Alkanes: The Simple Hydrocarbons 🔬

What Are Alkanes? 🤔

Alkanes are saturated open-chain hydrocarbons with only carbon-carbon single bonds. Their general formula is CnH2n+2 (e.g., methane CH4, ethane C2H6)

Key Properties ⚗️

  • Physical: Non-polar → insoluble in water. Boiling point ↑ as molecular mass ↑.
  • Chemical: Inert (don’t react with acids/bases) → nicknamed “paraffins” (low affinity).

Isomerism & Naming 🏷️

Chain isomers exist for alkanes with ≥4 carbons. Same formula, different structures!

• Butane (C4H10): 2 isomers
  – n-Butane (straight chain)
  – 2-Methylpropane (branched)

• Pentane (C5H12): 3 isomers
  – n-Pentane
  – 2-Methylbutane
  – 2,2-Dimethylpropane

IUPAC Rules: Longest chain = parent name. Number chain to give lowest numbers to substituents. Alphabetize groups (ignore prefixes like di-, tri-).

Making Alkanes 🧪

3 main preparation methods:

  1. Hydrogenation: Add H2 to alkenes/alkynes with catalyst (Pt, Ni):
    \( \ce{CH2=CH2 + H2 ->[Pt/Pd/Ni] CH3-CH3} \)
  2. Wurtz reaction: 2 alkyl halides + Na in dry ether → alkane (even carbons):
    \( \ce{2CH3Br + 2Na ->[dry~ether] CH3-CH3 + 2NaBr} \)
  3. Decarboxylation: Carboxylic acid salt + soda lime → alkane (1 less carbon):
    \( \ce{CH3COO^-Na^+ + NaOH ->[CaO] CH4 + Na2CO3} \)

Chemical Reactions ⚛️

Alkanes react under specific conditions:

  • Combustion (🔥): Burn in O2 → CO2 + H2O + heat
    \( \ce{CH4 + 2O2 -> CO2 + 2H2O} \)
  • Halogenation (free radical): Needs heat/UV light. Reactivity: F2 > Cl2 > Br2 > I2
    \( \ce{CH4 + Cl2 ->[hv] CH3Cl + HCl} \)
    Mechanism: Initiation → Propagation → Termination
  • Isomerization: Straight chain → branched chain with catalysts
    \( \ce{n-Hexane ->[Anhy.~AlCl3/HCl] 2-Methylpentane} \)

Conformations of Ethane 🔄

Rotation around C-C bond creates different 3D shapes:

  • Staggered: Most stable (H atoms farthest apart)
  • Eclipsed: Least stable (H atoms overlap)

Energy difference: ~12.5 kJ/mol (staggered preferred!)

NEET Super Focus! 🎯

Must-Know Concepts:

  1. General formula: \( \ce{C_nH_{2n+2} \)
  2. Isomers: C4H10 (2 isomers), C5H12 (3 isomers), C6H14 (5 isomers)
  3. Halogenation: Free radical mechanism & reactivity order (F2 > Cl2 > Br2 > I2)
  4. Combustion: Complete vs incomplete
  5. Wurtz reaction: Limited to even-carbon alkanes