Inductive & Resonance Effects

What are the factors that influence carbocation stability? The most common means of stabilizing an ion is by charge delocalization, either by inherent structural interactions or by solvation. As noted elsewhere, such structural interactions may usually be classified as inductive or resonanceeffects, and these may complement or oppose each other. Examples of both are given in the following diagram.

Alkyl groups have somewhat lower electronegativities and are more polarizable than hydrogen. If an alkyl group is bonded to the carbocation center, the electron pair of the C-C sigma bond will shift toward the positive charge, transferring a small part of that charge to the alkyl group. In the diagram on the left above, this inductive electron shift is designated by a light blue arrow head. Additional alkyl groups provide increased inductive charge dispersion, with each group assuming a share of the charge. Clearly, this analysis supports the stabilizing influence of alkyl substituents on carbocations.
Resonance stabilization by non-bonding electron pairs on adjacent heteroatoms is particularly strong, as shown on the right above. Such charge delocalization overcomes the potential inductive destabilization of these electronegative substituents. Similar stabilization is provided by an adjacent nucleophilic pi-electron function, such as a double bond. A phenyl substituent affords even greater charge delocalization than a double bond, as reflected by the position of a benzyl cation in the stability order. Resonance stabilization is generally stronger than inductive effects, and is the predominant factor stabilizing the tropylium and trityl cations.