Superacids and Liquid Ammonia: Non-Aqueous Solvent Chemistry

Chemistry in Superacids and Liquid Ammonia

Most of the chemistry we encounter daily takes place in water — the "universal solvent." But some of the most fascinating and powerful chemical transformations occur in media far removed from aqueous solutions. Two of the most important non-aqueous solvent systems in chemistry are superacids and liquid ammonia. These systems allow chemists to generate species that would be instantly destroyed in water, unlock reaction pathways otherwise impossible, and probe the very limits of acid-base theory.

2.1 What Is a Superacid?

The concept of a superacid was first formally defined by Ronald Gillespie in 1969 as:

An acid stronger than 100% sulfuric acid.

Since 100% H₂SO₄ has a Hammett acidity function (H₀) of approximately −12, any medium with H₀ < −12 qualifies as a superacid. To put this in perspective, superacids can be billions to trillions of times stronger than concentrated sulfuric acid.

The term "superacid" itself was coined earlier by James Bryant Conant in 1927, but it was George Olah who systematically explored their chemistry and earned the 1994 Nobel Prize in Chemistry for his work on carbocations in superacid media.

2.2 The Hammett Acidity Function

In superacid media, the traditional pH scale is meaningless — it was designed for dilute aqueous solutions. Instead, the Hammett acidity function (H₀) is used:

where B is a neutral base indicator and BH⁺ is its conjugate acid. The more negative H₀, the stronger the acid. For example:

A Hammett function of −31 means the medium is roughly 10¹⁹ times stronger than 100% sulfuric acid.

2.3 Major Superacid Systems

2.3.1 Fluorosulfuric Acid (HSO₃F)

Fluorosulfuric acid is one of the strongest known pure Brønsted acids. It is a colorless, fuming liquid with remarkable properties:

Dissociation in HSO₃F:

2.3.2 Magic Acid (FSO₃H·SbF₅)

The name "Magic Acid" arose from a famous incident in George Olah's laboratory at Case Western Reserve University around 1966–1967. A postdoctoral researcher placed a Christmas candle into the solution, and the paraffin wax (a saturated hydrocarbon — normally considered inert) dissolved completely, forming stable carbocation solutions. This was, at the time, nothing short of "magic."

Magic Acid is a mixture of:

The Lewis acid accepts a fluorosulfate anion, generating a weakly coordinating, highly stabilizing anion:

This removal of the conjugate base drives the autoprotolysis equilibrium further to the right, vastly increasing acidity.

H₀ ≈ −23 (at optimal 1:1 molar ratio)

2.3.3 Fluoroantimonic Acid (HF·SbF₅)

This is the strongest known superacid system, with a Hammett acidity function reaching approximately −28 to −31 depending on composition. It is a mixture of hydrogen fluoride (HF) and antimony pentafluoride (SbF₅).

The key equilibria:

The SbF₅ acts as a Lewis acid, sequestering F⁻ ions and generating the extraordinarily stable hexafluoroantimonate anion (SbF₆⁻). This drives the autoprotolysis of HF forward, liberating more H₂F⁺ — the strongest known Brønsted acid cation.

H₂F⁺ is so reactive that it can protonate virtually any substance, including materials considered completely inert under normal conditions.

Dangers: Fluoroantimonic acid is extraordinarily corrosive and can dissolve glass, most organic materials, and even many metals. It must be handled in PTFE (Teflon) containers.

2.3.4 Carborane Superacids (H(CHB₁₁Cl₁₁))

A more recent development, carborane acids are sometimes called the strongest known pure Brønsted acids (as opposed to mixed Brønsted-Lewis systems like fluoroantimonic acid). The carborane anion [CHB₁₁Cl₁₁]⁻ is one of the weakest, most chemically inert coordinating anions known:

These acids were extensively studied by Christopher Reed at UC Riverside.

2.3.5 Perchloric Acid (HClO₄) — A Milder Superacid

Anhydrous perchloric acid is sometimes classified as a superacid:

The perchlorate ion is an exceptionally weak base (its conjugate acid H₂ClO₄⁺ has never been isolated), making HClO₄ a very strong acid. However, it is less potent than the fluorosulfuric or fluoroantimonic systems.

Warning: Anhydrous HClO₄ is extremely dangerous and potentially explosive, especially when in contact with organic materials.

