carbocations

A carbocation is a positively charged carbon created by the removal of a pair of bonding electrons. This usually occurs when a leaving group covalently bonded to the carbon undergoes heterolysis.



In the above example, the bromine atom is the leaving group in the reactant. When its bond to carbon breaks, as shown by the red arrow, it takes the electron pair with it. This leaves carbon with a positive formal charge (hence the term carbocation).

Carbocations are not chemically stable. In fact, the formation of a carbocation is a reversible reaction - the leaving group has the potential to act as a nucleophile (the carbocation is now an electrophile) and the bond between them can re-form.

In order to chemically stabilize a carbocation, it needs to be protected, and its positive charge needs to be diffused.


 * Carbocation Stability **

It is nearly impossible to form a carbocation on a primary carbon (one that has 2 H's along with the leaving group). The reasons are many, including high reactivity (the reverse reaction rate is too high to be practical) and lack of enough stabilizing carbon-carbon bonds. //Any reaction mechanism requiring the formation of a carbocation will not work in this situation.// The only exception is if the carbocation is stabilized by pi conjugation.

Secondary carbocations are also not very practical in terms of carbocation formation. Again, lack of stability leads to this impracticability. This position is stabilized, however, if the carbocation forms adjacent to pi bonds.

The tertiary carbon is a fair position to form a carbocation. The three carbon-carbon bonds provide some electron density to the carbocation and thus increase its lifetime (it becomes less electrophilic).

The best situation for a carbocation is when it can benefit from pi conjugation. A carbocation takes the triangular planar shape, and will have an empty 2p orbital:



Recall that sp2 hybridized carbons also have 2p orbitals (used in pi bonding):



These 2p orbitals will conjugate (interact) and share, or distribute, the positive charge of the carbocation, thus stabilizing the compound and its chemical activity.

Here is a good visualization of the stabilizing effect of pi conjugation on a primary carbocation: