angelhair - 12-7-2008 at 17:01
I wanted the monohydrate from a dry Lab grade salt. So 150g of the supposed dry salt disolved after a while in 50ml of water. I thought more water was
needed. Anyway that was evaporated in a boiling water bath.
The ony way to get it to dry back to a salt was to gently blow on it while stirring. That happened very quickly.
The result after putting it in the dessicator was 148g. Did I just end up back with a dry salt? And what happened to the other 2 grams. Is this not
the way to the monohydrate?
Klute - 13-7-2008 at 01:43
Are you sure you have the anhydrous form in the first place? Anhydrous TsOH is incredebly hygroscopic, turning to a wet mass in a matter of seconds in
humid atmospher. The ease with which you got it to dry leads me to think you haev the monohydrate int he first place, either because it it's initial
form, or because the bottle wasn't sealed well enough and the acid abosrbed enough water to get to the monohydrate.
Try refluxing some of your TsOH with some toluene in a Dean Stark and see how much water is evolved.
But again, if you are going to work in semi aqueous conditions, there's no reason at all to partially hydrate your acid before.
not_important - 13-7-2008 at 06:19
Usual way to the mono-hydrate is by recrystallising from concentrated hydrochloric acid, or from cold aqueous solution by bubbling in HCl gas. Dry in
a desiccator over KOH.
Outside of the - agree with what Klute said, anhydrous p-toluene sulfonic acid picks up moisture readily. Heating an aqueous solution on a water or
steam bath is likely to give you the hydrate or hydrate plus a small amount of anhydrous acid.