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Which of the compounds has the highest percentage of enol?

  1. $\ce{CH3-CHO}$

  2. $\ce{Ph2-CH-CHO}$

  3. $\ce{Ph-CO-Ph}$

  4. Parabenzoquinone

How do I compare these?

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How do I compare these?

You could expect higher enol content in the following cases:

1.If there is an active methylene group ( or any other acidic $\ce{C-H}$ bond )

2.Extended conjugation in the enol form

3.Attainment of aromaticity after enolisation

4.Any intramolecular hydrogen bonding possibility in the enol form

Coming to your question:

3,4 - there's no α-hydrogen (no enolisable hydrogen).So, no tautomerisation takes place

1- has 99% keto content

2- is expected to have more enol content due to extended conjugation of the π-electrons of the double bond formed after enolisation with the phenyl rings.

ANS: 2 has the highest enol content.

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