# Does fumaric acid form maleic anhydride upon heating?

Maleic acid (the cis isomer of the dicarboxylic acid $\ce{C4H4O4}$) readily forms its anhydride on heating. Clayden's organic chemistry textbook mentions this as a point of distinction between it and fumaric acid (the trans isomer), which fails to dehydrate to its anhydride upon heating. However, I have discovered that at temperatures above 200 $^{\circ}\mathrm{C}$, fumaric acid forms maleic anhydride as well.

Which is the correct answer? I don't think it is likely that fumaric acid can form the anhydride - even at higher temperatures - given the trans configuration and restricted rotation.

• Some calculations (values from NIST) reveal that the formation of maleic anhydride and water from fumaric acid is an endothermic reaction ($\Delta H\approx +65.28 \frac{kJ}{mol}$) but the change in entropy is positive ($\Delta S\approx 134.24 \frac{J}{mol\cdot K}$). This means that the reaction becomes spontaneous for temperatures above $213^\circ C$, but this is only "mildly" spontaneous ($\Delta G\approx -5 \frac{kJ}{mol}$ at $523\ K$). So, the use of a dehydrating agent to remove the water from the reaction mixture and drive the equilibrium forward (Le Chatelier's principle) is necessary. – Eashaan Godbole Jun 9 '17 at 3:20

Also, dehydrating fumaric acid with a dehydrating agent also forms maleic anhydride. This paper describe how to convert $\ce{C^{14}}$ labelled fumaric acid to maleic anhydride.
Maleic anhydride-2-$\ce{C^{14}}$ is readily obtained from fumaric acid-2-$\ce{C^{14}}$ through dehydration using phosphorus pentoxide getting 96% yield under controlled condition.