Fluorine compounds, organic
Authors: Siegemund, G; Schwertfeger, W; Feiring, A; Smart, B; Behr, F; Vogel, H; McKusick, B; Kirsch, P
In Ullmann's encyclopedia of industrial chemistry. online: John Wiley and Sons, Inc.
HERO ID: 3980878
Organic fluorine compounds are characterized by their carbon–fluorine bond. Fluorine can replace any . . .
Organic fluorine compounds are characterized by their carbon–fluorine bond. Fluorine can replace any hydrogen atom in linear or cyclic organic molecules because the difference between the van der Waals radii for hydrogen (0.12 nm) and fluorine (0.14 nm) is small compared to that of other elements (e.g., chlorine 0.18 nm). Thus, as in hydrocarbon chemistry, organic fluorine chemistry deals with a great variety of species. When all valences of a carbon chain are satisfied by fluorine, the zig-zagshaped carbon skeleton is twisted out of its plane in the form of a helix. This situation allows the electronegative fluorine substituents to envelop the carbon skeleton completely and shield it from chemical (especially nucleophilic) attack. Several other properties of the carbon–fluorine bond contribute to the fact that highly fluorinated alkanes are the most stable organic compounds. These include low polarizability and high bond energies, which increase with increasing substitution by fluorine (bond energies: C–F bond in CH3F, 448 kJ/mol; C–H bond in CH4, 417 kJ/mol; C–Cl bond in CH3Cl, 326 kJ/mol; and C–F bond in CF4, 486 kJ/mol).
The cumulative negative inductive effect of the fluorine in perfluoroalkyl groups may reverse the polarity of adjacent single bonds (e.g., in the pair H3C ◅ I and F3C ▻ I) or double bonds (e.g., CH3CδH CδH2 and CF3 CδH CδH2). Fluorine substitution changes the reactivity of olefins and carbonyl compounds. Polyfluorinated olefins possess an electron-deficient double bond, which reacts preferentially with nucleophiles. Carboxy groups are affected by the presence of an adjacent perfluoroalkyl radical. In carboxylic acids, the acidity is markedly increased. In other carbonyl compounds, the reactivity is increased without any fundamental change in the chemistry of the compound. Correspondingly, the basicity of amines is reduced by the introduction of fluorine. Fluorine attached to the ring of aromatic compounds acts mainly as a paradirecting substituent, whereas perfluoroalkyl groups behave as meta-directing substituents.
Naturally, the influence of fluorine is greatest in highly fluorinated and perfluorinated compounds. The fact that these compounds have a high thermal stability and chemical resistance and are physiologically inert makes them suitable for many applications for which hydrocarbons are not. Properties that are exploited commercially include high thermal and chemical stability, low surface tension, and good dielectric properties, for example, in fluoropolymers, perfluorinated oils and inert fluids.
Individual fluorine atoms or perfluoroalkyl groups do not change the technical properties of a hydrocarbon fundamentally. However, this is not the case with physiological properties. A fluorine atom in a bioactive material may simulate a hydrogen atom, and although this does not prevent metabolic processes from occurring, the end products may be ineffective or toxic. Accordingly, such fluorine compounds are important in, for example, pesticides and pharmaceuticals.
A general overview of the scientific literature on organofluorine chemistry was published in 2013 ; commercial applications of fluorine products are reviewed in [2–4].