After we talk about what a carbocation is and the factors that affect carbocation stability, we're going to apply that to a very common type of problem that you might find on your exam: Rank the following carbocations in order of decreasing stability.
P.S. Check out my 24-minute-long lesson on carbocation stability if you’re not rushing!
A carbocation is a carbon atom with a positive charge. Its octet is not filled, it has an empty p-orbital, and it’s sp2-hybridized. They’re generally created when a leaving group dissociates in a substitution, elimination, or solvolysis reaction. Not all carbocations are created equal however; some of them are much more stable than others.
What affects the stability of carbocations? How many other carbon atoms they’re attached to. Other R-groups will actually donate electron density to the carbocation through a process called hyperconjugation. Carbocations are electron-deficient, so the more R groups one has attached the more stable it will be! Let’s check out the trend below.
The more R-groups a carbocation has attached, the more stable it is! That means that tertiary is more stable than secondary, secondary more stable than primary, and primary more stable than methyl. In general, you probably won’t see a primary or methyl carbocation in O-Chem 1.
Let’s go ahead and sort some carbocations in terms of their stability.
Let's quickly identify each carbocation as methyl, primary, and so on.
Now that we know what kinds of carbocation each one is, it should be really easy to place them in the right order!
Tertiary is on top since it’s the most stable due to its R-groups, and methyl is on bottom because it has no R-groups.
Okay, cool! We’ve sorted carbocations in order of decreasing stability! But what happens if a carbocation is allylic, i.e. adjacent to a double bond? My videos on carbocation stability go over that and more! Good luck studying!