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According to Wikipedia, $\ce{HeH+}$ and fluoroantimonic acid are the strongest.

According to a News article in Nature, Carborane acid is the strongest, but Wikipedia says fluoroantimonic acid is stronger.

Links:

So, which is the strongest ?

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    $\begingroup$ HeH+ is a very acidic ion, but in order to exist in a condensed form there would have to be a counterion X−. If the counterion gets protonated by the HeH+, we end up with a molecular acid HX. This then becomes only as strong as the pKa of that acid. You cannot have a bottle of HeH+ so calling it an acid is in a way kind of nonsense. I might as well talk about having a bottle of protons, which would be at least as acidic as HeH+. Note that the free proton does not exist in aqueous solution, what we call H+(aq) is really H3O+ . $\endgroup$
    – Level River St
    Commented Oct 6, 2014 at 13:09
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    $\begingroup$ What do you mean by "According to...even Google itself"? Remember that Google - a search engine - cannot itself be the source for any information. $\endgroup$
    – Gaurang Tandon
    Commented Jul 14, 2018 at 9:15

3 Answers 3

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The problem with this question is that the exact answer is "it depends..."

First off, it depends on your definition of acidity and how you measure it. Everyone seems to be using Brønstead acids (i.e. $\ce{H+}$ donors). I see two different measures in other answers:

  • Proton affinity: This is a gas-phase measurement of $\ce{A^{−} + H^{+} -> HA}$
  • Hammett acidity ($H_0$): This is a solution measurement, given by $\mathrm pK_{\ce{BH^+}} - \log\left(\frac{[\ce{BH^+}]}{[\ce{B}]}\right)$

Secondly, it depends on medium as mentioned by LDC3.

  • Leveling effect: The solvent leveling effect reflects the lowest possible $\mathrm pK_\mathrm a$ in a particular solvent, based on the basicity of the conjugate base. So you need $\ce{HF}$ or fluorosulfuric acid to reach low $\mathrm pK_\mathrm a$.

So the problem in my mind is that carboranes and fluoroantimonic acid are solution measurements, but $\ce{HeH+}$ is a gas-phase measurement. It does have the highest gas-phase proton affinity. But I'd put my money on things I can use in lab.

Incidentally, the Reed group prepared the fluorinated carborane acid this year: Angew. Chem. Int. Ed. 2014, 53 (4), 1131–1134. So that compound, $\ce{H(CHB11F11)}$ wins the crown for strongest solution-phase Brønstead acid (for the moment, at least).

There's also a nice review article "Myths about the Proton. The Nature of H+ in Condensed Media" by the same group in Acc. Chem. Res. 2013, 46 (11), 2567–2575.

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  • $\begingroup$ Well i reckon and also according to Wikipedia it has been discovered. See helium compounds in Wikipedia. $\endgroup$
    – NeilRoy
    Commented Oct 6, 2014 at 15:07
  • $\begingroup$ So, is it more acidic than carborane. Also Wikipedia suggested that NeH+ has also been seen. So since Neon is even more inert so will it be the strongest acid ever known? If you read the HeH+ ion article in wikipedia, it is written it is the 2nd strongest acid known. So i was wondering! $\endgroup$
    – NeilRoy
    Commented Oct 6, 2014 at 15:45
  • $\begingroup$ I think it depends on your definition of "acidity" as I mention above. Are you asking about proton affinity? The linked Wikipedia article gives the "strongest known acid" comment. The list of proton affinities gives He above Ne. $\endgroup$
    – Geoff Hutchison
    Commented Oct 6, 2014 at 15:50
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    $\begingroup$ Then I guess you'd have to go to the proton affinity definition, which points to $\ce{HeH+}$. But as I indicate in my answer, it's not like you can use it for actual chemistry. $\endgroup$
    – Geoff Hutchison
    Commented Oct 6, 2014 at 15:57
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    $\begingroup$ You can, however, (sort of) define a proton affinity for the null molecule $\varnothing$ consisting of no nuclei or electrons; its proton affinity is trivially just 0 kJ/mol, since the "reaction" $\ce{\varnothing+H+->H+}$ does not involve any actual reaction at all, and thus no change in energy. $\endgroup$
    – Ilmari Karonen
    Commented Oct 6, 2014 at 20:36
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From Wikipedia
It depends on the medium.

For plasma: helium hydride ion
For liquids: fluoroantimonic acid

From Wikipedia
Fluoroantimonic acid - $H_0$ Value = −31.3
Carborane superacid - $H_0$ Value = −18.0
100% Sulfuric Acid - $H_0$ Value = -12.0
I didn't find any information about $\ce{HeH^+}$, I guess it's because it only exist in a plasma and difficult to study.

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Carborane Acid is the strongest acid.

The reason this is technically the world’s strongest acid is because it’s the strongest acid as a single molecule. Much stronger acids- the acids that melt through tables- can be created by mixing certain antimony compounds with hydrofluoric acid ($\ce{HF}$). For example when you mix $\ce{SbF5}$ with $\ce{HF}$, you get an acid with a pH of -31.

Reference: Chemistrytwig

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  • $\begingroup$ What about that HeH+ ? I think it is stronger... $\endgroup$
    – NeilRoy
    Commented Oct 6, 2014 at 6:06
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    $\begingroup$ @neil_roy So we can say that HeH+ is strongest acid in ion form and Carborane Acid in molecule form. But it I can't figure which of them is the strongest. It seems HeH+ because it is in ion form! $\endgroup$
    – Freddy
    Commented Oct 6, 2014 at 6:27
  • $\begingroup$ yeah i agree that HeH+ is stronger also because it involves a noble gas... I was researching a li'l bit and found that NeH+ has also been found (Wikipedia : Neon Compounds) which makes it the strongest but it is so rare that...! $\endgroup$
    – NeilRoy
    Commented Oct 6, 2014 at 6:33
  • $\begingroup$ @NeilRoy The Ne proton affinity is lower than He. So no, NeH+ is not stronger than HeH+. $\endgroup$
    – Geoff Hutchison
    Commented Oct 6, 2014 at 15:58
  • $\begingroup$ I think single molecule is an important distinction since the strongest Lewis acid is probably a very positively charged cathode ;) $\endgroup$
    – ab initio in silico
    Commented Jun 1, 2017 at 3:27

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