# How does "hydrogen bonding" affect "normal boiling points"?

Oct 26, 2016

Consider water, $O {H}_{2}$. This is a very low molecular weight material that has quite an elevated normal boiling point. The strength and persistence of intermolecular hydrogen bonding is responsible for this involatility.
Contrast water's behaviour with solvents where hydrogen is not bound to a strongly electronegative atom. For the alkane series ${C}_{n} {H}_{2 n + 2}$, in which there is only marginal electronegativity difference between $C$ and $H$, the chain length must rise to $7$ before we reach a boiling point of $98$ ""^@C in heptane, ${C}_{7} {H}_{16}$. Here the predominant intermolecular force is the dispersion force, which increases with the chain length of the molecule.
On other hand, $H F$, and ammonia, $N {H}_{3}$, as the pure solvents, have boiling points of and $- 19.5$ ""^@C, $- 33.3$ ""^@C respectively, and these elevated boiling points also reflect substantial hydrogen bonding interactions. Compare these boiling points with the lower group homologues, where the boiling points are not so high.