Carbocation stability and reactivity

The stability of carbocations increases from primary>secondary>tertiary. Stability increases with the number of adjacent carbon atoms meaning tertiary carbocations - with the maximum possible three alkyl substituents - is the most stable.

Figure 1. Carbocation order of stability.

This stabilizing effect comes from hyperconjugation which creates stabilization through the donation of electrons from C-H sigma bonds into the empty pi orbital of the carbocation.

Carbocations can also be stabilized by neighbouring C-C double and triple bonds as the overlap between the empty p orbital of the carbocation and adjacent p orbitals of the C=C pi bond allows the charge to be spread across multiple atoms. This effect is known as delocalization.

Figure 2. Carbocations stabilized by adjacent C=C multiple bonds.

We can see this stabilizing effect by drawing resonance structures that illustrate the charge moving from atom to atom.

Figure 3. Carbocations stabilized by resonance.

Carbocations can also be stabilized by adjacent lone pairs that are able to donate a pair of electrons to the electron-poor carbonation.

Figure 4. Lone-pair stabilized carbocation.

Remember that this usually results in the formation of a double bond (pi bond) and the charge will migrate to the atom donating the lone pair. This type of stabilization often goes by the name ‘pi donation’.

Figure 5. Resonance drawings for lone-pair stabilized carbocations.