Fluorosulfuric acid ranks among the strongest known superacids (H₀ = -15.1). It is primarily used as a catalyst in highly specialized organic synthesis applications, although its extreme corrosivity and reactivity impose significant safety constraints that limit its routine use, particularly in biochemical contexts.

Chemical Properties

Fluorosulfuric acid (HSO₃F, molecular weight 100.07 g/mol) is a colorless, fuming liquid with a density of 1.73 g/cm³, a boiling point of 163°C, and a melting point of -87°C. Structurally, it contains a sulfur atom in the +6 oxidation state bonded to three oxygen atoms and one fluorine atom in a tetrahedral geometry (S=O bond length ~1.41 Å; S–F ~1.64 Å). Its Brønsted acidity exceeds that of sulfuric acid (H₀ ≈ -12), and it undergoes partial self-ionization according to the equilibrium:

2 HSO₃F ⇌ H₂SO₃F⁺ + HSO₃F⁻

The compound reacts violently with water, leading to hydrolysis and formation of hydrogen fluoride (HF) and sulfuric acid:

HSO₃F + H₂O → HF + H₂SO₄

This reaction releases highly toxic HF fumes, contributing to its hazardous profile.

Biochemical Applications

In organic synthesis, anhydrous fluorosulfuric acid is employed as a catalyst for electrophilic fluorination of aromatic compounds (typically 1–5 mol% at room temperature) and for dehydration reactions such as the conversion of aldoximes into nitriles. When combined with antimony pentafluoride (SbF₅) to form so-called “magic acid,” it is capable of protonating extremely weak bases, including alkanes, enabling advanced carbocation studies.

Its applications in biochemical systems remain very limited. In peptide chemistry, it may be used under strictly anhydrous conditions for N-Boc deprotection. In carbohydrate chemistry, it has been investigated for the activation of glycosyl fluoride precursors. However, due to the generation of hydrogen fluoride and its extreme reactivity, it is not suitable for standard protein or nucleic acid protocols.