Unit 9: Hydrocarbons

Learning Objectives

• Name hydrocarbons according to the IUPAC system.
• Recognize and write structures of isomers of alkanes, alkenes, alkynes, and aromatic hydrocarbons.
• Learn various methods of preparation of hydrocarbons.
• Distinguish between alkanes, alkenes, alkynes, and aromatic hydrocarbons based on physical and chemical properties.
• Draw and differentiate between various conformations of ethane.
• Appreciate the role of hydrocarbons as energy sources and for industrial applications.
• Predict formation of addition products of unsymmetrical alkenes and alkynes.
• Comprehend benzene structure, explain aromaticity, and understand benzene's electrophilic substitution reactions.
• Predict directive influence of substituents in monosubstituted benzene rings.
• Learn about carcinogenicity and toxicity.

Introduction to Hydrocarbons

• Composed of carbon and hydrogen.
• Used as fuels (LPG, CNG, LNG, petrol, diesel, kerosene, coal gas).
• Found in polymers like polythene, polypropene, polystyrene.
• Solvents for paints, starting materials for dyes/drugs.

Classification of Hydrocarbons

• Saturated: Single carbon-carbon bonds (alkanes, cycloalkanes).
• Unsaturated: Multiple carbon-carbon bonds (alkenes, alkynes).
• Aromatic: Special cyclic compounds with alternating single and multiple bonds.

Alkanes

• Saturated hydrocarbons with C-C single bonds.
• Methane (CH₄): First member, tetrahedral structure.
• General formula: CₙH₂ₙ₊₂.
• Structure: Methane forms a tetrahedral shape, with bond angles of 109.5°.

Nomenclature and Isomerism

• Carbon atoms in alkanes are primary, secondary, tertiary, or quaternary.
• Structural isomers: Compounds with the same molecular formula but different structures.
• Chain isomers: Differ in the carbon chain.

Preparation

• From unsaturated hydrocarbons via hydrogenation.
• Wurtz reaction with alkyl halides for higher alkanes.
• Decarboxylation of carboxylic acids.

Properties

• Physical: Non-polar, gases to solids depending on molecular size.
• Chemical: Inert, but can undergo substitution (halogenation) and combustion.

Conformations

• Free rotation around C-C bonds creates different conformations (eclipsed, staggered).

Alkenes

• Unsaturated hydrocarbons with at least one double bond.
• General formula: CₙH₂ₙ.
• Addition reactions due to pi bonds.

Structure of Double Bonds

• Made of a strong sigma bond and a weaker pi bond.
• Pi bonds are sources of mobile electrons, attacked by electrophiles.

Isomerism

• Structural and geometrical (cis-trans) isomerism.

Preparation

• From alkynes through partial reduction.
• Dehydrohalogenation of alkyl halides.

Properties

• Physical: Similar to alkanes, gases to solids.
• Chemical: Addition reactions including hydrogen, halogens, and water.

Alkynes

• Unsaturated with at least one triple bond.
• General formula: CₙH₂ₙ₋₂.
• First member: Ethyne (acetylene).

Structure

• Triple bond contains one sigma and two pi bonds.

Preparation

• From calcium carbide or vicinal dihalides.

Properties

• Acidic nature due to sp hybridization.
• Addition reactions with dihydrogen, halogens, water.

Aromatic Hydrocarbons

• Contain benzene rings.
• Exhibits resonance, making it stable and less reactive in addition reactions.

Benzene Structure and Stability

• Resonance: Delocalized pi electrons, intermediate bond lengths.

Preparation

• From ethyne polymerization, decarboxylation of benzoic acid, reduction of phenol.

Properties

• Physical: Non-polar, aromatic odor.
• Chemical: Primarily electrophilic substitution reactions.

Substituent Effects

• Directive influence on incoming groups (ortho/para, meta direction).

Carcinogenicity and Toxicity

• Benzene and polynuclear hydrocarbons can be carcinogenic.

Summary

• Hydrocarbons are key energy sources and chemical industry materials.
• Alkanes: Saturated, exhibit free radical substitution.
• Alkenes/Alkynes: Unsaturated, undergo addition reactions.
• Aromatic hydrocarbons: Exhibit aromaticity and electrophilic substitutions.