The Worldfs Strongest Acid

In chemistry, the invention of a new powerful reagent can often open up a whole new field by enabling formerly difficult processes. Since gacidich chemicals are known to mediate a number of reactions, therefs no wonder that the news gthe worldfs strongest acid createdh received a great deal of attention.

Strength/Weakness of an Acid
You might remember from high school chemistry class that an acid is ga substance capable of releasing a proton (H+).h Also, you probably learned acetic acid and carbonic acid as the examples of weak acid, and hydrochloric acid, nitric acid, and sulfuric acid as the examples of strong acid. So, do you know what determines the strength (or the weakness) of an acid?
Letfs compare acetic acid and ethanol as an example. Ethanol is a gneutralh compound according to high school textbooks, but precisely speaking it can be considered a very weak acid.
The difference between the two is the presence of a C=O double bond (carbonyl group) next to the hydroxyl group in acetic acid. The oxygen atom of the carbonyl group pulls electron density towards itself and weakens the O-H bond of neighboring hydroxyl group, which results in easier release of proton. The negative charge left in the molecule after the loss of proton is equally balanced between the two oxygen atoms, meaning itfs stabilized (Figure 1).


Fig 1 acetic aid


On the other hand, in ethanol the negative charge has to be shouldered by only one oxygen atom, so no stabilization in this case (Figure 2). This is why acetic acid is a much stronger acid than ethanol. The stabilization by charge distribution means that the compound is more willing to give up a proton and is more able to accommodate the charge, and that translates to stronger acidity. The acid strength of a given compound basically depends on whether or not it contains a group that can stabilize a negative charge.


Fig 2 ethanol

One of the measures of acid strength is pKa values. The value is one unit smaller when the acidity of the compound is ten times stronger. Ethanol has a pKa value of roughly 16, and that of acetic acid is about 4.8.
Nitric acid, an even stronger acid, contains two electron-withdrawing double-bonded oxygen atoms. A negative charge is stabilized more effectively, as it can be balanced among three oxygen atoms (Figure 3). As a result, nitric acid (pKa = -1.3) is some 106 times more acidic than acetic acid.


Fig 3 nitric acid

Sulfuric acid, another strong acid, has two hydroxyl groups and two double-bonded oxygen atoms on the central sulfur atom. The loss of the first proton results in a negative charge shared by the three oxygens, as was the case for nitric acid. But after the loss of the second proton, the resulting -2 charges must be shared by the four oxygen atoms, meaning that the second stabilization is less effective overall. This can explain the fact that the acidity of sulfuric acid is stronger for the first step than for the second step (1st pKa = -3.0, 2nd pKa = 2.0). With Figure 4, I hope this makes sense.


Fig 4 sulfic acid

The Effect of Fluorine
Oxygen isnft alone when it comes to the ability to stabilize negative charge(s) by electron-withdrawing property. For example, fluorine is actually even stronger in this regard and the addition of fluorine to a compound can amplify its acidity. Trifluoroacetic acid (TFA, Figure 3.32, pKa = -0.25), which is a triple-fluorinated version of acetic acid, is a hundred thousand times stronger acid than normal acetic acid.


Fig 5 trifluoroacetic acid (TFA)

The incorporation of fluorine atoms into already-strong sulfuric acid should make a super-strong acid. Indeed, the two acids shown in Figures 6 had been crowned as the gstrongest acids as single isolable compoundsh for a long time (pKa = -14, -16, respectively).


Fig 6 Fluoroslfonic acid, trifluoromethanesulfonic acid

The Arrival of Carborane Acid
The several decade-old record of acid strength was shattered in 2004 by the introduction of carborane acid, shown in Figure 7. This beautiful molecule with the icosahedrane core consisting of one carbon atom and eleven boron atoms was synthesized in the lab of Professor Christopher Reed at University of California, Riverside. Each boron atom is bonded to a chlorine atom and the whole cluster of boron, chlorine, and carbon atoms is paired with a proton, which is attached on the carbon (As you may have noticed, the boron and carbon atoms in this molecule have six bonding arms, but this is because of boronfs unique ability to form special type of bond called gthree-center two-electron bond.h The carbon in this case is sort of ggetting alongh with the borons. Boron is also known to form many different types of polyhedral clusters).


Fig 7 carborane acid

The structure of carborane acid appears very different from those of conventional acids, but the idea of charge distribution and stabilization is the same. The chlorine atoms on the surface are there not only for enhancing acidity but also for shielding the molecule to protect it against attacks from outside. The published paper doesnft mention pKa, but the acidity of carborane acid has been shown at least a million times stronger than concentrated sulfuric acid, and hundreds of times stronger than the previous record holder fluorosulfonic acid.
By the way, when I put the news on my homepage I received a question, gif itfs a million times more acidic than sulfuric acid, would a cup of carborane acid be enough to turn a lake into strongly acidic?h The answer is actually no. The aqueous solutions of sulfuric acid and carborane acid at the same concentration would have roughly the same pH.
The reason is because ga million times stronger acidh doesnft mean that it always produces a million times more number of protons, but it means that its ability to donate a proton to its reaction partner is a million times stronger. Since water is a pretty good acceptor of proton, the difference between two strong acids is simply not tangible with water as a medium. The difference would become clearer when the reaction partner was a much poorer proton acceptor such as hydrocarbons.
As an analogy, if you think a heavy-weight champion (carborane acid) and a light-weight champion (sulfuric acid), the former should be a lot stronger. But if the two champions boxed against an ordinary guy (water), both would probably knock him out within five seconds and you couldnft really tell which is stronger. If you wanted to find out, logically they would have to fight against a stronger opponent like a middle-weight champion (hydrocarbon).

Magic Acid
I wrote gstrongest acid as single isolable compoundh earlier, because there are in fact stronger ones outside that condition. Antimony pentafluoride (the left in Figure 8, SbF5), when mixed with other acidic compound, coordinates the acidfs oxygen atom to increase the acidity. In particular, the one-to-one mixture with fluorosulfonic acid, called magic acid, is known to be powerful enough to easily protonate poor proton acceptors including hydrocarbons.


Fig 8 magic acid

The effect was discovered after a Christmas party held by the group of Professor George Olah, when a student found that a left-over candle dissolved completely in a magic acid solution. The main component of candle is hydrocarbons, which are poor proton acceptors as mentioned before, but it wasnft quite poor enough for the extreme acidity of magic acid. Professor Olah used this superacid in his innovative research of carbocation chemistry, and became the sole recipient of the Nobel Prize in Chemistry in 1994.
So, carborane acid turns out not exactly the strongest after all, but it does have some important advantages over magic acid. Magic acid and fluorosulfonic acid partially decompose to produce fluoride ion (F-), which causes unwanted side reactions. Fluoride ion eats glassware and these acids are corrosive enough to destroy molecules like fullerene by tearing up carbon-carbon bonds. On the other hand, carborane acid can be stored in ordinary glass bottles and also forms stable one-to-one salt with fullerene. Carborane acid is powerful but non-corrosive at the same time, which is why itfs been called a gstrong yet gentleh reagent.

The chemistry of these superacids can be applied in industrial businesses such as petroleum cracking and the synthesis of pharmaceutical compounds, and is also potentially useful in areas like the development of special materials and the treatment of organic wastes.
However, Professor Reed seems focused on pursuing its pure academic aspect to explore new carbocation chemistry. He has said that hefs interested in ionizing the inert gas xenon, simply because gitfs never been done before.h It sounds like a research that would have an impact on various fields, but itfs interesting that what drives his research seems to be a childlike curiosity of wanting to make something that have never been made before.

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