Carbocation Stability

Carbocations are positively charged carbon ions that have a formal charge of positive one (i.e. carbon cations). They have three bonds to other atoms, zero lone-pair electrons, an empty p-orbital, and they have six instead of the eight electrons favored by the octet rule. As a result of this lack of electron density, carbocations are generally very reactive—especially toward nucleophilic addition reactions and elimination reactions.

Carbocations have three bonds and an empty p-orbital. They are sp2-hybridized and have a trigonal planar geometry.

Carbocation Stabilization

There are two main phenomena that affect the stability of carbocations: hyperconjugation and resonance.

Hyperconjugation

What’s especially interesting about carbocations is how they are stabilized by neighboring carbon atoms through a phonemenon called hyperconjugation. Hyperconjugation is a stabilizing effect (read: reactivity decreaser) that occurs via a kind of “spilling over” of the electrons comprising adjacent carbons’ sigma bonds into the carbocation’s empty p-orbital.

electron density “spills over” into the carbocation’s empty p-orbital from an adjacent carbon’s sigma-bond

Consequentially, the more carbons are attached to the carbocation (maximum of three), the more stable the carbocation. In other words, tertiary carbocations (those with three other carbon atoms attached) are the most stable while primary carbocations (those with just one other carbon atom attached) are very unstable—so much so that your professor might even tell you that they never form.

Stability increases from left to right, with 0º carbocations being the least stable and 3º being the most stable.

Zeroth/Methyl carbocations (those with no other carbon atoms attached) are even more unstable than primary carbocations, so if you find yourself drawing a carbocation with three hydrogen atoms attached, that’s a good sign that you might've made a mistake somewhere along the way!

Resonance

As important here as everywhere else in O-Chem, resonance (basically the molecule’s ability to distribute electron density evenly across its surface area) can play a huge role. Resonance greatly affects the stability of carbocations!

Rather than be localized to a single carbon atom, an allylic (adjacent to a pi-bond) carbocation has its positive charge distributed across at least two carbon atoms.

A 2º carbocation is less stable than an allylic 2º carbocation

The general rule is that the more resonance ways a carbocation can resonate (e.g. adjacent lone pairs and pi-bonds), the more stable the carbocation is.

An allylic 2º carbocation is less stable than a “double allylic” 2º carbocation

Though it varies among professors, most will tell you that allylic carbocations are just as stable as benzylic carbocations (adjacent to a benzene ring). Just make sure to check with your professor and follow what they say!

Allylic and benzylic carbocations are about as stable as each other. Double check with your prof!

So, that’s it for the most frequently tested aspects of carbocation stability! Next time, we’ll learn about carbocation rearrangements.