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Its formula is C6H4(CO)2O. It is an organic molecule that doesn’t have any branching or functional groups on carbon 4 or 1.

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First lets take a look at o-pthalic acid and it's anhydride. o-pthalic Acid o-pthalic anhydride

Now as you can see the ring formed in case of o-pthalic anhydride is sterically feasible. There is not very high angle strain and the aromaticity is still intact. Now, take a look at p-pthalic acid. p-pthalic acid

Now, if you try to dehydrate this molecule as you see there is a very big steric problem. To accomplish this dehydration, the molecule must take a Boat form, which looks something like this, (I am using image of cyclo-hexane just to show you the structure, benzene ring will never do this)

Boat conformer of cyclo-hexane

But to accomplish this conformation the benzene ring has to loose it's aromaticity and also create a very unstable system with the pi bond densities repelling each other. So, as this conformation is not Thermodynamically feasible, anhydride formation doesn't take place.

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  • $\begingroup$ Would p-phthalic acid dehydrate to form a polymeric anhydride (possibly a goo) or decompose? it does form polymers with dialcohols. $\endgroup$ – James Gaidis Apr 20 at 15:33
  • $\begingroup$ @James Gaidis I would assume yes as it gives very similar reactions in the production of PET(polyethylene terpthalate) and PBT(polybutylene terpthalate). $\endgroup$ – Nisarg Bhavsar Apr 20 at 15:45

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