Pictures of the Day CH320M328M

Carbocation hybridization is sp2, with an empty 2p orbital (as shown on the right). Thus, carbocations are trigonal planar, a fact that is important because it means carbocations can be attacked by nucleophiles from the “top” or the “bottom” with equal probability, leading to enantiomer products if a new stereocenter is created. The nucleophiles start reacting with carbocations by placing electron density (i.e. a lone pair of electrons) into the empty 2p orbital. Carbocations are extremely reactive because they possess both an unfilled valence shell and a full positive charge. Thus, you will only encounter them as relatively high energy reaction intermediates during the mechanisms of reactions.

Carbocations are stabilized by adjacent alkyl groups, so more highly substituted carbocations are more stable. Alkyl groups stabilize carbocations by a combination of an inductive effect (positively-charged carbon is electronegative, i.e. it wants electron density, so it withdraws some of the electron density of an alkyl group through sigma bonds) and hyperconjugation in which the empty 2p orbital partially overlaps in space with the C-H sigma bonds of the adjacent alkyl groups. In both cases, some electron density is placed on the positively-charged carbon atom from the alkyl groups, leading to a greater distribution of the positive charge around the molecule. The more alkyl groups, the more highly distributed is the positive charge. The more highly distributed the positive charge, the more stable the carbocation. You can see the dramatic differences in charge distribution quantitatively on the bottom row in which carbocations with more alkyl groups attached have less intense blue color on the positively-charged carbon atom, indicating more delocalization of positive charge and thus greater stability. Keep in mind that even tertiary (3°) carbocations are still carbocations and extremely reactive with even weak nucleophiles, only surviving for exceedingly short amounts of time during reaction mechanisms.