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Alkenes - Physical Properties of Alkenes

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Estimated time: 6 minutes
  • Key Points: Physical Properties of Alkenes
CBSE: Class 12
Maharashtra State Board: Class 12

Key Points: Physical Properties of Alkenes

1. Solubility

  • Alkenes are non-polar
  • Insoluble in water
  • Soluble in non-polar organic solvents (e.g., hexane, benzene)

2. Boiling Point (BP)

Increases with molecular mass

  • More electrons → stronger London dispersion forces

Straight-chain > Branched-chain

  • Straight chains have larger surface area → stronger intermolecular forces

Cis-alkenes > Trans-alkenes (usually)

  • Cis is more polar → dipoles do not cancel → higher BP

Alkenes vs Alkanes (same number of carbons)

  • Alkenes have slightly lower BP

Reason:

  • π-bond leads to less effective electron distribution for dispersion forces
  • Slightly weaker intermolecular attractions

3. Melting Point (MP)

Trans-alkenes > Cis-alkenes

  • Trans is more symmetrical → packs better in crystal lattice → higher MP

Cis-alkenes

  • Less symmetrical → poorer packing → lower MP

In trans-alkenes:

  • Bond dipoles cancel → non-polar
  • Leads to tighter packing in solid state
CISCE: Class 10

Example: Ethene

  • Ethene is a colourless, flammable gas with a faint sweet smell
  • Melting point: −169 °C
  • Boiling point: −102 °C
  • Slightly soluble in water, but highly soluble in organic solvents
  • Shows weak intermolecular forces, hence very low BP and MP
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