Stability order: 3 degrees > 2 degrees > 1 degrees > CH3+

Special carbocations (extra stable):

• Benzylic: Resonance with aromatic ring
• Allylic: Resonance with C=C
• Tropylium (C7H7+): Aromatic, 6 pi electrons
• Cyclopropenyl (C3H3+): Aromatic, 2 pi electrons

Rearrangements: Less stable carbocations rearrange to more stable ones via:

• 1,2-hydride shift: H migrates with its bonding electrons
• 1,2-methyl shift: CH3 migrates with bonding electrons
• Ring expansion: Cyclobutyl to cyclopentyl

JEE favorite: Neopentyl system — primary carbocation rearranges to tertiary via 1,2-methyl shift. This explains unexpected products in SN1/E1 reactions.