Monday, January 28, 2008

IIT JEE Revision - Alkenes - Physical properties

1. State: Ethene, propene and butene are gases at room temperature.
From pentene onwards till alkenes having 18 carbon atoms, they are liquids.
Still higher members of the family are solids.

2. Melting points: Alkenes have higher melting points than the corresponindg alkanes. Intermolecular forces of attraction in double bond are stronger.

The melting points increase with molecular mass of alkenes.

Among isomers, trans-alkenes have higher melting points than their corresponding cis alkenes.

3. Boiling points: The boiling points increase with increase in carbon atoms. The branched chain alkenes have lower boiling points than the corresponding straight chain alkenes.

Among geometric isomers, cis-alkenes have higher boiling points than the corresponding trans-isomers.

4. Dipole moments: Alkenes are weakly polar. Their dipole moments are higher than those of alkanes.

In case of geometricl isomers, symmetrical trans alkenes are nonpolar and have zero dipole moment due to symmetry. But unsymmetrical trans alkenes are polar. Cis isomers are polar and have dipole moments.

Unsymmetrical terminal alkenes such as propene and but-1-ene have some dipole moment.

5. Solubility: Alkenes are lighter than water. These are insoluble in water. They readily dissolve in organgic solvents like alcohol, benzenes, ether, carbon tetrachloride etc.

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