Electrochemical fluorination
Encyclopedia
Electrochemical fluorination (ECF), or electrofluorination, is a foundational organofluorine chemistry
method for the preparation of fluorocarbon
-based organofluorine compounds. The general approach represents an application of electrosynthesis
. The fluorinated chemical compounds produced by ECF are useful because of their distinctive solvating properties and the relative inertness of carbon–fluorine bond
s. Two ECF synthesis routes are commercialized and commonly applied, the Simons Process and the Phillips Petroleum Process. Additionally, it is also possible to electrofluorinate in various organic media. Prior to the development of these methods, fluorination with fluorine
, a dangerous oxidant, was a dangerous and wasteful process. Also, ECF can be cost effective but it may also result in low yields.
of a solution of an organic compound
in a solution of hydrogen fluoride
. An individual reaction can be described as:
In the course of a typical synthesis, this reaction occurs once for each C–H bond in the precursor. The cell potential is maintained near 5–6 V. The anode
is nickel
-plated. Simons discovered the process in the 1930s at Pennsylvania State College
(U.S.), under the sponsorship of the 3M Corporation. The results were not published until after World War II because the work was classified due to its relevance to the manufacture of uranium hexafluoride
. In 1949 Simons and his coworkers published a long paper in the Journal of the Electrochemical Society. The Simons process is used for the production of perfluorinated amine
s, ether
s, carboxylic acid
s, and sulfonic acid
s. For carboxylic and sulfonic acids, the products are the corresponding acyl
and sulfonyl
fluorides. The method has been adapted to laboratory-scale preparations. Two noteworthy considerations are (i) the hazards associated with hydrogen fluoride (the solvent
and fluorine source) and (ii) the requirement for anhydrous
conditions.
anodes in molten potassium fluoride
in hydrogen fluoride. The species KHF2
is relatively low melting, a good electrolyte, and an effective source of fluorine. The technology is sometimes called “CAVE” for Carbon Anode Vapor Phase Electrochemical Fluorination and was widely used at manufacturing sites of the 3M Corporation. The organic compound is fed through a porous anode leading to exchange of fluorine for hydrogen but not chlorine.
as the solvent. A typical fluoride source is (C2H5)3N
:3HF. In some cases, acetonitrile is omitted, and the solvent and electrolyte are the triethylamine-HF mixture. Representative products of this method are fluorobenzene
(from benzene) and 1,2-difluoroalkane
s (from alkene)s.
Organofluorine chemistry
Organofluorine chemistry describes the chemistry of organofluorine compounds, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil- and water-repellents to pharmaceuticals, refrigerants and reagents in catalysis...
method for the preparation of fluorocarbon
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...
-based organofluorine compounds. The general approach represents an application of electrosynthesis
Electrosynthesis
Electrosynthesis in organic chemistry is the synthesis of chemical compounds in a electrochemical cell The main advantage of electrosynthesis over an ordinary redox reaction is avoidance of the potential wasteful other half-reaction and the ability to precisely tune the required potential...
. The fluorinated chemical compounds produced by ECF are useful because of their distinctive solvating properties and the relative inertness of carbon–fluorine bond
Carbon–fluorine bond
The carbon–fluorine bond is a bond between carbon and fluorine that is a component of all organofluorine compounds. It is the strongest single bond in organic chemistry—and relatively short—due to its partial ionic character. The bond also strengthens and shortens as more fluorines are...
s. Two ECF synthesis routes are commercialized and commonly applied, the Simons Process and the Phillips Petroleum Process. Additionally, it is also possible to electrofluorinate in various organic media. Prior to the development of these methods, fluorination with fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
, a dangerous oxidant, was a dangerous and wasteful process. Also, ECF can be cost effective but it may also result in low yields.
Simons Process
The Simons Process entails electrolysisElectrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
of a solution of an organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
in a solution of hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
. An individual reaction can be described as:
- R3C–H + HF → R3C–F + H2
In the course of a typical synthesis, this reaction occurs once for each C–H bond in the precursor. The cell potential is maintained near 5–6 V. The anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
is nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-plated. Simons discovered the process in the 1930s at Pennsylvania State College
Pennsylvania State University
The Pennsylvania State University, commonly referred to as Penn State or PSU, is a public research university with campuses and facilities throughout the state of Pennsylvania, United States. Founded in 1855, the university has a threefold mission of teaching, research, and public service...
(U.S.), under the sponsorship of the 3M Corporation. The results were not published until after World War II because the work was classified due to its relevance to the manufacture of uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
. In 1949 Simons and his coworkers published a long paper in the Journal of the Electrochemical Society. The Simons process is used for the production of perfluorinated amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s, ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
s, carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s, and sulfonic acid
Sulfonic acid
Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...
s. For carboxylic and sulfonic acids, the products are the corresponding acyl
Acyl
An acyl group is a functional group derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids.In organic chemistry, the acyl group is usually derived from a carboxylic acid . Therefore, it has the formula RCO-, where R represents an alkyl group that is...
and sulfonyl
Sulfonyl
A sulfonyl group can refer either to a functional group found primarily in sulfones or to a substituent obtained from a sulfonic acid by the removal of the hydroxyl group similarly to acyl groups...
fluorides. The method has been adapted to laboratory-scale preparations. Two noteworthy considerations are (i) the hazards associated with hydrogen fluoride (the solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
and fluorine source) and (ii) the requirement for anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
conditions.
Phillips Petroleum Process
This method is similar to the Simons Process but is typically applied to the preparation from volatile hydrocarbons and chlorohydrocarbons. In this process, electrofluorination is conducted at porous graphiteGraphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
anodes in molten potassium fluoride
Potassium fluoride
Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali metal halide and occurs naturally as the rare mineral carobbiite...
in hydrogen fluoride. The species KHF2
Potassium bifluoride
Potassium bifluoride is the inorganic compound with the formula KHF2. This colourless salt consists of the potassium cation and the bifluoride anion. The salt is used in etchant for glass...
is relatively low melting, a good electrolyte, and an effective source of fluorine. The technology is sometimes called “CAVE” for Carbon Anode Vapor Phase Electrochemical Fluorination and was widely used at manufacturing sites of the 3M Corporation. The organic compound is fed through a porous anode leading to exchange of fluorine for hydrogen but not chlorine.
Other methods
ECF has also been conducted in organic media, using for example organic salts of fluoride and acetonitrileAcetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
as the solvent. A typical fluoride source is (C2H5)3N
Triethylamine
Triethylamine is the chemical compound with the formula N3, commonly abbreviated Et3N. It is also abbreviated TEA, yet this abbreviation must be used carefully to avoid confusion with triethanolamine, for which TEA is also a common abbreviation....
:3HF. In some cases, acetonitrile is omitted, and the solvent and electrolyte are the triethylamine-HF mixture. Representative products of this method are fluorobenzene
Fluorobenzene
Fluorobenzene is the chemical compound with the formula C6H5F, often abbreviated PhF. This species is a derivative of benzene, with a single fluorine atom attached. Its melting point is 44 °C lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular...
(from benzene) and 1,2-difluoroalkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s (from alkene)s.