Question

What happens when boiling point is less than melting point?

Answer 1

Sublimation is not boiling but there is a reason for listing it instead. Sublimation is direct vaporisation from the solid. The most familiar is solid dry ice producing CO22 gas. So if metaldehyde melts at 246 C then one might suppose that the boiling point is higher. However, look at the generic P vs. T diagram below. If we assume that at P = 1000 is normal atmospheric pressure then a compound with this P vs T sublimes. As we define boiling to occur only at normal pressure and be the liquid to vapour (gas) transition then this compound can never boil at normal pressure. To become a liquid high temperature and pressure is needed. So you can see why the table lists sublimation at normal pressure rather than boiling.