Physical properties of alkanes

Carbon and hydrogen have almost identical electronegativity, so the C-H and C-C bonds throughout an alkane are essentially non-polar covalent bonds, making alkane molecules as a whole non-polar. Because there is no significant charge separation to create dipole-dipole attraction, the only force holding separate alkane molecules together is the weak van der Waals (London dispersion) force, and the strength of this force scales with how much surface area two neighbouring molecules can bring into close contact. A straight-chain alkane can lie alongside its neighbours over its whole extended length, giving a comparatively large effective contact area and therefore stronger intermolecular attraction, whereas a branched isomer of the same molecular formula packs into a more compact, sphere-like shape with less surface area available for contact, and therefore weaker attraction. This single surface-area argument explains why straight-chain alkanes consistently have higher melting and boiling points than their branched isomers (visible directly in Table 15.2, where each same-formula pair or triplet always has the straight-chain member boiling highest), and why intermolecular forces have to be overcome only partly during melting but completely during boiling. It also explains alkanes' insolubility in water (a highly polar solvent that cannot form favourable interactions with a non-polar solute) and their good solubility in non-polar organic solvents such as chloroform or ether (like dissolves like). Alkanes themselves are colourle …