: 230
Alkanes are the least reactive organic compounds due to their strong, nonpolar C-C and C-H sigma bonds. Their primary reaction is free radical halogenation, proceeding through initiation (X2 → 2X· by UV/heat), propagation (H-abstraction → halogen transfer chain), and termination (radical coupling). The H-abstraction step is rate-determining and controls selectivity. Chlorine is reactive but unselective (3°:2°:1° = 5:3.8:1), while bromine is slow but highly selective (1600:82:1). This selectivity difference is explained by Hammond's postulate — the endothermic H-abstraction by Br· has a product-like transition state that discriminates between radical stabilities, while the exothermic Cl· step has an early TS with poor discrimination. To calculate monochlorination product ratios: multiply the number of each type of H by its relative reactivity, then normalize. Conformational analysis is another key topic: ethane has a 12.5 kJ/mol rotational barrier between staggered and eclipsed forms. Butane's conformations range from anti (most stable) to fully eclipsed (least stable, +19 kJ/mol). Cyclohexane adopts the chair conformation with axial and equatorial positions; bulky groups strongly prefer equatorial to avoid 1,3-diaxial strain.