118 Carbocations
Carbocations are carbon atoms in an organic molecule bearing a positive formal charge.
Therefore they arecarboncations. Carbocations have only six electrons in their valence
shell making them electron deficient. Thus, they are unstable electrophiles and will react
very quickly with nucleophiles to form new bonds. Because of their reactivity with het-
eroatoms, carbocations are very useful intermediates in many common organic reactions.
118.1 Carbocation Structure
The orbitals of carbocations are generally sp^2 hybridized so that the three full orbitals are
arranged in a trigonal planar geometry about the carbon nucleus. The remaining p orbital
is empty and will readily accept a pair of electrons from another atom. Because of the
symmetry of this geometric arrangement, nucleophilic attack is equally favorable above or
below the plane formed by the full orbitals.
118.2 Carbocation Stability
Carbocationsaregenerallyunstableandfairlyhardtoform. Theyusuallycannotbeisolated
from a reaction as they will react immediately to fill their empty p orbital. Because they
are electron deficient, attaching electron donating groups (such as alkyl groups) to the
carbocation will help stabilize the carbocation. In general:
Stability of Alkyl CarbocationsCH 3 +< RCH 2 +< R 2 CH+< R 3 C+Clearly, the tertiary carbocation is the most stable, as it is surrounded by three other
carbon atoms that share the burden of its positive charge. Primary and especially methyl
carbocations are rarely seen in organic reactions except under special circumstances like in
the case of benzylic or allylic cations.
Carbocations can also be stabilized through resonance by neighboring lone pairs or pi-
electrons. In general, this stabilization is greater than one degree of substitution, so a
secondary carbocation stabilized by resonance will be more stable than a tertiary carboca-
tion with no resonance stabilization, and a primary carbocation stabilized by resonance will
be more stable than a secondary carbocation with no resonance stabilization.