Carbocation stability & Hyperconjugation

The slides I showed in lecture today are available here.

I have three short comments to go with these:

  1. Check out the material on hyperconjugation in the Chapter 4 section of your Solutions Manual/Study Guide. I have only recently begun looking at this text and I’m impressed by the material inside it. I was going to lecture on MO models of hyperconjugation, but maybe I don’t have to.
  2. I drew two resonance structures for CH3CH2+ to show how hyperconjugation changes bond and charge patterns. Chemists do not normally draw these resonance structures. So when I ask you to “draw resonance structures” for some random carbocation, don’t include hyperconjugation. Instead, look for pi bonds and lone pairs that are poised for electron-sharing.
  3. My slides don’t say anything about the relative energies of these carbocations, but the ChemActivity that I handed out in lecture captures this point. Three weakly electron-donating methyl groups greatly stabilize a carbocation. Likewise, a single strongly electron-donating vinyl group. CH3CH2+, by comparison, is highly unstable and is never observed in routine chemical processes.
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