Rearrangements

The final fundamental mechanistic pattern we will explore is Rearrangements. While various types of rearrangements exist in organic chemistry, two specific types of carbocation rearrangements are particularly important in sophomore-level organic chemistry:

1,2-Hydride shift
1,2-alkyl shift (usually a methyl shift)

Key Principles of Carbocation Rearrangements:

Adjacency Requirement: Shifts can only occur from an atom (carbon) immediately adjacent to the carbocation center.
Stability Driven: Rearrangements only occur if they lead to the formation of a more stable carbocation. For instance, a secondary carbocation might rearrange to a more stable tertiary carbocation.
"1,2" Definition: The "1,2" designation refers to the migration of a group from one carbon atom to an adjacent carbon atom, which is the carbocation center.

Identifying Potential Rearrangements:

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

Let's consider an example: Is this secondary carbocation able to rearrange to a more stable tertiary carbocation?

Identify Adjacent Migratory Groups:
Identify any hydrogen atoms (H) or methyl groups ( $CH_{3}$ ) attached to the carbon atoms immediately adjacent to the carbocation.
Evaluate Shift Stability:
Determine if the shift of one of these identified groups (H or CH₃) would result in a more stable carbocation.

Note on Tertiary Carbocations:

Tertiary carbocations typically will not rearrange further unless the rearrangement would lead to an even more stable carbocation, such as one stabilized by resonance.Tertiary carbocations typically will not rearrange unless a resonance stabilized carbocation is formed.

Rearrangements to Relieve Ring Strain:

In addition to forming more substituted carbocations, rearrangements can also occur to relieve ring strain. Small rings, particularly three- and four-membered rings (like cyclopropane and cyclobutane), possess significant internal energy due to their bond angles being highly distorted from the ideal 109.5° tetrahedral angle. This "ring strain" makes these systems less stable.

When a carbocation is adjacent to such a strained ring, a ring expansion (or sometimes contraction) can occur, leading to a larger, less strained ring. This reorganization of the carbon skeleton, even if it results in a less substituted carbocation, is often highly favorable due to the considerable energy released by relieving the ring strain.

In this example, the ring strain is reduced and a more stable carbocation is formed. Double wammy!