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Why is 2,4 pentanedione more highly enolized than 1,3 cyclohexanedione?

Answer 1

Enolization of acetylacetone allows a 6-membered ring, stabilized by hydrogen bonding.

Conversely, when the cyclic dione enolizes, hydrogen bonding would happen between molecules rather than within because the ring restricts the oxygen centers.

I am trying to think how you would be able to test this hypothesis. Maybe the #pK_a# of each dione would be revealing.

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Answer 2

Easton Jones

The increased enolization of 2,4-pentanedione compared to 1,3-cyclohexanedione is attributed to the greater stability of the resulting enolate ion. In 2,4-pentanedione, the presence of a conjugated system between the two carbonyl groups facilitates resonance stabilization of the enolate ion, lowering its energy and favoring enolization. In contrast, 1,3-cyclohexanedione lacks this conjugation, resulting in a less stable enolate ion and reduced enolization tendency.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.