Conformations: Butane stability: Anti (180°) > Gauche (60°) > Eclipsed (120°) > Fully eclipsed (0°) — mnemonic: "A Gentle Evening Falls"; fully eclipsed is LEAST stable.

Radical Halogenation: 3 steps: Initiation → Propagation → Termination; selectivity 3° > 2° > 1°; $Br_{2}$ most selective; halogen reactivity $F_{2}$ > $Cl_{2}$ > $Br_{2}$ > $I_{2}$.

Markovnikov vs Anti-Markovnikov: HBr (no peroxide) → Markovnikov (2-bromopropane from propene). HBr + ROOR → anti-Markovnikov (1-bromopropane). ONLY HBr + peroxide — HCl and HI do NOT work.

Ozonolysis (reductive, Zn/$H_{2}O$): Cleaves C=C → aldehydes (–CH=) + ketones (–CR=); 2-butene → 2 × acetaldehyde.

Alkynes: Acidity sp > $sp^{2}$ > $sp^{3}$; terminal alkyne + $NaNH_{2}$ → acetylide; Lindlar's → cis-alkene (syn); Na/liq. $NH_{3}$ → trans-alkene (anti).