Electronic effects carbocations

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

Ionization reaction rates are subject to both electronic and steric effects. The most important electronic effects are stabilization of the carbocation by electron-releasing... 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

Which sigma complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects. As a carbocation, a substi-... 

It is interesting to see completely different regiochemistries in nucleophilic and electrophilic reactions to 1,1-difluoroethene as a model. Scheme 1.55 shows a typical electronic effect of the fluorine atom as a substituent. Electrophiles mostly attack the (3-carbon of 1,1-difluoroethene generating a-difluorinated carbocations as intermediates in contrast to regioselective nucleophilic additions on the a-carbon of 1,1-difluoroethene generating (3-fluorocarbanions. 

The formation of new stereogenic centers using chemical intermediates such as free radicals, carbanions, or carbocations can be influenced by many factors such as solvents, electronic effects, the electrophilicity or nucleophilicity of reagents, and the presence of Lewis acid. Of course, the intrinsic reactivity of the intermediate used will also affect the outcome of the reaction. Still, the general principles governing the induction of diastereoselectivity remain the same for all of the above-mentioned species. 

---