Overview
This lecture reviews carbocation rearrangements, focusing on how and why hydride and methyl shifts occur to increase carbocation stability.
Identifying Carbocations
- Carbocations are classified as primary, secondary, or tertiary based on the number of carbons bonded to the positively charged carbon.
- Count the carbons directly attached to the carbocation to determine its type.
Carbocation Rearrangement Mechanisms
- Carbocations rearrange via hydride (H shift) or methyl (CH₃ shift) shifts to become more stable.
- Secondary carbocations can undergo a hydride or methyl shift to form a more stable tertiary carbocation.
- The shift occurs when a neighboring carbon has a hydrogen or methyl group available to move.
Special Cases: Allylic Carbocations
- Even a tertiary carbocation can rearrange if it results in an allylic carbocation, which is stabilized by resonance from a nearby double bond.
- Resonance stabilization can make an allylic carbocation more stable than a regular tertiary carbocation.
Typical Trends in Rearrangement
- Most rearrangements involve less stable (primary or secondary) carbocations shifting to the more stable tertiary form.
- Rearrangement from tertiary to allylic carbocation is rare but possible if resonance stabilization is present.
Key Terms & Definitions
- Carbocation — A carbon atom bearing a positive charge.
- Primary/Secondary/Tertiary Carbocation — Classified by the number of carbons bonded to the charged carbon (1, 2, or 3).
- Hydride Shift — Movement of a hydrogen atom (with its electrons) from a neighboring carbon to the carbocation.
- Methyl Shift — Movement of a methyl group (CH₃) from a neighboring carbon to the carbocation.
- Allylic Carbocation — A carbocation adjacent to a carbon-carbon double bond, stabilized by resonance.
Action Items / Next Steps
- Review examples of carbocation rearrangements, focusing on identifying possible shifts and assessing resulting stability.
- Prepare for the next lecture on the following section.