Hydrogen fluoride

Structure and reactions

HF is diatomic in the gas phase, consisting of separate molecules. As a liquid, HF forms relatively strong hydrogen bonds, hence its relatively high boiling point. Solid HF consists of zigzag chains of HF molecules. The HF molecules, with a short covalent H–F bond of 95 pm length, are linked to neighboring molecules by intermolecular H–F distances of 155 pm. Liquid HF also consists of chains of HF molecules, but the chains are shorter, consisting on average of only five or six molecules.

Comparison with other hydrogen halides

Hydrogen fluoride does not boil until 20 °C (68 °F) in contrast to the heavier hydrogen halides, which boil between −85 and −35 °C (−121 and −31 °F). This hydrogen bonding between HF molecules gives rise to high viscosity in the liquid phase and lower than expected pressure in the gas phase.

Aqueous solutions

HF is miscible with water (dissolves in any proportion). In contrast, the other hydrogen halides exhibit limiting solubilities in water. Hydrogen fluoride forms a monohydrate HF·H₂O with melting point −40 °C (−40 °F), which is 44 °C (79 °F) above the melting point of pure HF.

Aqueous solutions of HF are called hydrofluoric acid. When dilute, hydrofluoric acid behaves like a weak acid, unlike the other hydrohalic acids, due to the formation of hydrogen-bonded ion pairs [H₃O⁺·F⁻]. However concentrated solutions are strong acids, because bifluoride anions are predominant, instead of ion pairs. In liquid anhydrous HF, self-ionization occurs:

3 HF ⇌ H₂F⁺ + HF−2

which forms an extremely acidic liquid (H₀ = −15.1).

Reactions with Lewis acids

Like water, HF can act as a weak base, reacting with Lewis acids to give superacids. A Hammett acidity function (H₀) of −21 is obtained with antimony pentafluoride (SbF₅), forming fluoroantimonic acid.

Synthesis

Hydrogen fluoride is typically produced by the reaction between sulfuric acid and pure grades of the mineral fluorite (calcium fluoride):

CaF₂ + H₂SO₄ → 2 HF + CaSO₄

About 20% of manufactured HF is a byproduct of fertilizer production, which generates hexafluorosilicic acid. This acid can be degraded to release HF thermally and by hydrolysis:

H₂SiF₆ → 2 HF + SiF₄

SiF₄ + 2 H₂O → 4 HF + SiO₂

High purity deuterium fluoride results from the reduction of AgF₂ in a D₂ atmosphere at 120 °C. Crystallized DF from this reaction, analyzed by neutron diffraction has been used to gain a better understanding of the hydrogen bonding in solid HF.

Use

In general, anhydrous hydrogen fluoride is more common industrially than its aqueous solution, hydrofluoric acid. Its main uses, on a tonnage basis, are as a precursor to organofluorine compounds and a precursor to synthetic cryolite for the electrolysis of aluminium.

Precursor to organofluorine compounds

HF reacts with chlorocarbons to give fluorocarbons. An important application of this reaction is the production of tetrafluoroethylene (TFE), precursor to Teflon. Chloroform is fluorinated by HF to produce chlorodifluoromethane (R-22):

CHCl₃ + 2 HF → CHClF₂ + 2 HCl

Pyrolysis of chlorodifluoromethane at 550–750 °C yields TFE.

HF is a reactive solvent in the electrochemical fluorination of organic compounds. In this approach, HF is oxidized in the presence of a hydrocarbon and the fluorine replaces C–H bonds with C–F bonds. Perfluorinated carboxylic acids and sulfonic acids are produced in this way.

1,1-Difluoroethane is produced by adding HF to acetylene using mercury as a catalyst.

HC≡CH + 2 HF → CH₃CHF₂

The intermediate in this process is vinyl fluoride or fluoroethylene, the monomeric precursor to polyvinyl fluoride.

Precursor to metal fluorides and fluorine

The electrowinning of aluminium relies on the electrolysis of aluminium fluoride in molten cryolite. Several kilograms of HF are consumed per ton of aluminium produced. Other metal fluorides are produced using HF, including uranium tetrafluoride.

HF is the precursor to elemental fluorine, F₂, by electrolysis of a solution of HF and potassium bifluoride. The potassium bifluoride is needed because anhydrous HF does not conduct electricity. Several thousand tons of F₂ are produced annually.

Catalyst

HF serves as a catalyst in alkylation processes in refineries. It is used in the majority of the installed linear alkyl benzene production facilities in the world. The process involves dehydrogenation of n-paraffins to olefins, and subsequent reaction with benzene using HF as catalyst. For example, in oil refineries "alkylate", a component of high-octane petrol (gasoline), is generated in alkylation units, which combine C₃ and C₄ olefins and isobutane.

Solvent

Hydrogen fluoride is an excellent solvent. Reflecting the ability of HF to participate in hydrogen bonding, even proteins and carbohydrates dissolve in HF and can be recovered from it. In contrast, most non-fluoride inorganic chemicals react with HF rather than dissolving.

Lasers

High power HF lasers have been constructed but their operating wavelength is strongly absorbed by water in the atmosphere. Systems like the short-lived Tactical High Energy Laser instead rely on deuterium fluoride lasers. Such lasers can produce 4.5W average power with 4000 pulses per second. These lasers rely on combustion reactions of H₂ or D₂ with dissociated fluorine gas F₂, producing excited vibrational and rotational states of HF:

F + H₂ → HF^* + H

The vibrationally excited state of HF laze around 2.7 μm and DF around 3.8 μm.

Health effects

Hydrogen fluoride is highly corrosive toward tissue. It can cause blindness by rapid destruction of the corneas.