Read Which Compounds Can Actually Tautomerise
Keto-enol tautomerism needs an α-hydrogen next to a carbonyl — no α-H, no tautomer, regardless of how reactive the carbonyl otherwise looks.
Tautomerism and Enolisation is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Requiring an α-hydrogen for keto-enol tautomerism
Acetone (has α-hydrogens) readily tautomerises to its enol form; 2-methylpropanal-type aldehydes with no available α-hydrogen on the relevant side CANNOT tautomerise via that pathway — always check specifically for a hydrogen on the carbon adjacent to the carbonyl before assuming tautomerism is possible.
Enolisation is enhanced by additional stabilisation of the enol form
A conjugated cyclic enone-type system shows MAXIMUM enolisation among a comparison set, because the resulting enol gains extra conjugative stabilisation beyond a simple isolated enol — ranking 'which shows maximum enolisation' means identifying which candidate's enol tautomer is most stabilised, not just which candidate has an α-hydrogen at all.
Urea and nitroalkanes show tautomerism through different mechanisms
Urea exhibits tautomerism between its amide (keto-like) and iminol forms — not classified as chain, position, or geometrical isomerism, since tautomers are constitutional isomers that interconvert rapidly. Nitromethane similarly tautomerises between its nitro and aci-nitro forms via a mechanism analogous to keto-enol tautomerism.
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- ✓Keto-enol tautomerism requires an α-hydrogen next to the carbonyl — without one, that pathway is unavailable.
- ✓Extra conjugative stabilisation of the enol form (as in a conjugated enone) increases the degree of enolisation.
- ✓Urea and nitroalkanes tautomerise too, via mechanisms analogous to keto-enol tautomerism, not through chain/position/geometrical isomerism.
Don't assume every carbonyl compound can tautomerise — always check for an actual α-hydrogen on a carbon adjacent to the carbonyl first; some aldehydes and ketones genuinely have none available.
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