Hi,
It's my 1st message on that forum, please accecpt my appologies for my poor english !
So I wanted to convert phthalic acid to anhydride. I know a widely used method that consists in heating acid to remove a water molecule & them get
the anhydride.
I tried that method but it's quite long (on heating &stirring plate) anhydride condensates & forms tiny bright cristals.
but the output is quite ridiculous.
I experimented another way using conc. sulfhydric acid.
I took the phthalic acid cristals and place in H2SO4 and heat a bit for the whole amount to "dissolve"
Then I poor liquid water & got a white cristalline precipitate which wasn't soluble into water (as well as anhydride I prepared by heating)
But don't know if I got the desired compound.
How could I do to know it ?
I do chem. at home & don't have physical tools to identify it !!!
any idea to do that or another way to get anhydride ?
Thanks for allSauron - 25-5-2007 at 12:44
Phthalic acid, per se, is actually rather uncommon and expensive, while phthalic anhydride is a chemical commodity and very cheap.
How did you obtain the free acid?
Most dehydrating agents such as concentrated sulfuric acid or acetic anhydride will abstract one mol H2O from the phthalic acid and yield the
anhydride.
You ought tobe able to find a freely available reference on the Internet with a detailed procedure.
I would suggest that you relocate your post to the thread about obtaining phthalic acid from napthalene as those members might have the information at
their fingertips. A new thread on this is not necessary.pbmineral - 25-5-2007 at 12:48
Originally posted by pbmineral
But don't know if I got the desired compound.
How could I do to know it ?
I do chem. at home & don't have physical tools to identify it !!!
any idea to do that or another way to get anhydride ?
It wouldn't have hurt you if you searched these by yourself but anyway: mp of phthalic anhydride: 129-133°C ; bp: 284°C
mp of phthalic acid: 210-211 °C (dec.)
As for the best method to dehydrate phthalic acid, I can only advise you to stick to the checked procedures and everything should work fine even if
you are not particularly skilled in practical chemistry. Note also that hot phthalic anhydride sublimes terribly and the vapors are irritating.pbmineral - 26-5-2007 at 01:23
Ok
Thank u for your answer !
But I wondered if H2SO4 could change the phthalic acid molecule for instance remove carboxylic group or to do a sulfonation on benzene ring.not_important - 26-5-2007 at 01:54
Sulfuric acid is overkill in this case. Heat a mixture of technical xylene and phthalic acid, distilling off the xylene-water azeotrope. Condense
the azeotrope, return the lighter xylene layer to the still. When only one layer comes over, with no cloudiness or small droplets forming in the
condensate, all the phthalic acid has be converted to anyhydride. Concentrate the xylene solution under reduced pressure until crystals of the
anhydride form, then cool and filter off the anhydride. Some of the anhydride will remain in the xylene, you can either repeat the concentration
under reduced pressure, or just save the xylene for dehydrating another batch of the acid.
As noted, the hot anhydride sublimes and is an irritant. The xylene solvent reduces the tendency to sublime, but expect some to do so anyway. If
crystals form in the upper section of the distilling apparatus, warm it with a hot air gun until the crystals melt and run back into the still pot.
Normally the refluxing portion of the distillate will dissolve those crystals, so no extra effort is needed.pbmineral - 1-6-2007 at 09:16
Thank u for that message
I tried with my anhydride and when I added soda I got a bright purple colour
great !!
THE remaining question is "Do other compounds react the same ?"
(for instance sulfonated derivatives or I don't know !)