2.4 Types of Acidity in Superacid Media

Superacids are not limited to Brønsted acidity. In these media, all three classical types of acidity are amplified:

Additionally, Olah introduced the concept of:

2.5 Chemistry in Superacid Media

2.5.1 Generation and Study of Stable Carbocations

This is the crown jewel of superacid chemistry. In superacid media, even simple hydrocarbons can be protonated to form carbocations — species that exist only fleetingly in normal solvents.

Protonation of alkanes:

The methonium ion (CH₅⁺) — a pentacoordinate carbon species — was first observed in mass spectrometry and subsequently studied in superacid solutions. This was revolutionary because it demonstrated that even saturated C–H bonds could be protonated.

Protonation of molecular hydrogen:

The trihydrogen cation (H₃⁺) is one of the most abundant molecular ions in interstellar space and was studied extensively in superacid media.

Classical and non-classical carbocations:

Superacid chemistry at low temperatures allowed the direct observation (by NMR, IR, and X-ray crystallography) of:

Alkyl cations studied in SbF₅/HSO₃F at low temperature:

2.5.2 Protolytic Cleavage of C–C and C–H Bonds

In superacid media, even the strongest bonds in organic chemistry can be cleaved:

C–H bond cleavage:

This is formally an oxidative process (the substrate is oxidized, H⁺ is reduced to H₂), but it occurs readily in superacid media due to the enormous thermodynamic driving force.

C–C bond cleavage:

These reactions are of enormous importance in petroleum chemistry, as they model the cracking processes used industrially.

2.5.3 Superacid-Catalyzed Reactions

Superacids serve as extraordinarily powerful catalysts:

2.5.4 Onium Ions and Protonated Species

Superacid media stabilize onium ions — species formed by protonation of lone-pair-containing molecules:

In superacid media, even very weak bases can be protonated:

2.5.5 Heterogeneous Superacids

Solid superacids have enormous industrial importance:

These solid superacids combine the extraordinary acid strength of superacid chemistry with the practical advantages of heterogeneous catalysis (easy separation, recyclability).

2.6 Historical Milestones

3.1 Properties of Liquid Ammonia

Liquid ammonia (NH₃) is one of the most important and widely studied non-aqueous solvents. Its properties differ significantly from water:

Key observations:

3.2 Autoionization of Liquid Ammonia

Like water, liquid ammonia undergoes self-ionization:

The ionic product is:

This extremely small value means:

3.3 Acid-Base Chemistry in Liquid Ammonia

3.3.1 Brønsted Acid-Base Reactions

An acid in liquid ammonia is any substance that increases the concentration of NH₄⁺:

A base in liquid ammonia is any substance that increases the concentration of NH₂⁻:

The neutralization reaction is:

This is perfectly analogous to:

Substances that are acids in water may not be acids in liquid ammonia, and vice versa. The leveling effect works differently:

Acetic acid, for example, is a weak acid in water but behaves as a strong acid in liquid ammonia because the equilibrium:

lies essentially completely to the right.

3.3.2 Lewis Acid-Base Chemistry

Ammonia is an excellent Lewis base (nucleophile) due to the lone pair on nitrogen. This makes liquid ammonia an outstanding medium for:

Metal ammine complexes are among the most extensively studied coordination compounds, and many were first prepared in liquid ammonia:

3.4 Solvent Properties and Dissolved Metals

3.4.1 Dissolving Power

Liquid ammonia dissolves a wide range of substances:

3.4.2 Alkali Metal Solutions — The Crown Jewel of Liquid Ammonia Chemistry

When alkali metals (Li, Na, K, Rb, Cs) dissolve in liquid ammonia, they produce some of the most remarkable solutions in all of chemistry:

Dilute solutions (< 0.5 M) are deep blue and contain:

The solvated electron in liquid ammonia is one of the simplest chemical species and has been studied extensively since the discovery by Sir Humphry Davy (first observations) and later Charles A. Kraus (systematic studies in the 1900s–1920s).

Properties of dilute metal-ammonia solutions:

Concentrated solutions (> 1 M) are bronze-colored and metallic in character:

At very high concentrations (~5 M at −33°C), the solution separates into two immiscible phases — a dilute blue phase and a concentrated bronze/gold phase — similar to liquid-liquid phase separation.

The metal-nonmetal transition in alkali metal–ammonia solutions has been a central problem in condensed matter physics and physical chemistry for over a century and is still actively studied.

Decomposition of metal-ammonia solutions:

These solutions are metastable. They slowly decompose:

Catalysts (transition metal salts, Fe₂O₃) dramatically accelerate this decomposition. In the absence of catalysts, dilute solutions can persist for days or weeks.

