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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

c

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.

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    $\begingroup$ $\ce{CH5+}$ is cheating? $\endgroup$
    – Karsten
    Commented Nov 28, 2019 at 3:39
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    $\begingroup$ See also en.wikipedia.org/wiki/Methanium. But the OP may mean a neutral molecule. $\endgroup$
    – Poutnik
    Commented Nov 28, 2019 at 5:55
  • $\begingroup$ @KarstenTheis yep ;) Nice try though. I will read about methanium. Also edited the question accordingly. $\endgroup$ Commented Nov 28, 2019 at 10:09
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    $\begingroup$ @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. $\endgroup$ Commented Nov 28, 2019 at 21:13
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    $\begingroup$ related chemistry.stackexchange.com/questions/9212/… chemistry.stackexchange.com/questions/20283/… $\endgroup$
    – Mithoron
    Commented Nov 28, 2019 at 22:15

1 Answer 1

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In this answer a hydrocarbon anion called Kuhn's anion (1) has the formula $\ce{C67H_{39}^-}$ and 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):

enter image description here

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  • $\begingroup$ Ehhm, but the question is about neutral hydrocarbons… $\endgroup$
    – andselisk
    Commented Nov 29, 2019 at 13:56
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    $\begingroup$ But the neutral hydrocarbon is the acid. It's mentioned here as the conjugate of the anion. $\endgroup$ Commented Nov 29, 2019 at 14:25
  • $\begingroup$ 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? $\endgroup$
    – andselisk
    Commented Nov 29, 2019 at 14:41
  • $\begingroup$ Not sure how to modify the anion, which would involve using CH in the center instead of the carbanion. Would need a desktop I think and I cannot get to one this weekend. $\endgroup$ Commented Nov 29, 2019 at 14:51
  • $\begingroup$ 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. $\endgroup$ Commented Nov 29, 2019 at 16:54

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