Sciencemadness Discussion Board

Dean-stark method annoying azeotropes

Cou - 12-6-2020 at 00:32

Today I attempted to make octyl butyrate with the dean-stark technique and fischer esterification. However water was not cleanly falling to the bottom of the trap. I found out that butyric acid and water form an azeotrope that boils at about 99 C. For comparison, the boiling point of the toluene-water azeotrope is 84.1 C. When a dean-stark trap gets contaminated with other organic compounds, separation of water from toluene fails.

Unfortunately I found that valeric acids have azeotropes too. You probably aren't in the clear until 6-carbon acids and heavier.

I am wondering, would the dean-stark method work if I use benzene or hexane, instead of toluene? The benzene-water and hexane-water azeotropes are in the 60s C, even further from the water-butyric acid azeotrope. I'm wondering if this means that when water forms in the fischer esterification, the water will take almost entirely benzene/hexane with it, and not butyric acid.

You guys might not be able to answer this, but I might try it with hexane and see if the dean-stark technique still screws up. If it still doesn't work, i'll just have to accept the lower yields from normal fischer esterification. I believe in industry, they accept the lower yields from fischer b/c the methods that go to completion are so expensive that it's not worth it.

[Edited on 12-6-2020 by Cou]

Sigmatropic - 12-6-2020 at 15:58

In industry butyryl chloride is commercially available and we would not bother starting from the acid, but then again we are not interested in such simple esters.

SWIM - 12-6-2020 at 19:25

Benzene and hexane ought to work.
Benzene is more dangerous, but smells nicer so you can really enjoy the exposure.

Molecular sieves in the trap might do the job even if the distillate doesn't form two layers.
I have read of their use in Clevenger traps in situations like this but I don't recall if it was an ester they were making.

Cou - 12-6-2020 at 19:39

Another technique I am trying out is a "pseudo dean stark technique" where I use fractional distillation to distill over the toluene-water azeotrope (84.1 C), but not the butyric acid-water azeotrope (100 C). So the toluene-water azeotrope distills into a beaker. You can make it work like the dean stark trap by regularly pipetting the toluene layer back into the flask.

EDIT: still doesn't frikin work. I've decided to accept my losses, some wastage from using an excess of C-4 and C-5 acids in fischer esterifications, instead of messing around with this dean stark crap.

[Edited on 13-6-2020 by Cou]