The Electrical Effect of the Trifluoromethyl Group

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 Hückel 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.