: 230
Carbocations are sp2 hybridized, trigonal planar species with an empty p-orbital. Stability order: 3 degree > 2 degree > 1 degree > methyl, governed by hyperconjugation (alpha-H count) and inductive (+I) effects. Special carbocations — tropylium (C7H7+, 6 pi e-, aromatic), benzylic (resonance into ring), allylic (resonance with C=C), and cyclopropenyl (C3H3+, 2 pi e-, aromatic) — gain extraordinary stability from delocalization. Less stable carbocations rearrange to more stable ones via 1,2-shifts: hydride shift (H:- migrates) or methyl shift (CH3:- migrates). The neopentyl system is a classic JEE example — a primary cation rearranges to tertiary via 1,2-methyl shift. Ring expansions (cyclobutyl to cyclopentyl) follow the same principle. Hammond's postulate connects stability to reaction rates: for endothermic carbocation-forming steps, the transition state resembles the product, so more stable carbocations form faster. This is the theoretical basis for Markovnikov's rule in electrophilic additions. Anti-Markovnikov addition (via radical mechanism) forms the more stable radical intermediate instead. Vinyl cations (sp hybridized, poor stability) are generally avoided in reaction pathways — electrophilic addition to alkynes prefers pathways that avoid vinyl cation intermediates when possible.