Rearrangements

The final pattern is rearrangements.  While there are many types of rearrangements, two types of carbocation rearrangements are of importance in sophomore organic chemistry.

 

  • 1,2-Hydride shift
  • 1,2-alkyl shift (usually a methyl shift)
Take NoteShifts can only occur from an adjacent carbon and only occur if a more stable carbocation results.  The 1,2 refers to something shifting from a carbon to an adjacent carbon.

 

When you encounter a carbocation, you must consider if rearrangement (Hydride and methyl shift) could result in a more stable carbocation.

This is a secondary carbocation. Could it rearrange to a more stable tertiary carbocation?

1) Identify any H and CH3 attached to adjacent carbons.  

2) Determine if the shift of one of these results in a more stable carbocation.

 

 

Tertiary carbocations typically will not rearrange unless a resonance stabilized carbocation is formed.