This is a β-hydroxyacid, but why doesn't it get dehydrated to give an α,β-unsaturated acid on heating?
The mechanism which I think should occur is:
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1$\begingroup$ What you've drawn isn't a dehydration, it's a retro-aldol reaction. You're losing formaldehyde instead of water. And your product isn't an alpha,beta-unsaturated acid. $\endgroup$– orthocresol ♦Apr 24, 2017 at 16:11
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$\begingroup$ @orthocresol I'm not sure of the mechanism that causes the conversion from beta hydroxy acid to alpha beta unsaturated acid. Could you give me a link? The mechanism in the image is just a guess... $\endgroup$– user38977Apr 24, 2017 at 16:14
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$\begingroup$ Isn't the carbocation more likely to be captured by the carboxy group giving a 4-membered lactone? $\endgroup$– WaylanderApr 24, 2017 at 16:35
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$\begingroup$ @Waylander Which carbocation? $\endgroup$– user38977Apr 24, 2017 at 16:44
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$\begingroup$ Depends on which O you think gets protonated first. If the carboxy group protonates the the alcohol OH can attack similar to acid-catalysed ester formation. The resulting B-lactone is a strained species,yet B-lactones exist. $\endgroup$– WaylanderApr 24, 2017 at 16:47
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1 Answer
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In order to obtain an α-β-unsaturated carbon acid from a β-hydroxy acid, you need a secondary or tertiary carbon in α position, because otherwise you would have to break a carbon-carbon bond, which requires a lot more energy then simply heating.
The mechanism is basically self catalysed, which is why this may happen easily with a little bit of heat. The acid may protonate intramolecular (or intermolecular) the hydroxy group. You have water as a nice leaving group, creating a carbocation, and abstraction of a proton in α-position leads to the olefin.
If you do not have that α-hydrogen, you are stuck with the carbocation. You might be able to obtain a lactone,like Waylander suggests, but I doubt this would easily happen with a little bit of heating, as spiro-compounds with four-membered rings tend to be quite unstable.
In your attempt you choose the less acidic hydrogen to transfer and then break a carbon-carbon bond resulting in a very unstable molecule which would rearrange to cyclohexanecarboxylic acid. I'd suspect you would need insane temperatures to overcome the activation barrier.
answered Apr 25, 2017 at 12:42
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$\begingroup$ In the last but one structure you have drawn, we could've even removed CO2 and H2O in a single step, forming a new exocyclic double bond, leading to an alkene. Can you please explain why this would be incorrect? $\endgroup$ Feb 16, 2021 at 14:20
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$\begingroup$ @SpaceSkiier No I cannot. Sorry. You might want to How to Ask a follow up question and someone else answers that. $\endgroup$– Martin - マーチン ♦Feb 16, 2021 at 21:57