chemrox - 1-12-2008 at 21:32
The syntheses of p-toluenesulfonic acid I've found either produce the Na salt or employ gaseous HCl to push the equilibrium in favor of the acid
formation. The latter procedure is outlined in Vogel's 3rd. I'm wondering if in lieu of gaseous HCl one might make a saturated solution of HCl in
heptane, hexane,... something soluble in toluene.. and use that in place of operating the gas generator during the reaction to make the acid. It's a
question of available facilities right now I'm dealing with and it would be more convenient, for the time being, to make an HCl reagent instead of
generating the gas during the reaction.
[Edited on 1-12-2008 by chemrox]
not_important - 2-12-2008 at 00:13
The gaseous HCl is to tie up the water and force the toluenesulfonic acid out of solution, leaving H2SO4 in the aqueous phase. You can't do that with
an organic solution of HCl.
When I wanted to make a large amount of pTSA I used a related procedure to that in Vogel. I boiled toluene and passed the vapour into H2SO4 at
150-160 C, condensing and collecting the distillate. When the amount of aqueous layer indicated that 90-95 percent of the H2SO4 had reacted, I allowed
it to cool somewhat, added enough hot water to keep the reaction mix a slush or slurry so I could get it out of the flask, and poured it into ice cold
concentrated hydrochloric acid. Stir well for some minutes, let sit packed in ice or snow for a couple of hours, filter, wash with several small
amounts of ice cold con HCl. This seemed to remove the H2SO4 satisfactorily.
The amount of water I added was no where near enough to dissolve all the pTSA, the amount of con HCL just swamped it out and left only a small amount
of the product in solution.
Klute - 2-12-2008 at 09:00
Why not use a Dean Stark, and keep the solution in toluene of anhydrous acid? Depedning on what you need it for, you can directly use it as is, the
amount of residual H2SO4 being minimal after removing all the formed water.
Otherwise you can add 1 eq of water, stir at ice cold temps, and filter off the mono hydrate. You can always re-dry it again with a Dean Stark when
need be.
chemrox - 2-12-2008 at 09:28
@not-important: indeed, why not use a Dean-Stark? I read it was an equilibrium thing but I trust you on this so what do you think about Klute's
idea?