Alkanes Cycloalkanes Physical Properties

Physical Properties of Alkanes and Cycloalkanes

Physical Properties of Alkanes and Cycloalkanes

Alkanes and cycloalkanes are nonpolar hydrocarbons whose physical properties depend on size, shape, and ring constraints. Dispersion forces dominate, so boiling/melting trends track molecular surface area and compactness.


Boiling and Melting Point Trends

  • Chain length: longer chains → higher boiling/melting points (stronger dispersion).
  • Branching: lowers boiling point (more compact, less surface contact); often raises symmetry which can raise melting point.
  • Example: n-pentane (36 °C) > isopentane (27 °C) > neopentane (9.5 °C) for boiling points.
Boiling point trends with chain length and branching

Solubility and Density

  • Solubility: insoluble in water; soluble in nonpolar/weakly polar solvents.
  • Density: < 1 g/mL; liquids float on water.
  • Small alkanes gases at RT; mid-length liquids; very long chains waxy solids.
Solubility and density overview for alkanes

Cycloalkane Comparisons

  • Rings reduce flexibility; similar C count vs open chain often gives slightly higher boiling/melting points (tighter packing, larger surface).
  • Chair cyclohexane is strain-free with ~109.5° angles; ring size and substituent orientation influence stability/physical data.
Cycloalkane strain and stability comparison

Summary

  • Dispersion forces control alkane/cycloalkane physical behavior.
  • Longer chains ↑ boiling/melting; branching ↓ boiling (often ↑ symmetry for melting).
  • Nonpolar, low-density, water-insoluble; cycloalkanes often boil/melt slightly higher than their open-chain counterparts.