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What's the strongest known organic acid? Is it maybe trifluoromethanesulfonic acid or tautomer of pentacyanocyclopentadiene?

EDIT: Since the question was reactivated I thought about formalising it. I can think of a few categories of compounds: neutral (actually created or only studied in silico) and cationic (also two options). Second Dave's answer and ANM's one do the trick for two groups, but two other options remain (still prefer answers without boro/silico/etc. stuff).

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    $\begingroup$ This has been discussed recently. Enter "strongest acid" in the search box up above and you'll find your answer. $\endgroup$ – ron Dec 3 '14 at 22:46
  • $\begingroup$ I'm asking specifically for organic acid not inorganic like carboborane or hexafluoroantimonic. $\endgroup$ – Mithoron Dec 3 '14 at 23:20
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    $\begingroup$ I wouldn't consider triflic acid to be an organic molecule, it behaves more like the derivative of sulfuric acid than methane. $\endgroup$ – Martin - マーチン Dec 4 '14 at 1:57
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    $\begingroup$ Well, sulfonic acids are organic compounds. That there's only one carbon and there are 3 fluors doesn't change this. $\endgroup$ – Mithoron Dec 4 '14 at 9:54
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enter image description here

According to "The Strongest Isolable Acid" Angewandte Chemie International Edition 43 (40): 5352–5355, carborane acids are the strongest, with $\ce {H(CHB11Cl11)}$ being the strongest.

Depending upon ones preferred definition of "organic" this might not be considered organic, but the derivative $\ce{H(CHB11Me5Br6)}$ is strong enough to fully protonate benzene.

$\ce{H(CHB11Me11)}$ is stronger than $\ce{H(CHB11Me5Br6)}$ and only slightly weaker than $\ce {H(CHB11Cl11)}$, so if $\ce {H(CHB11Cl11)}$ is disregarded as no being organic, then $\ce{H(CHB11Me11)}$ is strongest. If $\ce{H(CHB11Me11)}$ still isn't organic enough, then protonated benzene or a protonated alkane.

(A later article and the article in Nicolau's comment report $\ce {H(CHB11F11)}$ and $\ce {H(CEtB11F11)}$ which may be even stronger than the above)

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    $\begingroup$ The same group has since isolated the fluorinated derivative of carborane superacid, $\ce{H(CHB11F11)}$, which is even stronger still. $\endgroup$ – Nicolau Saker Neto Dec 5 '14 at 13:57
  • $\begingroup$ @NicolauSakerNeto Cool! $\endgroup$ – DavePhD Dec 5 '14 at 14:00
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    $\begingroup$ Also, protonated methane (methanium, $\ce{CH5+}$) is a significantly stronger acid than protonated benzene (comparing proton affinities; the less positive the value, the stronger the acid), though it quickly tends to decompose into the methylium cation ($\ce{CH3+}$) and hydrogen gas. The cation $\ce{CH6^{2+}}$ is also known, and is arguably an even stronger acid. Of course, none of these can be isolated in a neutral compound, as they will protonate essentially any anionic conjugate base. $\endgroup$ – Nicolau Saker Neto Dec 5 '14 at 14:07
  • $\begingroup$ I have just one more comment. To be clear, the picture shown is only for the conjugate base, $\ce{CHB11Cl11^{-}}$. The acidic proton is not shown (i.e. it's not the hydrogen atom attached to the carbon). Presumably it stays somewhat spread over the entire ion, staying only weakly coordinated to the chlorine atoms. $\endgroup$ – Nicolau Saker Neto Dec 5 '14 at 19:24
  • $\begingroup$ Like I commented earlier, I wanted tu exclude carboborane acids... Also non-cationic species would be welcome unless you know something that would be stronger than triprotonated methane and still stable :) $\endgroup$ – Mithoron Dec 8 '14 at 21:39
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I think the recent article by Zenaidee et al.1 answers this question well.

