is it unstable

2 Answers

In acidic medium a proton adds to -OH and it leaves, forming a vinyl carbocation. That is extremely unstable. Hence it is unstable in acidic medium.

R=-CH₃ is maleic acid
less stable than trans isomer

Maleic acid and fumaric acid can normally not be interconverted because rotation around a carbon carbon double bond is not energetically favourable.

However it can be isolated.

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