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The answer for the above question is three. But I am able to draw four structures:

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Where am I getting it wrong?

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    $\begingroup$ The last structure does not really exist in slightly acidified solution of the given compound. $\endgroup$ – J_B892 Sep 8 at 8:02
  • $\begingroup$ Coming to this question won't even cis and trans also contribute to Number of tautomers? $\endgroup$ – J_B892 Sep 8 at 8:03
  • $\begingroup$ yes I was confused whether last structure is correct or not. but don't know the reason to eliminate it. why it doesn't exist in acidified solution? also if we make the cis and trans structure as well then we will have more than 4 structures which are not in the option. that means the question is just asking the standard structures. $\endgroup$ – Garima Singh Sep 8 at 8:16
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    $\begingroup$ Once you do a single tautomerism there's a hydrogen bonding (6 membered ring) formed between -OH and the oxygen. This gives stability to the compound and prefers not to further tautomerise $\endgroup$ – J_B892 Sep 8 at 8:38
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    $\begingroup$ I consider your structures correct. I agree that there should be E/Z isomerism considered, too, even though equilibration will probably be fast. $\endgroup$ – Martin - マーチン Sep 8 at 20:34
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In your attempt the 4th compound is not reported to have significant percentage in solution (acidic or basic). This is because once the tautomerisation occurs for example let's say the 1st compound, there is a tautomer having a six-membered conjugated ring involving H-bonding. This thereby increases the stability of the compound and consequently increasing the activation energy for the tautomerisation you proposed in compound 4. This is why compound 4 is not formed.

Also keep in mind that stereo-chemistry is also important while finding number of possible compounds.

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    $\begingroup$ This is a typical example of a exercise asked in the wrong way. It is obvious that all the structures OP wrote are correct, therefore the question itself must ask an entirely different thing. If it were stated that high energy conformers are to be excluded, then your answer would (at least partially) be correct. Your reasoning should exclude structure 2, too, because it is a very high energy structure compared to 1 and 3. (Please don't use tag lines in your answers.) $\endgroup$ – Martin - マーチン Sep 8 at 20:30
  • $\begingroup$ "Not reported" by who? $\endgroup$ – orthocresol Sep 8 at 21:00
  • $\begingroup$ thank u, sir ... I understood ur point $\endgroup$ – Garima Singh Sep 9 at 12:01
  • $\begingroup$ can u please look at this question as well:chemistry.stackexchange.com/questions/120080/… $\endgroup$ – Garima Singh Sep 9 at 12:05
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When a tautomer is formed from a diketone, a six-membered pesudo-ring structure is formed that stabilizes the structure through H-bonding. This structure is responsible for not letting any tautomerization of the second carbonyl.

enter image description here

Note: the structure I've shown on the top is made from MS Paint so its quite messy but you'll get the point.

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