Fluoroantimonic acid is widely recognized as the strongest acid known.
The title of the strongest acid falls to fluoroantimonic acid. As per the reference, it is a superacid mixture comprising of antimony pentafluoride (SbF₅) and hydrofluoric acid (HF). This powerful combination makes it millions of times stronger than concentrated sulfuric acid, which itself is considered a very strong acid.
Superacids are defined as acids with an acidity greater than that of 100% pure sulfuric acid. Fluoroantimonic acid achieves its extraordinary strength by effectively donating protons (H⁺ ions) with unparalleled ease.
Why is Fluoroantimonic Acid So Potent?
The incredible strength of fluoroantimonic acid stems from the synergistic interaction between its two components:
- Hydrofluoric Acid (HF): While a strong acid itself, HF alone does not classify as a superacid.
- Antimony Pentafluoride (SbF₅): This compound acts as a powerful Lewis acid, meaning it is an electron-pair acceptor. When mixed with HF, SbF₅ readily abstracts fluoride ions (F⁻) from HF, forming the stable, non-coordinating anion [SbF₆]⁻.
This process liberates highly reactive, "naked" protons (H⁺) from the HF, as the resulting [SbF₆]⁻ anion is exceptionally weak as a base and does not readily re-protonate the H⁺. This unique characteristic allows fluoroantimonic acid to protonate even very weak bases, including hydrocarbons, which are typically unreactive with conventional acids.
Key Characteristics of Fluoroantimonic Acid
This formidable superacid possesses several defining features:
- Composition: A mixture of antimony pentafluoride (SbF₅) and hydrofluoric acid (HF). The common molar ratio is 1:1, but others are possible.
- Extreme Acidity: Its Hammett acidity function (H₀) is estimated to be as low as -31.3, making it the most acidic known chemical compound.
- Corrosivity: It is extremely corrosive and reacts violently with water and many organic compounds.
- Applications: Due to its immense protonating power, fluoroantimonic acid is primarily used in specialized chemical research, particularly in organic chemistry, for reactions that require exceptionally strong acidic conditions, such as the formation of carbocations.
Relative Acid Strengths
To put the strength of fluoroantimonic acid into perspective, consider the following simplified comparison:
| Acid | Type | Relative Strength (H₀) | Notes |
|---|---|---|---|
| Fluoroantimonic Acid | Superacid | ~ -31.3 | Strongest known acid |
| Carborane Acid | Superacid | ~ -18 | Strongest single compound acid |
| Sulfuric Acid (100%) | Strong Acid | ~ -12 | Benchmark for superacid definition |
| Hydrochloric Acid | Strong Acid | ~ -1.8 | Common strong laboratory acid |
| Acetic Acid | Weak Acid | ~ 4.76 (pKa) | Found in vinegar |
Safety and Handling
Given its extreme reactivity and corrosive nature, fluoroantimonic acid must be handled with the utmost care under highly controlled laboratory conditions. It necessitates specialized equipment and robust safety protocols to prevent severe chemical burns and dangerous reactions.
