Physical Properties of Haloalkanes and Haloarenes

Full Notes

General Characteristics

Most alkyl halides and aryl halides are colourless liquids at room temperature.

Iodo compounds may appear coloured.

They are generally denser than hydrocarbons of similar molar mass.

Many haloarenes are crystalline solids.

Melting and Boiling Points

Boiling points of haloalkanes increase with:

• Increasing molecular mass
• Increasing size of halogen atom (C–I > C–Br > C–Cl > C–F)
• Increase in number of halogen atoms

Boiling points follow this trend due to greater van der Waals forces and dipole–dipole interactions.

For isomeric haloalkanes, the straight-chain isomers have higher boiling points than their branched counterparts.

Melting points, particularly for haloarenes, are influenced by symmetry. Para-isomers pack more efficiently into crystal lattices, so they have higher melting points than ortho- and meta-isomers.

Example: Melting point order for dichlorobenzenes: para > meta > ortho

Density

Density increases with:

• Molar mass
• Presence of heavier halogen atoms

Order of density: RI > RBr > RCl > RF

Among isomeric compounds of similar molecular weight, density variation is minimal.

Solubility

Haloalkanes and haloarenes are practically insoluble in water.

Reason: They are unable to form hydrogen bonds with water molecules and the energy released in forming new attractions is less than the energy required to break existing H-bonds in water.

They are soluble in organic solvents like ether, chloroform, and benzene due to similar van der Waals forces.

Summary

• Boiling points increase with molecular size, halogen mass, and number of halogens.
• Melting points are influenced by isomerism and molecular symmetry.
• Densities rise with increasing molecular weight and atomic number of the halogen.
• These compounds are insoluble in water but soluble in non-polar organic solvents.