Why is the compound 3,4-dihydroxy-3-cyclobutene-1,2-dione more acidic than acetic acid?

Title compound

I don't get how its resonance stabilization is far greater than that of acetic acid despite the fact that the conjugate base has high angular strain. How is it more stable than acetate ion? In the consideration of acidity, why is only the resonance dispersion of charge taken into account and not the angular strain?

For stability comparisons of the anions so formed I am stuck at the resonance structure too:

resonating structures

Finally, I want to know whether it would be more acidic that formic acid?


1 Answer 1


This molecule is commonly known as Squaric acid. The Wikipedia entry gives the possible resonance structures of the conjugate base (added here for convenience): Squaric acid dianion resonance structures

You can see that there are four possible resonance structures for the conjugate base, resulting from the proximity of two ketone groups and two hydroxyl groups around the ring. This proximity allows movement of electrons around the square ring and between the ring and each oxygen atom.

On the other hand, acetic acid (or rather, the conjugate acetate anion) has only two resonance structures, as seen here: Acetate anion resonance structures

Since the principle is that a molecule with more resonance structures is more stable than a molecule with less resonance structures, squaric acid anion is more stable than acetate anion. Since a more stable conjugate base makes an acid stronger, squaric acid is considered to be stronger than acetic acid.

This conclusion is also evident by the Pka values. This table provides Pka values for many acids, including squaric, acetic and formic acid. You can see that squaric acid has two values for two acidic protons - 1.5 for the first proton and 3.5 for the second proton. acetic acid has a Pka value of 4.75 and formic acid has a Pka value of 3.75. This means that the strongest acid of the three is squaric acid, followed by formic acid, and the least acidic of them is acetic acid.

Squaric acid > Formic acid > Acetic acid

  • $\begingroup$ Can you help in the resonating structures of first anion formed when only one proton is extrascted $\endgroup$ Feb 20, 2017 at 16:03
  • $\begingroup$ It is as you drew it in your notebook. Keep in mind protons are constantly associating and dissociating from charged oxygen atoms (any of the four oxygen atoms) when equilibrium has been reached. If we imagine those brief moments when the molecule is resonating in a monoprotic form, it is as you drew it. $\endgroup$
    – Don_S
    Feb 20, 2017 at 16:14
  • $\begingroup$ then it has got only two resonating forms for the first ionisation then why are those more stable than those of acetate ion $\endgroup$ Feb 21, 2017 at 13:43

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