Nonaqueous Solvents, the Leveling Effect, and Superacids

Core Idea

The solvent sets the limits of acid-base chemistry. In water, every acid stronger than H₃O⁺ ionizes completely, so HCl, HBr and HClO₄ all look equally strong; this is the leveling effect. Less basic solvents such as glacial acetic acid can tell these acids apart, and more basic solvents such as liquid ammonia level even weak acids. Superacids, which are more acidic than 100% sulfuric acid, extend the scale far beyond what water allows.

How It's Best Learned

Begin with water's autoionization and ask what the strongest acid and strongest base are that can survive in it. Repeat the exercise for liquid ammonia (NH₄⁺ and NH₂⁻) and for acetic acid. Place several acids on one pKa line and mark each solvent's "window" to see which acids are leveled and which are differentiated.

Common Misconceptions

• HCl and HClO₄ are equally strong acids. (They only appear so in water, which levels both to H₃O⁺; in acetic acid, perchloric acid is measurably stronger.)
• Acid strength is a property of the acid alone. (Ionization is a proton transfer to the solvent, so the solvent's basicity decides how far it goes.)
• pH and Ka values carry over unchanged from water to any solvent. (Each solvent has its own autoionization constant and its own acidity scale.)

Explainer

When an acid dissolves, it hands a proton to the solvent. The extent of that transfer depends on two things: how readily the acid gives up the proton and how strongly the solvent accepts it. Water accepts protons well enough that any acid stronger than H₃O⁺ ionizes completely. As a result, H₃O⁺ is the strongest acid that can exist in water in appreciable amounts, and OH⁻ is the strongest base. Hydrochloric, hydrobromic, hydroiodic, nitric and perchloric acids all reduce to the same species, H₃O⁺. This is the leveling effect, and water is called a leveling solvent for these acids.

To rank such acids, use a solvent that is a weaker base. Glacial acetic acid accepts protons reluctantly, so strong acids ionize only partly in it, and their differences show up. In acetic acid, perchloric acid is clearly stronger than hydrochloric acid. A solvent that reveals these differences is called a differentiating solvent.

The reverse happens in a strongly basic solvent. Liquid ammonia, which boils at about −33 °C, autoionizes as 2NH₃ ⇌ NH₄⁺ + NH₂⁻. In it, NH₄⁺ plays the role of H₃O⁺ and the amide ion NH₂⁻ plays the role of OH⁻. Because ammonia is far more basic than water, even acetic acid ionizes completely. The solvent-system definition generalizes this: an acid is a solute that raises the concentration of the solvent's own cation, and a base raises the concentration of its anion. Liquid ammonia also dissolves alkali metals to give blue solutions that contain solvated electrons, a strong reducing medium used in organic synthesis.

Going in the acidic direction leads to superacids, defined as media more acidic than 100% sulfuric acid. Mixing a strong Brønsted acid with a strong Lewis acid gives the most extreme examples. Fluorosulfuric acid combined with antimony pentafluoride is called magic acid, and HF with SbF₅ forms fluoroantimonic acid. Because pH does not apply to such media, acidity is measured with the Hammett acidity function, H₀; pure sulfuric acid has H₀ near −12, and superacids lie well below that. These media can protonate alkanes and hold carbocations long enough to record their NMR spectra, the work for which George Olah received the 1994 Nobel Prize in Chemistry.

The general lesson is that every solvent defines a window of usable acidity. Choosing the solvent is a way to choose which acid-base reactions are possible.