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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.

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    $\begingroup$ 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. $\endgroup$ – Eashaan Godbole Jun 9 '17 at 3:20
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Fumaric acid does form maleic anhydride due to partial combustion. From wikipedia article of fumaric acid:

Fumaric acid converts to the irritant maleic anhydride, upon partial combustion.

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.

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I agree with your reasoning but it really seems that fumaric acid forms the anhydride.

According to the Wikipedia page on maleic acid the isomerization process (from maleic to fumaric) is rather easy.

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