Physical properties

Pure alkyl halides are colourless, but bromides and iodides turn coloured in light. Many volatile halogen compounds smell sweet.

At room temperature CH₃Cl, CH₃Br, C₂H₅Cl and some chlorofluoromethanes are gases; the larger members are liquids or solids.

Being polar and heavier, halogen derivatives have stronger dipole-dipole and van der Waals attractions than the parent hydrocarbons, so chlorides, bromides and iodides boil much higher than hydrocarbons of similar molecular mass.

For one alkyl group, boiling points run RI > RBr > RCl > RF, because a bigger, heavier halogen gives larger van der Waals forces. Among isomers, more branching means a lower boiling point: 2-bromo-2-methylpropane boils lowest of the three C₄H₉Br isomers NCERT compares.

Isomeric dihalobenzenes boil at nearly the same temperature, but the para isomer melts higher than ortho and meta because its symmetry packs better into the crystal lattice.

Haloalkanes are only very slightly soluble in water: dissolving must break the haloalkane-haloalkane attractions and water's hydrogen bonds, and the new haloalkane-water attractions release less energy. In organic solvents the attractions formed and broken are similar, so they dissolve.

Bromo, iodo and polychloro compounds are denser than water. Density rises with more carbon atoms, more halogen atoms and heavier halogen: n-C₃H₇Cl 0.89, n-C₃H₇Br 1.335, n-C₃H₇I 1.747, CH₂Cl₂ 1.336, CHCl₃ 1.489, CCl₄ 1.595 g/mL.