Alkane properties

The picture on the facing page shows melting points measured for alkanes. (Note that even when alkanes are listed in chemical databases—as discussed above—their melting points may not be given.) Unbranched alkanes yield melting points that increase smoothly for n even and for n odd. Highly symmetrical branched alkanes tend to have high melting points, presumably because they pack well in space. No reliable general method for predicting melting points is however known (see note above), and in fact for large n alkanes tend to form jellies with no clear notion of melting.

Things appear somewhat simpler with boiling points, and as noticed by Harry Wiener in 1947 (and increasingly discussed since the 1970s) these tend to be well fit as being linearly proportional to the so-called topological index given by the sum of the smallest numbers of connections visited in getting between all pairs of carbon atoms in an alkane molecule.