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.

A series of unbranched alkanes in which successive members differ by a −CH₂− group is called a homologous series, and each alkane in this series is a homologue.

Alkanes are nonpolar due to the small electronegativity difference between carbon and hydrogen, implying that only weak dispersion forces exist between their molecules.

The strength of these forces increases proportionally with carbon chain length. The first four alkanes are gases at room temperature and atmospheric pressure; moderate carbon chains, from C₅ to C₁₇, are liquids. Homologues beyond C₁₇ are solids.

Dispersion forces also influence the different physical properties of alkanes.

Within a homologous series, as surface area increases, the dispersion forces between molecules also increase. Because more energy is required to separate the molecules, the boiling points of unbranched alkanes rise with each additional carbon.

Unlike straight-chain alkanes, branched alkanes are compact and more spherical, which reduces the area of interaction and the strength of the intermolecular forces. Therefore, compared to the unbranched form, branched isomers boil at lower temperatures.

While the boiling points of straight-chain alkanes rise gradually with the number of carbons, their melting points do not increase steadily. Instead, the trend alternates between the even and odd members of the homologous series.

In the crystalline state, even alkanes pack closely in a zig-zag arrangement. Consequently, the molecules experience stronger attractions, leading to higher melting points.

In comparison, odd alkanes pack less tightly due to their parallel arrangement, resulting in weaker interactions and lower melting points.

The melting points of branched alkanes are dictated by their molecular symmetries.

Branched alkanes with substantial symmetry melt at temperatures higher than the unbranched hydrocarbon. In comparison,  asymmetrical branching leads to a lower melting point.

Alkanes are insoluble in water due to their inability to form hydrogen bonds. Additionally, as their densities are lower than 1 g/cm³, they float on water.