Which of the following has higher enol content?

enter image description here

The enol form of (A) and (B) is:

enter image description here

Both the enols have hydrogen bonding.

If the oxygen were to accept the pi electrns from carbon forming a carbocation like this (a resonance structure):

enter image description here

the compound becomes stabilized due to resonance as well. So isn't (B) the answer?

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    $\begingroup$ You think right. It is the question itself that is wrong (too easy). It should have contained a double bond in (A) as well, and that's where things would start getting somewhat interesting. $\endgroup$ Dec 25, 2015 at 9:02
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    $\begingroup$ If there was another double bond in (A), then its resonance structure will be anti aromatic, making it unstable. Is that correct? $\endgroup$
    – Aditya Dev
    Dec 25, 2015 at 9:04
  • 3
    $\begingroup$ Exactly! While (B) is aromatic and feels quite well. $\endgroup$ Dec 25, 2015 at 9:06
  • $\begingroup$ @AdityaDev - Since you are correct, consider expanding on your explanation and posting it as an answer to your own question. If you are able to do so, you are encouraged to answer your own question. $\endgroup$
    – Ben Norris
    Dec 25, 2015 at 12:12
  • $\begingroup$ OK. I will post an answer. $\endgroup$
    – user23939
    Dec 25, 2015 at 12:13

1 Answer 1


The two main factors affecting the stability of the enol forms of (A) and (B) are

  1. Hydrogen bonding
  2. Aromaticity

In the enol form of both compounds, hydrogen bonding is present:

enter image description here

But the resonance structure of enol form of (B) is aromatic. It contains $6$ $\pi$-electrons:

enter image description here

The enol form of (A) lacks this stability. Hence, the enol content of (B) is more than that of (A)


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