Physical Properties of Alkenes

The physical properties of alkenes, like boiling point, melting point, and solubility, are primarily influenced by their molecular structure, particularly the presence of the carbon-carbon double bond, and the resulting intermolecular forces.

Boiling Point

  • Comparison to Alkanes: Alkenes generally have boiling points very similar to those of alkanes with the same number of carbon atoms. The presence of the double bond doesn't drastically change the boiling point.
  • London Dispersion Forces: The primary intermolecular forces in alkenes (and alkanes) are London dispersion forces (instantaneous dipole-induced dipole interactions).
  • Effect of Size: Boiling points increase with increasing molecular size (number of carbon atoms) due to stronger London dispersion forces.
  • Branching: Branching in the carbon chain *decreases* the boiling point, similar to alkanes, as it reduces the surface area available for London dispersion forces.
  • Cis vs. Trans Isomers: For alkenes exhibiting cis/trans isomerism, the *cis* isomer often has a slightly higher boiling point than the *trans* isomer. This is because the *cis* isomer's shape can lead to a slightly larger dipole moment, resulting in weak dipole-dipole interactions in addition to London dispersion forces. However, this difference is usually small.

Melting Point

  • Comparison to Alkanes: Melting point trends for alkenes are somewhat more complex than boiling point trends.
  • Factors Affecting Melting Point: Melting points are influenced by how well molecules pack together in the solid state. Symmetry plays a role.
  • Cis vs. Trans Isomers: *Trans* isomers often have *higher* melting points than *cis* isomers. The more symmetrical *trans* isomers tend to pack more efficiently in the crystal lattice, leading to stronger intermolecular interactions and higher melting points.

Solubility

  • Insoluble in Water: Alkenes are essentially *insoluble* in water due to their nonpolar nature. The dominant intermolecular forces are weak London dispersion forces. They cannot form hydrogen bonds with water.
  • Soluble in Organic Solvents: Alkenes are soluble in most nonpolar organic solvents (e.g., hexane, diethyl ether).

Density

  • Alkenes are generally less dense than water.

Summary Table (Illustrative Trends)

Compound Type Boiling Point Trend Melting Point Trend Water Solubility Trend Key Intermolecular Force
Alkane Increases with size Increases with size (influenced by packing) Insoluble London Dispersion
Alkene Similar to alkanes Often *trans* > *cis* (influenced by packing) Insoluble London Dispersion (sometimes weak dipole-dipole in *cis*)

 

Key Takeaways
  • Alkenes have physical properties similar to alkanes with the same number of carbons.
  • Boiling points are primarily determined by London dispersion forces and increase with size. Branching decreases boiling points.
  • Melting points are influenced by molecular packing, and *trans* isomers often have higher melting points than *cis* isomers.
  • Alkenes are nonpolar and therefore insoluble in water but soluble in organic solvents.