What is the most acidic neutral hydrocarbon?

This is a question I've been thinking about quite frequently. I had seen that this hydrocarbon has a low $$\mathrm pK_\mathrm a$$ with reference to the highlighted H-atom

Loss of the central gives aromatic character to the three 5-membered rings in the compound, contributing to its acidity.

Are there any other neutral hydrocarbons which are stronger acids? How do their $$\mathrm pK_\mathrm a$$'s stack up against, say another organic acid such as acetic acid?

EDIT: As pointed out in the comments, $$\ce{CH_5^+}$$ (methanium) is a superacid, but is not a neutral hydrocarbon.

• $\ce{CH5+}$ is cheating? – Karsten Theis Nov 28 '19 at 3:39
• See also en.wikipedia.org/wiki/Methanium. But the OP may mean a neutral molecule. – Poutnik Nov 28 '19 at 5:55
• @KarstenTheis yep ;) Nice try though. I will read about methanium. Also edited the question accordingly. – Aniruddha Deb Nov 28 '19 at 10:09
• @AniruddhaDeb Well, what I had in mind was for the initial structure to be completely non-aromatic. For reference, cyclopentadiene is approximately as acidic as 9-phenyl-9H-fluorene, according to Hans Reich's pKa tables in DMSO, even though the latter has far more extended conjugation. It's better for aromaticity to suddently appear, instead of existing aromatic systems merely being conjugated. – Nicolau Saker Neto Nov 28 '19 at 21:13
• – Mithoron Nov 28 '19 at 22:15

In this answer a hydrocarbon anion called Kuhn's anion has the formula $$\ce{C_{67}H_{39}^-}$$ has reported $$pK_b=8.1$$ which would correspond to the neutral hydrocarbon having $$pK_a=5.9$$. The anion, together with several hydrocarbon cations with which it forms stable salts, is shown below (taken from the answer referenced above; primary reference J. Org. Chem. 1990, 55 (3), 996–1002):
• Right, but the illustration showing only ions looks confusing to me. Why not to draw alongside the actual compound you are posting $\mathrm{p}K_\mathrm{a}$ value for? – andselisk Nov 29 '19 at 14:41
• How is this hydrocarbon so acidic? I understand that the $pK_a$ was derived via a practical experiment but how would one go about explaining it theoretically? Is it merely because of $\pi$-resonance? I don't see any aromatic rings forming/breaking to contribute to such a pKa. – Aniruddha Deb Nov 29 '19 at 16:54