Question

I am looking for the melting point of caffeine. Every site I go to says the temp is higher than the boiling point, which is impossible.

Answer 1

perhaps they mean higher than the boiling point of water?

Answer 2

I believe you will find your answer here: http://www.chemistrydaily.com/chemistry/Caffeine#Physical_properties

Answer 3

Remember, the boiling point is a PHYSICAL property

Answer 4

brb boiling some coffee to check :)

Answer 5

Tricky. As best as I can find, caffeine sublimates at 1 atm, meaning turns directly from solid to vapor, like CO2, at about 180C. Under pressure, for example in a sealed melting point tube, it will melt at about 240C. The key seems to be that the triple point lies above 1 atm, so that it will only melt (turn to liquid) at all under pressure. I believe it does have a theoretically-derived boiling point (also under pressure) above 400C, but I'm not sure, and it may decompose first, as many organic compounds do.

Answer 6

That link I posted had the answer on it.

Answer 7

Unfortunately, @jwheele1, it did not. The site to which you pointed is one of the many that (carelessly, in my opinion) lists the "boiling" point of caffeine at 180C and the melting point at 240C. As @ChmE correctly points out, this is impossible.

The site does write "sublimes" in parentheses to correct the (false) impression that the phrase "boiling point" gives, which is that caffeine turns from a liquid to a gas at that temperature.

However, the essential question then remains: how can one logically say a substance turns from solid to gas at 180C and then from solid to liquid at a still higher temperature, 240C? This does not compute.

As I said, the answer appears to be that at 1 atm, in an open container, caffeine will sublimate (turn from solid to gas) at 180C. If, on the other hand, the caffeine is confined, e.g. in a melting tube, so that as it sublimates the pressure rises steeply, then the triple pressure will be exceeded and the *remaining* solid caffeine will melt -- turn from a solid to a liquid -- at 240C and some (unknown, unfortunately) pressure higher than 1 atm.

This at least makes sense of the reported data, but I haven't been able to find a phase diagram of caffeine that would confirm the hypothesis.