Hydrocarbons

1) Hydrocarbons – Classification

• Hydrocarbons: Organic compounds containing only carbon and hydrogen.
• Saturated: Alkanes (single bonds)
• Unsaturated: Alkenes (C=C), Alkynes (C≡C)
• Aromatic: Benzene and its derivatives

2) Alkanes (Paraffins)

• General formula: C_nH_2n+2
• Hybridisation: sp³
• Bond angle: 109.5°
• Least reactive due to non-polar C–C and C–H bonds

Preparation of Alkanes

• Decarboxylation: R–COONa + NaOH → RH + Na₂CO₃
• Wurtz reaction: 2R–X + 2Na → R–R + 2NaX
• Hydrogenation: Alkene/Alkyne + H₂ (Ni/Pt/Pd)
• Kolbe electrolysis: Sodium salt of carboxylic acid
• Clemmensen reduction: Zn–Hg / HCl
• Wolff–Kishner reduction: NH₂NH₂ / base

Chemical Reactions of Alkanes

• Free radical substitution: Halogenation (UV light)
• Combustion: CO₂ + H₂O + heat
• Cracking: Formation of alkenes and smaller alkanes
• Isomerisation: Straight chain → branched

3) Alkenes

• General formula: C_nH_2n
• Hybridisation: sp²
• Bond angle: 120°
• Reactive due to π-bond

Preparation of Alkenes

• Dehydration of alcohols: Conc. H₂SO₄ / Al₂O₃
• Dehydrohalogenation: Alkyl halide + alc. KOH
• Partial hydrogenation of alkynes: Lindlar / Na–NH₃

Addition Reactions

• HX addition: Markovnikov’s rule
• Peroxide effect: Anti-Markovnikov (HBr only)
• Hydrogenation: H₂ / Ni, Pt, Pd
• Halogenation: Br₂ / CCl₄ (test for unsaturation)
• Hydration: Acid-catalysed
• Hydroboration-oxidation: Anti-Markovnikov, syn addition

Oxidation of Alkenes

• Cold KMnO₄: Vicinal diols
• Ozonolysis: Carbonyl compounds
• Hot KMnO₄: Cleavage to acids / ketones

4) Alkynes

• General formula: C_nH_2n−2
• Hybridisation: sp
• Bond angle: 180°
• Terminal alkynes show acidic character

Preparation of Alkynes

• CaC₂ + H₂O → C₂H₂
• Double dehydrohalogenation: Vicinal dihalides
• Alkylation of acetylide ions

Reactions of Alkynes

• Addition of HX, X₂, H₂O
• Hydration: HgSO₄ / H₂SO₄ → Ketone
• Oxidation: KMnO₄, ozonolysis
• Acidic hydrogen reactions: NaNH₂, AgNO₃, Cu₂Cl₂

5) Aromatic Hydrocarbons (Benzene)

• Planar, cyclic, conjugated system
• Follows Hückel rule: (4n + 2) π electrons

Electrophilic Substitution Reactions

• Nitration: HNO₃ / H₂SO₄
• Sulphonation: Fuming H₂SO₄
• Halogenation: FeCl₃ / FeBr₃
• Friedel–Crafts: Alkylation & Acylation

Side Chain Reactions (Toluene)

• Oxidation: KMnO₄ → Benzoic acid
• Halogenation: At benzylic position