The stability of carbocations increases as we go from primary to secondary to tertiary carbons. There’s two answers as to why this is. The age-old answer that is still passed around in many introductory textbooks points to carbons (alkyl groups in particular) as being “electron-releasing” groups through inductive effects. That is, a carbon (electronegativity 2.5) connected to hydrogen (electronegativity 2.2) will be electron rich, and can donate some of those electrons to the neighboring carbocation. In other words, the neighboring carbon pays the carbocation with electrons it steals from the hydrogens. The second, (and theoretically more satisfactory explanation) ishyperconjugation, which invokes stabilization through donation of the electrons in C-H sigma bonds to the empty p orbital of the carbocation.Carbocations are stabilized by neighboring carbon-carbon multiple bonds.Carbocations adjacent to another carbon-carbon double or triple bond have special stability because overlap between the empty p orbital of the carbocation with the p orbitals of the π bond allows for charge to be shared between multiple atoms. This effect, called “delocalization” is illustrated by drawing resonance structures where the charge “moves” from atom to atom. This is such a stabilizing influence that even primary carbocations – normally very unstable – are remarkably easy to form when adjacent to a double bond, so much so that they will actually participate in SN1 reactions.3) Carbocations are stabilized by adjacent lone pairs.The key stabilizing influence is a neighboring atom that donates a pair of electrons to the electron-poor carbocation. Note here that this invariably results in forming adouble bond(π bond) and the charge will move to the atom donating the electron pair. Hence this often goes by the name of “π donation”.

The strength of this effect varies with basicity, so nitrogen and oxygen are the most powerful π donors. Strangely enough, even halogens can help to stabilize carbocations through donation of a lone pair.The fact that atoms that we normally think of as electron-wthdrawing (nitrogen, oxygen, chlorine) can actually be electron-donor groups is probably one of the most difficult factors to wrap your head around in Org2This effect is tremendously important in the reactions of aromatic rings and also in enolate chemistry, where double bonds attached to donating groups (nitrogen and oxygen in particular) can be millions (or billions) of times more nucleophilic than alkenes that lack these groups.