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Superacids are often characterized as acids that are stronger than 100% sulfuric acid. However, this definition is somewhat arbitrary, and accurately determining the acid strength in concentrated solutions, non-aqueous solvents, or other phases such as gas and liquid can be quite challenging.

Furthermore, it is not clear, at least to me, how one would compare the acid strength of a solid or gas with that of a solution containing 100% sulfuric acid.

So I was wondering if there is one property that is common to all superacids. I know that superacids can stabilize carbocations, although it is unclear whether this is a property common to all superacids.

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I don't believe there is a 100% correct answer for what you are looking for. You could try using the Hammett acidity function as suggested by @ivan-neretin, where an acid with $\ce{H0≤−12}$, which is the value of 100% sulfuric acid, would indicate superacidity. However, obtaining H0 for non-liquid acids is challenging at best.

Another approach you could try is to compute the deprotonation energy (DPE) of your acid, which is the energy required to remove the H+ ion. DPE can be calculated as:

$$\ce{DPE = E_{H+} + E_{A-} + E_{HA}}$$ asuming the species $\ce{H+}$, $\ce{A-}$ and $\ce{HA}$ from the following chemical equilibrium: $$\ce{HA <=> H+ + A-}$$

However, this method might also be time-consuming and making meaningful comparisons, especially when comparing acids in different phases such as solid, liquid, and gas, could be difficult.

You have made a valid observation that the definition of superacid is somewhat arbitrary, and this is supported by George Olah in his book "Superacid Chemistry"

Gillespie proposed an arbitrary but widely accepted definition of superacids, defining them as any acid system that is stronger than 100% sulfuric acid, that is, H0 ≤ - 12.

While it is true that one common property of superacids is the ability to prepare and stabilize carbocations, which earned George Olah a Nobel Prize, I'm not entirely sure if this is a distinctive feature of superacids, nor I'm sure if all superacids meet the criteria of being stronger than 100% sulfuric acid can make long-lived carbocations.

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    $\begingroup$ Not all carbocations require superacids to stabilize. Cyclopropenyl cation was precipitated from a nonpolar solvent. Carbocations generally require superacids to stabilize them if the positive charge is not incorporated into or conjugated with aromatic rings. $\endgroup$ Apr 4, 2023 at 21:55
  • $\begingroup$ Is the block quote exact? The missing negative sign in $H_0\le-12$ is a major error. $\endgroup$ Apr 5, 2023 at 1:38
  • $\begingroup$ @OscarLanzi you are absolutely correct. I will change it. Thanks $\endgroup$
    – cbornes
    Apr 5, 2023 at 7:53
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    $\begingroup$ @OscarLanzi I'm not stating that carbocations are only stabilized by superacids, just that often superacids can stabilize carbocations. But according to the example you gave where $\ce{C3H3+}$ is stabilized after reacting with $\ce{SbCl5}$ which is not a superacid, means that this is clearly not a property that should be used to identify a superacid $\endgroup$
    – cbornes
    Apr 5, 2023 at 8:00
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    $\begingroup$ I don't believe there is a 100% correct answer for what you are looking for. // This may actually be the 100% correct answer. :-) $\endgroup$
    – Poutnik
    Apr 5, 2023 at 8:06
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Since superacids are defined as more acidic than 100% $\ce{H2SO4}$, one possible characteristic is the ability to form salts of the conjugate acid $\ce{H3SO4^+}$.

Such a synthesis has been carried out on $\ce{HF +SbF5}$ by Minkwitz and Seelbinder[1]. Instead of $\ce{H2SO4}$ itself, the substrate used was bis(trimethylsilyl) sulfate, $\ce{((CH3)3Si)2SO4}$, yielding the salt $\ce{[H3SO4^+][SbF6^-]}$.

Reference

  1. Minkwitz, Rolf & Seelbinder, Raphael & Schöbel, René. (2002). "Protonated Sulfuric Acid: Preparation of Trihydroxyoxosulfonium Hexafluoroantimonate H3SO4+SbF6−". Angewandte Chemie International Edition. 41. 111 - 114. https:// doi.orf/10.1002/1521-3773(20020104)41:1<111::AID-ANIE111>3.0.CO;2-6.
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