Hydrocarbons

Definition: Compounds consisting of only carbon and hydrogen atoms.

Classification: Based on nature of bonds:

• Saturated (Alkanes): Single bonds only (C-C).
• Unsaturated (Alkenes/Alkynes): Contain double or triple bonds.

Physical Properties: Non-polar, insoluble in water, soluble in organic solvents. Low molecular mass hydrocarbons are gases, moderate are liquids, high are solids.

Definition: Saturated hydrocarbons with general formula C_nH_2n+2. Called Paraffins (Least reactive).

Preparation:

• Hydrogenation of Alkenes/Alkynes: Addition of H₂ in presence of Ni/Pt catalyst.
• Reduction of Alkyl Halides: Replacement of Halogen with Hydrogen using Zn + dilute acid (HCl/CH₃COOH).

Reactions:

• Combustion: CH₄ + 2O₂ -> CO₂ + 2H₂O + Heat (Exothermic). In limited supply, produces CO (Carbon Monoxide).
• Halogenation (Substitution): React with Cl₂/Br₂ in diffused sunlight to form chloro-derivatives (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄). In direct sunlight, reaction is explosive (C + HCl).

Uses of Methane: Fuel (Natural Gas, CNG), Manufacturing Carbon Black (Shoe polish, tyres), Fertilizers (Urea), Chloroform (Solvent/Anaesthesia).

Definition: Unsaturated hydrocarbons containing at least one double bond (>C=C<). General Formula C_nH_2n. Called Olefins (Oil forming).

Preparation:

• Dehydration of Alcohols: Removal of water using Conc. H₂SO₄ or Al₂O₃ (at 350°C).
• Dehydrohalogenation of Alkyl Halides: Removal of HX using alcoholic KOH.

Reactions (Addition):

• Hydrogenation: Add H₂ (formation of margarine).
• Halogenation: Add X₂ (Br₂ water decolorization test).
• Hydrohalogenation: Add HX.
• Oxidation (KMnO₄): Forms Glycol (Baeyer's Test for unsaturation). Discharge of pink color.

Uses of Ethene: Ripening fruits, Manufacture of Polythene, Anaesthetic, Mastering gas.

Definition: Unsaturated hydrocarbons containing at least one triple bond (-C≡C-). General Formula C_nH_2n-2.

Preparation:

• Dehydrohalogenation of Vicinal Dihalides: Using alcoholic KOH.
• Dehalogenation of Tetrahalides: Using Zn dust.

Reactions (Addition):

• Halogenation: Add X₂ (forms Tetrahalides).
• Oxidation (KMnO₄): Forms Oxalic Acid.

Uses of Acetylene: Oxy-acetylene welding torch, Ripening fruits, Manufacture of PVC, Neoprene.