PHYSICAL PROPERTIES OF HALOALKANES

Physical & Chemical Properties of Haloalkanes and Haloarene

Physical Properties of Haloalkanes

Haloalkanes are hydrocarbons in which hydrogen in a normal alkane is replaced by a halogen (group 17 elements). In this article, we will discuss the physical properties of haloalkanes. The physical properties of haloalkanes are mostly like a normal covalent compound. 

Halogens not being much reactive functional group as a carboxyl group or aldehyde doesn’t affect the overall physical properties by much. Still, few differences can be seen as we move down in the homologous series of haloalkanes group due to the difference in atomic masses of the compound.

1. Notable Fact

Physical properties of any compound depend largely on

1. It’s mass
2. The type of intermolecular and intramolecular forces of attraction

2. Melting Point and Boiling Point of Haloalkanes

There is a large electronegativity difference between halogens and carbon resulting in highly polarized molecules. The higher molecular mass and greater polarity as compared to the parent hydrocarbon results in stronger intermolecular forces of attraction (dipole-dipole and van der Waals) in the halogen derivatives. 

Boiling Point depends upon the intermolecular forces of attraction and hence the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass.

As we go down in homologs series of haloalkanes, the forces of attraction become stronger due to the increase in molecular size and its mass, hence the boiling point increases down the homologs series. But the boiling point decreases with branching.

Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids.

The melting point of a compound depends upon the strength of the lattice structure of a compound. The melting point also follows the same trend as the boiling point. An exception to this is para-isomers. The para-isomers have higher melting as compared to their ortho and meta-isomers. It is due to the symmetry of para-isomers that fits in crystal lattice better as compared to ortho– and meta-isomers.

3. Density of Haloalkanes

Density is directly proportional to the mass of the compound, hence down the homologous series, density increases, also fluoro derivatives are lesser dense than chloro derivatives; chloro derivatives are less dense than bromo derivatives and so on.

4. Solubility of Haloalkanes

Haloalkanes are slightly soluble in water. This is because of the relatively larger amount of energy required to break the bond between halogen and carbon and the smaller amount of energy released when the bond is formed after dissolution ion and water.

Chemical Properties of Haloalkanes

Haloalkanes are quite reactive compounds. They undergo substitution, elimination and reduction reactions. They also react with some metals to form organometallic compounds. The reactivity of haloalkanes is mainly due to the polar nature of the carbon halogen bond.

The stability of alkyl halides generally decreases as the strength of the C-X bond decreases. Iodides generally liberate iodine due to their low stability and acquire brown or violet color.

2R-I → R-R + I2

Iodoform shows the antiseptic properties due to the liberation of iodine.