I couldn’t figure out how to turn the five membered ring into 6 membered ring. Pls help
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2$\begingroup$ Where is your attempt? $\endgroup$– cngzz1Jan 2, 2021 at 13:45
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$\begingroup$ Protonate -OH, after which water leaves, generating a carbocation, which rearranges, and then an elimination occurs. You should have a look at E1 mechanism. $\endgroup$– Aditya MishraJan 2, 2021 at 15:48
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$\begingroup$ I would like to see a solution to this, it looks less than straightforward. $\endgroup$– WaylanderJan 2, 2021 at 15:58
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2$\begingroup$ Looks like an acid treatment of tetrahydrofurfuryl alcohol leads to 2,3-dihydropyran. That's all good. But I'm not sure how the inclusion of a methyl in the reactant molecule as above leads to the resulting molecule above instead of 6-Methyl-3,4-dihydro-2H-pyran. I'd like to see the solution as well indeed. $\endgroup$– z1273Jan 2, 2021 at 16:40
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Despite the OP having made no effort to answer this, I was intrigued and wanted to see a solution.
The secondary alcohol is protonated by the strong acid, water leaves to generate a secondary cation. With assistance from the ring oxygen, a methylene shift occurs to generate a more stable cation centred on the oxygen. Solvent removes the methine proton to generate the observed dihydropyran product. This only works because of the presence of the oxygen.
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$\begingroup$ Please explain how the 'oxygen-centred' cation is more stable? I have very basic knowlege regarding this and I've learnt that positive charge on less electronegative atom would be much more stable than on a more electronegative one. Here, oxygen should be the least stable place the positive charge could be put on, right? $\endgroup$– DesaiJan 2, 2021 at 19:57
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1$\begingroup$ Oxygen has electron pairs that it can donate, enough to make this process viable $\endgroup$ Jan 2, 2021 at 20:39