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Carbocation Stability Revisited

Back in organic chemistry 1 you learned a bit about carbocations.

For the next week or so, the *stability* of carbocations is going to be REALLY important.

From Org 1 you might recall that

  • carbocation stability increases in the order  primary < secondary < tertiary
  • carbocations are stabilized by resonance.

Today we’re going to fill in a bit more of the picture.

First of all, the key point: carbocations are electron poor.

  • They are stabilized by neighbors that donate electrons
  • They are destabilized by neighbors that withdraw electrons.

“When you’re poor, it’s good to have rich neighbors”. So if a carbocation is next to a group that can *give* it electrons, it takes ’em. This helps to stabilize it. By the opposite token, a carbocation that is next to an electron withdrawing group is going to be even more electron poor – and more unstable!
Let’s look at these 5 carbocations.

Easy ones first.

  • From org 1, you know that carbocation B will be more stable than carbocation C, since it’s tertiary and C is secondary. But what about carbocation A?
  • Carbocation A is even more stable than carbocation B. This is weird, because we usually think of oxygen as electron withdrawing. But because it can donate a pair of electrons to give the resonance form below, in which all atoms have a full valence shell, this “pi donation” effect outweighs the electronegativity of oxygen! (By the way, we know this because experimental evidence tells us this – rationalizations come after!)
  • What about carbocation E? It’s the most unstable of all! The electron withdrawing fluorines connected to the carbon pull electrons away from the carbocation, making it more unstable. Furthermore, there are no resonance forms available to stabilize the carbocation. [see note]
  • The final (and trickiest case) is D. Fluorine is also electron withdrawing, but unlike CF3, it can also donate a pair of electrons to stabilize the carbocation. However since fluorine is more electronegative than oxygen we expect the resonance form where it donates electrons to be much less important.  Experiments tell us that carbocation D is more stable than E, but less stable than C.

This is really important stuff! Next we’ll see how to apply this to figure out important trends in reactions of aromatic compounds.

Tomorrow: let’s talk about the most important reaction of aromatic compounds you’re going to learn – electrophilic aromatic substitution.

Thanks for reading! James

Note – carbocation  E is technically tertiary, but the electron withdrawing groups destroy its ability to stabilize adjacent positive charge.