Rearrangement in Carbocations: 1,2-Hydride Shift
Carbocations are reactive intermediates that often undergo rearrangement to form a more stable carbocation before forming the final product. The 1,2-Hydride Shift is one of the most common types of such rearrangements.
1. Driving Force: Stability Order
The primary reason for rearrangement is to achieve greater stability. A less stable carbocation will rearrange to a more stable one if structurally possible.
Stability Order
$3^\circ > 2^\circ > 1^\circ > \text{Methyl}$
This stability is due to the Inductive Effect (+I) of alkyl groups and Hyperconjugation ($\alpha$-H atoms).
2. 1,2-Hydride Shift Mechanism
In this process, a hydride ion ($H^-$) migrates from an adjacent carbon (C-2) to the positively charged carbon (C-1). This shift carries the bonding electron pair with the hydrogen.
How it Works
- Step 1: Formation of an initial carbocation (e.g., $1^\circ$ or $2^\circ$).
- Step 2: Check adjacent carbons. If shifting an $H^-$ creates a higher degree ($3^\circ$ or benzylic/allylic) carbocation, the shift occurs.
- Step 3: The nucleophile attacks the new, stable carbocation.
Example: n-Propyl to Isopropyl
Figure: Visual Guide to Carbocation Rearrangements
3. Migratory Aptitude
When multiple groups can migrate, the group that best stabilizes the transition state migrates first.
General Order
$H > \text{Aryl (Ph)} > \text{Alkyl (R)}$
Note: Hydride shift is generally faster than Methyl shift because the 'H' atom is small and the C-H bond is easier to break heterolytically in the transition state context compared to C-C.
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