Hydrocarbons

Unit 9 Overview

• Hydrocarbons contain only carbon and hydrogen.
• They are categorized into alkanes, alkenes, alkynes, and aromatic hydrocarbons.
• Used as fuels (LPG, CNG) and in industrial applications.
• Key study topics:
  • IUPAC naming
  • Structure and isomers
  • Preparation methods
  • Physical and chemical properties
  • Benzene structure and reactions
  • Aromatic hydrocarbons
  • Carcinogenicity

Classification

• Saturated Hydrocarbons: Alkanes and cycloalkanes with single bonds.
• Unsaturated Hydrocarbons: Alkenes with double bonds, alkynes with triple bonds.
• Aromatic Hydrocarbons: Special cyclic compounds like benzene.

Alkanes

• Saturated hydrocarbons (single carbon-carbon bonds).
• General formula: CₙH₂ₙ₊₂.
• Tetrahedral geometry (109.5° bond angles).
• Common reactions:
  • Combustion: Produces CO₂, H₂O, releases energy.
  • Substitution: Reactions with halogens.
  • Pyrolysis: Breakdown into smaller hydrocarbons.
• Naming: Based on carbon chain length (methane, ethane, propane).

Isomerism in Alkanes

• Structural Isomers: Different connectivity (e.g., butane, isobutane).
• Chain isomers: Variations in carbon chain arrangement.

Conformations

• Single bonds allow rotation, leading to different conformations (staggered, eclipsed).
• Staggered is the most stable due to minimal torsional strain.

Alkenes

• Unsaturated hydrocarbons with double bonds.
• General formula: CₙH₂ₙ.
• Display both structural and geometric (cis-trans) isomerism.
• More reactive due to π-bond electrons.
• Common reactions:
  • Electrophilic addition: Halogens, hydrogen, water.
  • Polymerization: Formation of polymers like polythene, polypropylene.

Alkynes

• Unsaturated with triple bonds.
• General formula: CₙH₂ₙ₋₂.
• Simplest alkyne is ethyne (acetylene).
• More acidic than alkenes and alkanes due to sp hybridization.
• Common reactions:
  • Addition: Hydrogen, halogens.
  • Polymerization: Produces compounds like benzene.

Aromatic Hydrocarbons

• Benzene: C₆H₆, stable due to resonance.
• Undergo substitution rather than addition.
• Hückel's Rule: Aromaticity for compounds with (4n+2) π electrons.
• Reactions:
  • Nitration, halogenation, sulphonation.
  • Friedel-Crafts alkylation/acylation.

Carcinogenicity

• Some polycyclic aromatic hydrocarbons (PAHs) are carcinogenic.

Important Concepts

• Markovnikov's Rule: In additions, the negative part of the addendum attaches to carbon with fewer hydrogens.
• Peroxide Effect: Anti-Markovnikov addition due to free radicals.
• Directive Influence: Groups can activate/deactivate and influence substitution positions on benzene rings.

Exercises

• Practice problems on naming, isomer identification, and reaction prediction.

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These notes encapsulate the key concepts and reactions involving hydrocarbons, focusing on their structure, types, reactions, and applications. Understanding these principles is crucial for mastering organic chemistry.