BP & MP of Caffeine: 178°C & 236.2°C

[mnmman]

Pubchem says the BP of caffine is 178 deg C (sublimes) and the melting point is 236.2 deg C. Below is the only phase diagram I found for caffeine. Aren't the MP and BP measured at 1 atm? If so according to this phase diagram it shouldn't sublimate correct? Assuming the phase diagram is wrong, how does one attain the melting point if it really does sublimate at 178 deg C?
http://www2.volstate.edu/CHEM/2010/Labs/caffeine-6.gif

 

[Bystander]

Aren't the MP and BP measured at 1 atm?

"Usually."

it shouldn't sublimate correct?

"Sublimation" is a phase change from solid to gas, and can occur anywhere along the solid-vapor line in the phase diagram.
Compounds that sublime at temperatures greater than 1 atmosphere pressure, carbon dioxide, aluminum chloride, adamantane can have their melting points determined at higher pressures (carefully).

 

[mnmman]

OK, you think they would put the pressure they measure at if it wasn't at 1 atm.

 

[Bystander]

think they would put the pressure

Chemists can be very sloppy sometimes. At the same time, tabulations of physical properties aren't what some areas of chemistry really focus on.

 

[DrClaude]

I wouldn't trust the Pubchem data. For one, you can't sublimate at a lower temperature than the melting point! The source cited by Pubchem is the Merck Index, so it is a secondary source. I trust much more the NIST Chemistry WebBook, where you will find that the melting point is 509.3 K (with proper references), which is the same melting point you found. The triple point is at 508.3 K, so the phase diagram you got would make sense.

 

[Bystander]

For one, you can't sublimate at a lower temperature than the melting point!

Come again?

 

[DrClaude]

Come again?

I meant at a given pressure. As I was answering, I forgot you had already discussed the fact that the pressure might be different from atmospheric pressure, so my comment indeed sounds a bit silly.

 

[Bystander]

Macht es nichts. We chemists have to stick together in defense of our sloppy traditions and prerogatives.

 

[DrClaude]

Macht es nichts. We chemists have to stick together in defense of our sloppy traditions and prerogatives.

My background is in chemistry, so I'm not dissing anyone here! Full discloser: I love the Merck Index! But I don't have my copy with me, so I can't look up where they took that number for sublimation. If the number is right, I'm not sure I understand why anyone would report the sublimation point of caffeine at an arbitrary low pressure.

 

[Bystander]

What I remember from TA'ing a bunch of nursing students through a watered down organic lab is that purification for medical purposes (APC) is done by vacuum (water aspirators) sublimation to reduce decomposition (that horrible smell when the coffee pot boils dry at work). The vapor pressure of lab water supplies is a fairly common pressure for boiling points and other reduced pressure data in CRC.

Didn't happen in my lab, but next door really did get to witness "the cold finger." It is not an urban myth.