This lesson covers the four major families of hydrocarbons—compounds made exclusively of carbon and hydrogen. Their structures, bonding, reactions, and uses form the core of organic chemistry.

1. Core Concepts (Short Notes)

12.1 What Are Hydrocarbons?

Hydrocarbons are organic compounds made of only carbon and hydrogen. They are classified into:

• Alkanes (single bonds)

• Alkenes (double bonds)

• Alkynes (triple bonds)

• Aromatic hydrocarbons (benzene rings)

These categories differ in structure, reactivity, and physical properties.

12.2 Alkanes (Saturated Hydrocarbons)

General formula: CₙH₂ₙ₊₂

• Only single C–C bonds.

• Tetrahedral geometry (109.5°).

• Low reactivity due to strong σ-bonds.

Key reactions:

• Combustion: Produces CO₂ and H₂O.

• Substitution reactions with halogens under UV light.

Example: CH₄ + Cl₂ → CH₃Cl + HCl

12.3 Alkenes (Unsaturated Hydrocarbons)

General formula: CₙH₂ₙ

• Contain C=C double bonds.

• Planar around the double bond.

• More reactive than alkanes due to π-bond.

Key reactions:

• Addition reactions (hydrogenation, halogenation, hydration).

Example: C₂H₄ + Br₂ → C₂H₄Br₂

Cis/Trans Isomerism: Occurs when groups around a C=C bond differ.

12.4 Alkynes (Triple-Bond Hydrocarbons)

General formula: CₙH₂ₙ₋₂

• Contain C≡C triple bond.

• Linear geometry (180°).

• Very reactive.

Key reactions:

• Addition of hydrogen/halogens.

• Formation of metal acetylides.

12.5 Aromatic Hydrocarbons (Arenes)

Example: Benzene (C₆H₆)

• Planar ring with delocalized π-electrons.

• Very stable.

Key reactions:

• Undergo electrophilic substitution (NOT addition).

• Nitration, halogenation, alkylation.

Example: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

12.6 Physical Properties of Hydrocarbons

• Non-polar.

• Insoluble in water.

• Soluble in organic solvents.

• Boiling point increases with molar mass.

2. Key Formulas & Reactions to Memorize

General Formulas:

• Alkanes: CₙH₂ₙ₊₂

• Alkenes: CₙH₂ₙ

• Alkynes: CₙH₂ₙ₋₂

Combustion Equations:

Complete: Hydrocarbon + O₂ → CO₂ + H₂O Incomplete: Hydrocarbon + O₂ → CO + C + H₂O

Tests for Hydrocarbons:

• Bromine water test:

  • Decolorized → alkene/alkyne (addition).

  • No change → alkane.

Hydrogenation:

Alkene + H₂ → Alkane (Ni catalyst)

Aromatic Substitution:

Benzene + electrophile → substituted benzene + H⁺

3. Tips & Tricks for Exams

• Identify saturation: If it has a double or triple bond → unsaturated.

• Alkenes are more reactive because π-bonds break easily.

• Benzene does not decolorize bromine water unless a catalyst is present.

• For addition reactions, think: double bond breaks → new atoms add.

• In combustion questions, check oxygen balance carefully.

• Remember: aromatic compounds resist addition reactions.

4. Important Points to Remember

• Hydrocarbons form the skeleton of organic chemistry.

• Reactivity increases from alkanes → alkenes → alkynes.

• Aromatic compounds show unique stability due to delocalization.

• Addition reactions occur only with unsaturated hydrocarbons.

• General formulas help identify homologous series quickly.