Sciencemadness Discussion Board - phthalic acid -> phthalic anhydride

phthalic acid -> phthalic anhydride

Brominator - 6-3-2012 at 09:22

Would it be possible to dehydrate phthalic acid to phthalic anydride via sulphuric acid as the dehydrating agent,
I have no refrences for this hense why it is in beginnings,
i know that sulphuric acid does not carbonise the phthalic acid and i have thought up a possible mechanism for the reaction and was woundering if anybody else could shed some light on the subject
Thank you

Hexavalent - 6-3-2012 at 09:40

Watch this; http://www.youtube.com/watch?v=58Ve69s0qD0

Brominator - 6-3-2012 at 09:44

I've seen nurdrages video and i know phthalic acid can be dehydrated using heat i was wondering if sulphuric acid could be used more out of interest that practicality

AirCowPeaCock - 6-3-2012 at 10:00

Try it, with just a little, id like to know if it works too

[Edited on 3-6-2012 by AirCowPeaCock]

ItalianChemist - 6-3-2012 at 10:54

Try dissolving phthalic acid in hot sulphuric acid, let it cool down to room temperature and then pour the solution in water slowly!
phthalic anhydride should precipitate out!

Brominator - 6-3-2012 at 11:22

Surely the phthalic anhydride will simply hydrolyse in the water converting back to phthalic acid which won't be of much help ?

AirCowPeaCock - 6-3-2012 at 11:34

Phthalic anhydride should hydrolize rather slowly at room temperature.
Although, correct me if I'm wrong the sulfuric acid may speed that process up.

[Edited on 3-6-2012 by AirCowPeaCock]

entropy51 - 6-3-2012 at 16:34

Quote: Originally posted by Brominator

I've seen nurdrages video and i know phthalic acid can be dehydrated using heat i was wondering if sulphuric acid could be used more out of interest that practicality

In additon to the video, Magpie has posted a very straight forward method:

Quote: Originally posted by Magpie

Having given up on being able to get phthalic anhydride (PAN) from suppliers (they send me phthalic acid [PA] instead), I have developed a reasonably facile procedure for converting PA to PAN:

In a hood or outside:
Place 25g of the PA in a 250 mL beaker supported by an iron ring attached to a ringstand. Place the beaker in an oil bath so that the PA is heated by the oil. Set the bath temperature at 200C. As the PA is heated above its mp (191C) PAN and water will vaporize. Some of the PAN will be lost to the atmosphere and some will accumulate as fine needles in a wool along the sides of the beaker. Gather the wool as it is generated and drop it back into the melt. Continue this until the melt has turned from milky white to clear. Then pour the melt into an evaporating dish and place in a dessicator to cool. When cooled scrape out the melt and grind to a powder with a mortar and pestle. Yield = 82%.

I have done this twice. Both times the mp of the resulting PAN was 131.5-132.0C, as it should be.

Magpie - 6-3-2012 at 17:23

UnintentionalChaos - 6-3-2012 at 18:45

Sulfuric acid merely dissolves phthalic acid by protonating it, I believe. The anhydride does not form this way.

Brominator - 7-3-2012 at 07:02

I heated phthalic acid in conc sulphuric acid to get it to dissolve and then kept at this heat for 30 minutes i then slowly let it cool then cooled it in an ice bath and needle like crystals of what i think are phthalic anhydride crystallized out.
i will now try magpies method to achieve a bulk amount of phthalic anhydrise
Thanks

bfesser - 7-3-2012 at 16:45

In my experience the the 1,2-dicarboxylic acid tends to form platelets whereas phthalic anhydride tends to form needles when crystallizing out of solution. Therefore, I concur that your product is likely the anhydride. Also, from experience, the anhydride has a repulsive odor while the pure acid is typically odorless. Why would you want to waste sulfuric acid on this, however, is beyond me. The conversion requires nothing but heat and patience--it's one of the most trivial preparations imaginable.

In fact, I've converted some of my phthalic anhydride from Chemsavers Inc. (fairly poor quality, probably technical grade) into the dicarboxylic acid, to increase purity and eliminate the stink--also the anhydride tends to get 'static-y' where the acid does not. My thoughts were; why work with the anhydride in situations where the acid is sufficient? And, the lower vapor pressure of the acid should help to minimize inhalation exposure.

[Edited on 3/8/12 by bfesser]