Physical Properties of Haloalkanes

Melting and Boiling Points

• Depend on Van der Waals dispersion forces and dipole–dipole interactions.
• Boiling point ∝ size of halogen atom and number of electrons: R–I > R–Br > R–Cl > R–F (for the same carbon chain)
• Boiling point ∝ surface area ∝ no. of carbons in chain (longer chain → higher B.P.)
• Branching reduces B.P.: B.P. ∝ 1/branching (isomers go from primary → tertiary, B.P. falls)
• Para-isomers of dihalobenzenes have higher melting points than ortho- and meta-isomers due to symmetry fitting better in the crystal lattice.

Density

• Density ∝ no. of halogen atoms / molecular mass.
• Bromo, iodo, and polychloro derivatives are heavier than water: Density: R–I > R–Br > R–Cl > R–F
• For isomers of chlorobenzene: density ∝ molecular mass → benzene < chlorobenzene < dichlorobenzene < bromochlorobenzene.

Solubility

• Haloalkanes are very slightly soluble in water (attraction between alkyl halide molecules is stronger than attraction between alkyl halide and water, and they fail to form H-bonds with water).
• Solubility order in water: R–F > R–Cl > R–Br > R–I
• Haloalkanes dissolve readily in organic solvents (due to similar intermolecular forces).

Alkyl iodide is so unstable that it decomposes in sunlight: 2R–I → 2R + I₂ (violet vapours)