What is the most powerful base? I know about the most powerful acid, but after a good amount of searching, the best I could come up with was tert-butyllithium, and I was wondering if there was a base even more powerful than that one.
The answer to this question will depend on your exact definition of “powerful” and whether or not you need something that is commercially available. There is actually a numerically defined Strongest Base™, but you’re going to need to be writing about some fairly advanced chemical research before any of your characters might encounter it, though I suppose it could be useful as a tidbit of trivia.
Before we get to the actual Strongest Base™, it would be prudent to briefly review the defining characteristics of a chemical base and to discuss how basicity is measured. Here I will be using Brønsted-Lowry acid-base theory to define a base as a substance that can accept a proton from an acid, which in turn is anything that can donate a proton to a base. Upon accepting a proton a base becomes a conjugate acid (because now it has a proton that it could in theory give to something else), and acids that give up protons become conjugate bases. It may seem recursive, but it works – the generalized reaction looks like HA + B ⇌ A- + HB+. As an example, dissolving large quantities of ammonia (NH3) in H2O can be described as NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq). In this case NH3 is a base that becomes its conjugate acid NH4+ by accepting a proton, while H2O is an acid that becomes its conjugate base OH- by giving up a proton. The strength of a base can be measured a few different ways, but we’ll focus on the pKa of the conjugate acid, and when that fails us we’ll look at the absolute proton affinity.
In the generalized equation above for a Brønsted acid, the equilibrium constant (a measure of how far the reaction proceeds to one side or the other) is referred to as the acid dissociation constant, Ka. This is a decent quantitative measure of the strength of an acid or base in a solution – an acid that dissociates completely has a very large Ka, while the conjugate acid of a base that dissociates a negligible amount and has a very small Ka. These constants theoretically vary over more than 130 orders of magnitude, and to avoid writing a ridiculous amount of zeros chemists have defined the negative logarithm of the constant (i.e. -log10Ka) as pKa, reducing the span of numbers we have to deal with down to a range from around -60 to +70. Lower pKa values denote stronger acids, while higher values denote weaker acids (meaning the conjugates are stronger bases). The NH4+ cation has a pKa of 9.25 which makes NH3 a weak base in water, while the pKa for HCl (a strong acid) is somewhere around -6, though it is very hard to measure accurately. In aqueous solutions there is something known as a leveling effect – any acid stronger than water will simply donate a proton to create the hydronium ion (H3O+), and any base stronger than water will abstract a proton and generate the hydroxide ion (OH-). As such, H3O+ (pKa = -1.7) and OH- (pKa of H2O = 14) are respectively the strongest acid and base that can exist in water. A pKa lower than -1.7 is a strong acid (like HCl), while a pKa higher than 14 indicates a strong base. These substances can’t be measured experimentally in water, but through the use of other solvents or theoretical calculations there are ways to measure or estimate the pKa of very strong acids or bases.
The strongest acid known to modern science (which may or may not be what you know as the most powerful acid) is the helium hydride ion, HeH+. It forms during the radioactive decay of tritium, and it has an estimated pKa of -63. Recall that the pKa scale is logarithmic, so that makes it roughly 1057 times more acidic than HCl. On the other end of the spectrum you have things like the isobutane anion (C4H9-), where the estimated pKa of its conjugate acid isobutane (C4H10) is 71, making it approximately 1057 times more basic than OH-. Unfortunately for chemists who would like to rip hydrogen atoms off of pesky organic molecules like benzene, you can’t actually use the isobutane anion by itself under bulk synthetic conditions – the closest you can really get is taking isobutane and replacing one of the hydrogens with lithium, creating your suggested strong base t-butyllithium, or t-BuLi for short.
Despite t-BuLi having a partially ionic bond between the lithium and the alkyl group, it doesn’t behave in quite the same fashion as just the isobutane cation and the effective pKa is estimated to be ~53. This is plenty strong enough to grab a proton off of most organic molecules, but it still won’t work for benzene. There is a more active mixture called the Lochmann-Schlosser base, which is made from n-BuLi (the straight-chain version of t-BuLi) and potassium t-butoxide (t-BuOK). This base will deprotonate benzene, but there aren’t any good estimates of its pKa because the actual reactive species in the mixture is not fully understood.
As a brief departure from the wonders of acid-base chemistry, I must point out that the compounds included in this casual discussion are extremely dangerous to work with. For most chemists in need of a strong base, a trip to the reagent cabinet to grab the bottle of sodium hydroxide (NaOH) is all that is required. It comes as solid pellets and other than being caustic there aren’t too many safety considerations -- it’s even available to the general public as lye, usually to be used as a drain cleaner. If an inexperienced chemist were to pry the sure-seal septum cap off a bottle of t-BuLi and pour some out, the contents of said bottle would immediately erupt into a catastrophic fireball and the unfortunate individual would be lucky to survive the event. All work done with these reagents is performed under some kind of inert atmosphere, usually nitrogen or argon, in order to prevent violent reactions with oxygen and moisture in normal air. While a full description of proper air-free technique is way beyond the scope of this ask, be aware that any handling mistakes (dropping a bottle, pressurizing the wrong cannula, pulling the plunger out of a syringe, etc.) could put your characters and anyone nearby in a hospital (or worse) and potentially burn down the fume hood, lab, or building. These materials are not to be taken lightly, and tragic accidents have caused numerous injuries and claimed lives in the real world.
Now back to our search for the Strongest Base™ -- in terms of practical use in a laboratory, t-BuLi and the Lochmann-Schlosser mixture are the strongest bases you can get, but they are not the strongest bases known. To go even stronger one has to abandon solution chemistry and go straight to the gas phase, where some very unstable compounds can be trapped and analyzed. In these cases the pKa doesn’t really apply, so instead chemists will calculate the absolute proton affinity (PA), or the amount of energy released when a base accepts a proton in the gas phase, with higher values denoting stronger bases. As a point of reference the weakest base known is helium (He + H+ ⇌ HeH+) with a PA of 178 kJ/mol, and the value for water is 690 kJ/mol. The isobutane cation has a PA of ~1730 kJ/mol, and there are very few things that can top it. For a long time the most basic substance known was the methanide anion (CH3-) with a PA of 1742 kJ/mol, until it was displaced in 2008 by the lithium monoxide anion (LiO-) with a PA of 1782 kJ/mol. This too was beaten just last year by the ortho-diethynylbenzene dianion (o-DEB2-), which has a PA of 1843 kJ/mol.
So there you have it – if you want the Strongest Base™ and you don’t care if it can only exist in the ion trap of a mass spectrometer, the answer is the ortho-diethynylbenzene dianion. Your research did in fact lead you to the strongest base you can buy (t-BuLi), though mixing n-BuLi and t-BuOK to get the Lochmann-Schlosser base is slightly more potent.
~J
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