Ortho acids like orthoformic acid (methanetriol, $\ce{HC(OH)3}$), orthocarbonic acid ($\ce{C(OH)4}$), and orthoacetic acid (ethane-1,1,1-triol, $\ce{H3C\bond{-}C(OH)3}$) are known to be unstable and decompose spontaneously.

For example, orthoformic acid spontaneously decomposes into formic acid and water, while orthocarbonic acid spontaneously decomposes into carbonic acid and water. As a matter of fact, these two acids have never been isolated.

$$\ce{HC(OH)3 -> HCOOH + H2O}$$ $$\ce{C(OH)4 -> H2CO3 + H2O}$$

Interestingly enough, orthoesters are stable and can be isolated, unlike ortho acids, just that orthoesters are easily hydrolyzed, especially with a mild acid present. For instance, trimethyl orthoformate, the simplest orthoester, can be synthesized. Why is that ortho acids are very unstable, while orthoesters are stable? Is the presence of hydroxyl groups of ortho acids a direct contributor of their instability?

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    $\begingroup$ This question chemistry.stackexchange.com/questions/10722/…₂ should answer your question. $\endgroup$ – Pritt says Reinstate Monica Jun 3 '17 at 3:31
  • $\begingroup$ Aside from the factors discussed in the answer to the above-referenced question, carboxylic acids have an energy advantage over their orthoacid forms due to delocalization of the electrons on the OH group into the carbonyl group. $\endgroup$ – iad22agp Jun 3 '17 at 10:25