Hyperconjugation substitution

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

When the /)-positions are considered it is seen that they follow the sequence of inductive effects, and not of hyperconjugation. In this respect nitration is unusual amongst electrophilic substitutions. ... 

Secondary isotope effects at the position have been especially thoroughly studied in nucleophilic substitution reactions. When carbocations are involved as intermediates, substantial /9-isotope effects are observed. This is because the hyperconjugative stabliliza-... 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

In addition to the nucleophilic displacement of the halogen atom, subsequent substitution of the heteroarylium moiety has been observed in numerous cases. Tliese substitution reactions beneht from the increasing importance of the negative hyperconjugation (Scheme 13). Such a pathway has been intensively exploited to synthesize (fused) hve-, six-, and seven-membered heterocycles. 

Hyperconjugation (Sections 6.6, 6.9) An interaction that results from overlap of a vacant p orbital on one atom with a neighboring C-H a bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes. 

This stereoelectronic effect also explains the contrasting regioselectivity of cis- and fra x-2-fluoro-4-f-butylcyclohexanone.235 As a result of a balance between its polar effect and hyperconjugation, the net effect of a fluoro substituent in acyclic systems is small. However, in 2-fluorocyclohexanones an unfavorable dipole-dipole interaction comes into play for the cis isomer and preferential migration of the fluoro-substituted carbon is observed. 

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