What are Carbocations?
are species bearing a formal "+" charge on carbon.
They have sp2 hybridization and trigonal
planar geometry, with an empty p orbital on carbon,
perpendicular to the plane containing the substituents
(see diagrams shown to the right). Carbocations are
"hypovalent" species which have only three shared
pairs of electrons around the carbon, instead of the
usual four. This incomplete octet around carbon makes
carbocations very unstable and very reactive. Nevertheless,
carbocations are known to be formed as intermediates
in many types of organic reactions. The relatively
high energy of a carbocation, however, means that
it will usually be formed in the rate-determining
step for a reaction, and so it is important to understand
how the substituents -- shown as R1, R2,
and R3 in the diagrams -- can act to stabilize
a carbocation, thus making it easier to form from
a neutral molecule in the first place.