）
The strong meta-orienting influence of the trifluoromethyl and trichloromethyl groups in electrophilic aromatic substitution reactions is usually attributed to an electron-attracting effect which is least strongly felt at the position meta to the orienting group. The preferential induction of cationic charge at the o- and p-positions may be ascribed to contributions of resonance forms of two rather different types. Type I is a variety of “no-bond" resonance analogous to that postulated for alkyl groups (hyperconjugation Resonance of type I might be expected to be considerably more effective for a
trifluoromethyl group than hyperconjugation is for a methyl group because of the magnitude of the electronegativity difference between fluorine and carbon as compared to that between carbon and hydrogen. A second mode (II) of preferential relay of the electron-attracting influence of the trifluoromethyl group to the o- and p-positions may be inferred from explanations for the metadirecting power of --N^+(CH_3)_3 and similar groups. Alternatively, II may be expressed by the resonance structures, III, which denote induction of a positive charge on C-1 by the electron-attracting group (IIIa) and distribution of the charge on C-1 over the o- and p-positions (IIIb and IIIc). Quantum mechanical treatments of resonance of type III have been made by Huckel and by Wheland and Pauling. Irrespective of whether I or II represents more accurately the electrical effect of the trifluoromethyl group, the possibility may be suggested of at least some degree of conjugation or interaction between the trifluoromethyl group and a strong electron-donating group, such as -NR_2, located at the o- or p-position. This interaction might be represented by resonance structures such as IV which are analogous to the resonance forms (V) used to symbolize the interaction between the groups of p-nitroanilines.