Electrophilic substitution, aromatic hyperconjugation

Electrophilic substitution, aromatic, 31, 130-167, 381 1,2-ti. 1,4-addition, 195 as addition/elimination, 133 complexing with substituent, 160 deuterium exchange, 131,158 electronic effects in, 148, 158, 159 energetics of, 132, 136 field effect in, 152 hyperconjugation in, 153 inductive effect in, 22,152,153,156, 160... 

Hyperconjugation also predicts that the R3SiCH2 group will be electron releasing at the ortho and para positions of an aromatic ring, and activating to electrophilic aromatic substitution, as shown in equation 4. 

The spectroscopic manifestation of symmetry effects in cyclic hyperconjugation was reported in electron spin resonance (ESR) spectra. " Symmetric LUMOs show cooperative effects to hyperconjugative interactions, while interactions with antisymmetric LUMOs are cancelled by symmetry (Figure 8.28). Such effects can be extended to spin-paired molecules, Wheland intermediates of electrophilic aromatic substitution, metal cation/arene complexes, and n -cyclopentadieneylmetal compounds. ... 

Scheme 1.2 shows a hyperconjugative effect of the -CF3 group in an aromatic ring. Because of its hyperconjugative effect, the -CF3 group makes the o-and p-carbons electron deficient. Therefore, the electrophile attacks the relatively less electron-deficient m-carbon to give an m-substitution product in electrophilic aromatic substitution. 

Theoretical Considerations.—Several papers have appeared on the application of 1-arylethyl ester pyrolyses to the study of electrophilic aromatic reactivities. By this means, accurate values have been determined for para-cyclohexyl and t-butyl substituents. The effect of mera-t-butyl substitution in the same reaction and in the protiodetritiation in TFA at 70 °C has been measured. The results indicate that the Baker-Nathan order in solvolysis reactions arises from steric hindrance to solvation of the transition state and that C-C is more important than C-H hyperconjugation. This approach has also been used to study non-additivity of methyl and chlorine" substituent effects. 

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