1. Alkane General Formula: Cₙ$H_{2}$ₙ₊{2} (saturated, all $sp^{3}$). Alkene: Cₙ$H_{2}$ₙ (one C=C, $sp^{2}$ at double bond). Alkyne: Cₙ$H_{2}$ₙ₋{2} (one C≡C, sp at triple bond).

2. Ethane Conformations: Staggered (60° dihedral) = most stable; Eclipsed (0° dihedral) = least stable. Stability difference ~12 kJ/mol due to torsional strain (C–H orbital repulsion).

3. Butane Conformational Order (most to least stable): Anti (180°) > Gauche (60°) > Eclipsed (120°) > Fully eclipsed (0°). Mnemonic: "A Gentle Evening Falls." Fully eclipsed is least stable — methyl groups eclipse each other at 0°.

4. Free Radical Halogenation Steps: Initiation ($X_{2}$ → 2X• by UV/heat) → Propagation (R–H + X• → R• + HX; then R• + $X_{2}$ → R–X + X•) → Termination (radical + radical → stable molecule). H-abstraction order: 3° > 2° > 1°.

5. Halogen Reactivity vs Selectivity: Reactivity: $F_{2}$ > $Cl_{2}$ > $Br_{2}$ > $I_{2}$. Selectivity (3° vs 1°): $Br_{2}$ >> $Cl_{2}$ > $F_{2}$. $Br_{2}$ is the most useful in practice (selective but not explosive).

6. Markovnikov's Rule: In ionic addition of HX to an unsymmetrical alkene, H adds to the carbon with MORE H atoms. The driving force is formation of the MORE STABLE carbocation (3° > 2° > 1°). Propene + HBr (no peroxide) → 2-bromopropane (CC(Br)C).

7. Anti-Markovnikov (Kharasch Effect) — THE most-tested rule: HBr + organic peroxide (ROOR) → free radical mechanism → Br• adds to LESS substituted carbon → more stable radical → H adds to radical carbon → 1-bromopropane (CCCBr from propene). ONLY HBr works. HCl: Cl• addition to alkene is endothermic (chain fails). HI: I• causes premature termination (chain fails).

8. Ozonolysis (Reductive, Zn/$H_{2}$O): Cleaves C=C → each C=C carbon becomes C=O. C with H → aldehyde; C without H → ketone. 2-Butene → 2 × acetaldehyde. Used to locate double bond position.

9. Alkyne Acidity Order: sp (50% s) > $sp^{2}$ (33.3%) > $sp^{3}$ (25%). pKa: alkyne ~25 < alkene ~44 < alkane ~50. Terminal alkyne + NaN$H_{2}$ → sodium acetylide RC≡C^{-}$$Na^{+}.

10. Alkyne Reduction:

• Lindlar's (Pd/$CaCO_{3}$ + Pb(OAc)_{2} + quinoline) + $H_{2}$ → syn-addition → cis-alkene
• Na/liq. $NH_{3}$ → anti-addition (radical anion) → trans-alkene
• $H_{2}$/Pd–C (excess) → complete reduction → alkane