They managed to generate highly protonated cytochrome c molecules under MS-ESI conditions, which are acidic enough to protonate noble gases such as Ar according to the equation:

$$\ce{[cyt $c$, 23H]^23+ (g) + Ar (g) -> [cyt $c$, 22H]^22+ (g) + ArH+ (g)}$$

Of course such protein molecules can not be isolated under normal conditions (not yet at least). More details can be found in the article. The abstract is as follows:

The basicity of highly protonated cytochrome c (cyt c) and myoglobin (myo) ions were investigated using tandem mass spectrometry, ion–molecule reactions (IMRs), and theoretical calculations as a function of charge state. Surprisingly, highly charged protein ions (HCPI) can readily protonate non-polar molecules and inert gases,including Ar, O2, and N2 in thermal IMRs. The most HCPIs that can be observed are over 130 kJ mol−1 less basic than the least basic neutral organic molecules known (tetrafluoromethane and methane). Based on theoretical calculations, it is predicted that protonated cyt c and myo ions should spontaneously lose a proton to vacuum for charge states in which every third residue is protonated. In this study, HCPIs are formed where every fourth residue on average is protonated. These results indicate that protein ions in higher charge states can be formed using a low-pressure ion source to reduce proton-transfer reactions between protein ions and gases from the atmosphere.

  1. Zenaidee, M. A.; Leeming, M. G.; Zhang, F.; Funston, T. T.; Donald, W. A. Highly charged protein ions: the strongest organic acids to date. Angew. Chem. Int. Ed. 2017, 56 (29), 8522–8526. DOI: 10.1002/anie.201702781.
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  • $\begingroup$ This may be a good answer to the question, but posting bare references with a screen capture of the abstract is frowned upon on the site. It's hard to read, and completely unsearchable. Could you replace it with some sort of summary of your own? You could absolutely quote the key bits. $\endgroup$ – hBy2Py Feb 25 '18 at 12:42
  • $\begingroup$ Oh, agreed, I don't have a plagiarism concern here. The image of the paper contents is not searchable. Were you to type up a description, including quotations, then the quoted material and your commentary would be searchable by the site's search feature and other engines like Google, Bing, etc. As it is, the only searchable text in your answer is the citation itself, in which the only real keywords are the authors' names...not particularly topical. $\endgroup$ – hBy2Py Feb 25 '18 at 13:13
  • $\begingroup$ Even a sentence or two like what you put in your comment would strongly improve the post -- you could copy-paste part or all of the abstract into a blockquote, too. Wouldn't take long. $\endgroup$ – hBy2Py Feb 25 '18 at 13:14
  • $\begingroup$ Btw, please don't take the above critique the wrong way: it's a great find, and a great answer to the question. Just needs some buffing. :-) $\endgroup$ – hBy2Py Feb 25 '18 at 13:26
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    $\begingroup$ That works! The idea is that the post itself will show up in searches. Before, if someone searched 'protonated cytochrome c' on Google or wherever, it would never be in any results list. Now, it can be. $\endgroup$ – hBy2Py Feb 25 '18 at 14:25
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There is a recent article Design of the strongest organic Brønsted acids in gas phase Chemical Physics Letters (2017) Volume 681, Pages 50-55.

enter image description here

In this work, a new class of superacids containing poly enol groups was designed and their acidities were examined theoretically in gas phase. The conjugate bases of the superacids were stabilized due to formation of an aromatic ring and a hydrogen bonding network. To enhance the acidity, the electron withdrawing groups, F and CN, were substituted into the structures so that some cyano-derivative acids with $ΔH_\mathrm{acid} = \pu{223–243}$ and $ΔG_\mathrm{acid} = \pu{215–236 kcal/mol}$ were obtained. These thermodynamic data show that these compounds are the strongest organic acids in gas phase that have already been reported. Although $\ce{CB11(CF3)11H1H}$ and $\ce{B12(CF3)12H2}$ are stronger than the proposed acids they are not organic compounds.

In Table 2 the article lists theoretical $\mathrm{p}K_\mathrm{a}$ values in DMSO, the lowest of which is $−28.4$ for this class of compounds.

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A relatively strong acid that does not employ borane, cyano, or halogen functions, containing only carbon, hydrogen and oxygen, is croconic acid. The structure of this acid from Wikipedia is given below:

enter image description here

Croconic acid has two stages of dissociation, given in Wikipedia as $pK_{a1}=0.80, pK_{a2}=2.24$. The second stage dissociation especially stands out, being about two $pK_a$ units or more stronger than most polycarboxylic acids (e.g., oxalic, malonic tartaric, citric) and also stronger (in both stages) than other "ring" acids such as deltic or squaric.

The relatively strong diprotic acid character of this compound comes from the di-anion having aromatic character (mentioned in the Wikipedia article) plus the ability of a five-membered conjugate ring to take up negative charge, which is also compatible with aromatic coupling. The latter leaves less excess negative charge on the oxygens than is typical in acids containing carbon, hydrogen and oxygen only.

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