# Why does naphthalene have a higher melting point than both water and diphenylamine?

We learn that polar compounds have higher melting and boiling points than nonpolar compounds due to strong intermolecular forces such as hydrogen bonds and dipole-dipole interactions.

Naphthalene is an extremely simple and nonpolar compound, yet it has a significantly higher melting point than diphenylamine (a similar enough structure that is not only heavier, but has a dipole-inducing amine) and water (famously super polar).

What's going on here?

• High symmetry, and molecular mass, pi-stacking. – Mithoron Apr 22 '17 at 21:32
• @Mithoron Could you go into a bit of details about each of those in your answer? Like, why does high symmetry and pi-stacking increase melting point (from a molecular perspective)? – Nova Apr 22 '17 at 21:45
• – Mithoron Apr 22 '17 at 21:55
• en.wikipedia.org/wiki/Stacking_(chemistry) – Mithoron Apr 22 '17 at 21:56
• Pi stacking is explained in answer to this question chemistry.stackexchange.com/questions/60055/… – porphyrin Apr 23 '17 at 11:09

In the case of naphthalene and the amine, clearly the naphthalene can stack nicely and so increase its attractive interaction relative to repulsive ones, whereas the amine is twisted and cannot so easily pack meaning that repulsion can be stronger and attractive interactions weaker (even with dipole) and melting points lower. As an example consider the $\ce{C6H14}$ isomers, n-hexane mp -95 C, 2-methyl-pentane mp -154 C but boiling points are 69 & 60 C respectively and $\ce{C8H18}$ isomers, n-octane mp -57, 2-me-heptane mp -120 and bp 126 & 119 C respectively.