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I read that atleast one alpha carbon has to be sp3 wrt the functional group (and has to have atleast 1 alpha Hydrogen atom) for it to show tautomerism.

I am able to see that in the keto isomers of phenol.

However looking at phenol (enol isomer), how does one predict it can show tautomerism?

I'm wondering if some method can be used to identify that.

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1 Answer 1

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All enols exhibit tautomerism, at least to some extent, but not all ketones can tautomerise.

Conditions for showing tautomerism (for ketones) are listed below:

  1. Presence of at least one alpha H at the $\ce{sp^3}$ hybridized alpha C (i.e. C just next to the carbonyl carbon)
  2. In case of alpha-beta unsaturated ketones/aldehydes at least one alpha H should be present at the gamma carbon (i.e. third to the carbonyl C).

Also keep in mind some basic rules such as Bredt's Rule.

For better understanding refer to this.

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  • $\begingroup$ The absolute comment on ketones is troublesome. $\endgroup$
    – jimchmst
    Commented Dec 27, 2022 at 20:03
  • $\begingroup$ @jimchmst What do you mean? $\endgroup$
    – CHEMUMAN
    Commented Dec 28, 2022 at 5:25
  • $\begingroup$ I was thinking something like benzaldehyde or benzophenone could have an ionic enol to give a hydroxy enol. but di-t-butyl ketone cannot without a methyl shift to give an alpha hydroxy alkene [not an enol]. very unstable! $\endgroup$
    – jimchmst
    Commented Dec 28, 2022 at 7:08
  • $\begingroup$ @jimchmst not an ionic enol but it seems carbene enol which I don't think would be stable(why would it loose aromaticity , forming carbene). Also any of the mentioned compounds doesn't satisfy the conditions. $\endgroup$
    – CHEMUMAN
    Commented Dec 28, 2022 at 7:48

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