3.4: Physical Properties of Alkanes
Alkanes are nonpolar molecules due to the presence of only carbon and hydrogen atoms. The electronegativity difference between carbon and hydrogen is minimal, and hence alkanes have a zero dipole moment. This leads to the presence of only dispersion forces between the molecules. The strength of dispersion forces is dependent on the surface area of the molecules on which they act. Since the surface area increases with the molecular length for straight-chain alkanes, the dispersion forces also increase across the homologue with the increasing carbon chain length.
The dispersion forces affect the physical properties of the alkanes and vary their physical state. Based on the number of carbon atoms, the straight-chain alkanes exist in different physical states at a given temperature and pressure. Thus, the boiling point for straight-chain alkanes is directly proportional to their chain length and, in turn, is proportional to the dispersion forces. In contrast, the melting point shows an odd-even behavior, i.e., the odd and even membered alkane form a different melting point trend with increasing chain length.
Branched-chain isomers of alkanes show significant variations in their properties due to the differences in their shape and size compared to the straight-chain alkanes. For instance, in pentane, the melting point varies drastically between the straight-chain and branched forms. The straight-chain n-pentane melts at a temperature of −129.8°C, whereas the branched form iso-pentane melts at −161.0°C. Neopentane, the symmetrically branched isomer, melts at a much higher temperature of −16.5°C.