Carbocation Intermediate Stability

Now we’re going to discuss the most important intermediate of all organic chemistry, the carbocation.

Carbocation Stability

Carbocations are stabilized by a phenomenon called hyperconjugation.

Hyperconjugation is the delocalization of charge by the interaction of an empty p-orbital with an adjacent, eclipsed σ-bond. Basically it’s like resonance for single bonds (more complicated than that but don’t worry about it). 

Since this is only possible with -R groups, the more substituted the carbocation, the more stable

Example: Which of the following leaving groups would generate the most stable carbocation?

2m

Carbocation Rearrangements

Carbocations will rearrange to an adjacent, more stable possible if possible. These have different names based on which atoms are rearranging. 

a. 1,2-Hydride Shift occurs when there is a hydrogens located on an adjacent, more stable carbon.

b. 1,2-Alkyl Shift occurs when only small alkyl groups (methyl or ethyl) are located on an adjacent, more stable carbon.

  • This only occurs when there are NO hydrogens in the adjacent position.
  • Methyl is easier to move than ethyl. Only shift ethyls if you have no other options!

c. Ring Expansion occurs when a carbocation is adjacent to a 3, 4 or 5- membered ring.

  • This only occurs when the carbocation is NEXT TO the ring, not directly on it!

Example: Would the following carbocation rearrange? If so, draw the rearranged structure.  

1m

Example: Would the following carbocation rearrange? If so, draw the rearranged structure.  

1m

Example: Would the following carbocation rearrange? If so, draw the rearranged structure.  

1m