3.5 Ammonia as a Medium for Specific Chemical Reactions

3.5.1 Acid-Base Reactions and Amide Chemistry

Sodium amide (NaNH₂) is one of the most important bases in liquid ammonia:

Preparation:

Sodium amide is a powerful base, used extensively in organic chemistry for:

3.5.2 The Birch Reduction

The Birch reduction is one of the most important reactions performed in liquid ammonia. It uses dissolved alkali metals in liquid ammonia to reduce aromatic rings to 1,4-cyclohexadienes:

Mechanism:

The Birch reduction proceeds with remarkable regioselectivity. Electron-donating groups (e.g., methoxy) direct reduction to the position meta to the donor, while electron-withdrawing groups direct to the position bearing the substituent. This predictability makes it invaluable in synthetic organic chemistry.

Why liquid ammonia? The medium is essential because:

3.5.3 Nucleophilic Substitution

Liquid ammonia is an excellent medium for nucleophilic substitution reactions because:

Ammonolysis of alkyl halides:

This reaction is used industrially to produce amines, though competing over-alkylation to secondary and tertiary amines is a common problem.

Gabriel synthesis modifications use potassium phthalimide in liquid ammonia:

3.5.4 Precipitation Reactions

Many ionic compounds have very different solubilities in liquid ammonia compared to water. This allows precipitation reactions impossible in aqueous solution:

In water, BaCl₂ is soluble and AgCl is insoluble — the exact opposite of their behavior in liquid ammonia. This reversal of solubility is a powerful demonstration of how solvent choice controls chemistry.

Similarly:

Unlike in water, AgI dissolves readily in liquid ammonia due to the formation of [Ag(NH₃)₂]⁺ complexes.

3.5.5 Oxidation-Reduction Chemistry

The wide electrochemical window of liquid ammonia allows redox chemistry involving species that would react with water:

3.6 Liquid Ammonia in Industrial and Practical Applications

3.6.1 The Haber-Bosch Process (Indirect)

While the Haber-Bosch process produces ammonia (N₂ + 3H₂ → 2NH₃), the resulting ammonia is itself used as a chemical feedstock for:

3.6.2 Ammonia as a Refrigerant

Before the rise of chlorofluorocarbons (and their subsequent phase-out due to ozone depletion), ammonia was one of the most widely used refrigerants. It has excellent thermodynamic properties (high latent heat of vaporization) and is still used today in industrial refrigeration systems. It is designated R-717.

3.6.3 Ammonia in Rocket Propulsion

Liquid ammonia was explored as a rocket fuel (notably in the X-15 experimental aircraft with its XLR99 engine). It served as both fuel and coolant.

3.6.4 The Solvay Process (Modified)

The Solvay process for sodium carbonate production uses ammonia as an intermediary:

The NaHCO₃ is then calcined to Na₂CO₃. While this occurs in aqueous medium, ammonia's role is critical.

3.7 Analogies Between Water and Liquid Ammonia Chemistry

The deep structural parallels between water and ammonia allow us to draw systematic analogies:

However, there are crucial differences:

3.8 Metal Ammoniates — Solid Adducts

Many salts form ammoniates — crystalline compounds with coordinated ammonia molecules, analogous to hydrates:

These ammoniates often have different coordination geometries and properties compared to the corresponding hydrates, making them valuable in coordination chemistry research.

These two solvent systems sit at opposite extremes of the acid-base spectrum:

Together, they illustrate a profound truth: the choice of solvent is not merely a practical convenience — it fundamentally determines which reactions are possible, which species are stable, and what chemistry can be explored.

The chemistry of superacids and liquid ammonia represents two of the most dramatic expansions of the chemist's toolkit beyond the familiar world of aqueous solutions. Superacids revealed that molecules as mundane as methane and hydrogen could be protonated, that carbocations could be trapped and studied, and that acid catalysis could be pushed to extraordinary extremes. Liquid ammonia revealed the solvated electron — one of nature's simplest and most beautiful quantum objects — and provided a medium where alkali metals dissolve like sugar, where aromatic rings can be selectively reduced, and where the normal rules of solubility are rewritten.

Both systems remind us that the chemistry we learn in water is just one chapter of a much larger story. By venturing into unfamiliar solvents, chemists have discovered new reactivity, new bonding paradigms, and entirely new classes of chemical species — achievements that continue to shape modern chemistry, materials science, and catalysis.