Carbon–fluorine Bond
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The carbon–fluorine bond is a polar
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
between
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
that is a component of all
organofluorine compound Organofluorine chemistry describes the chemistry of the organofluorines, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil and water repellents to pharmaceuticals, refri ...
s. It is one of the strongest single bonds in
chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
(after the B–F single bond, Si–F single bond, and H–F single bond), and relatively short, due to its partial ionic character. The bond also strengthens and shortens as more fluorines are added to the same carbon on a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
. As such, fluoroalkanes like
tetrafluoromethane Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a ...
(carbon tetrafluoride) are some of the most unreactive organic compounds.


Electronegativity and bond strength

The high
electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
of fluorine (4.0 for fluorine vs. 2.5 for carbon) gives the carbon–fluorine bond a significant
polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of ord ...
or dipole moment. The electron density is concentrated around the fluorine, leaving the carbon relatively electron poor. This introduces ionic character to the bond through
partial charge A partial charge is a non-integer charge value when measured in elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+. Partial charges are c ...
s (Cδ+—Fδ−). The partial charges on the fluorine and carbon are attractive, contributing to the unusual bond strength of the carbon–fluorine bond. The bond is labeled as "the strongest in organic chemistry," because fluorine forms the strongest single bond to carbon. Carbon–fluorine bonds can have a
bond dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...
(BDE) of up to 130 kcal/mol. The BDE (strength of the bond) of C–F is higher than other carbon–
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
and carbon–
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
bonds. For example, the BDEs of the C–X bond within a CH3–X molecule is 115, 104.9, 83.7, 72.1, and 57.6 kcal/mol for X = fluorine,
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
,
chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
,
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simila ...
, and
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
, respectively.


Bond length

The carbon–fluorine bond length is typically about 1.35
ångström The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
(1.39 Å in
fluoromethane Fluoromethane, also known as methyl fluoride, Freon 41, Halocarbon-41 and HFC-41, is a non-toxic, liquefiable, and flammable gas at standard temperature and pressure. It is made of carbon, hydrogen, and fluorine. The name stems from the fact that ...
). It is shorter than any other carbon–halogen bond, and shorter than single carbon–
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and carbon–
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
bonds. The short length of the bond can also be attributed to the ionic character of the bond (the electrostatic attractions between the partial charges on the carbon and the fluorine). The carbon–fluorine bond length varies by several hundredths of an ångstrom depending on the hybridization of the carbon atom and the presence of other substituents on the carbon or even in atoms farther away. These fluctuations can be used as indication of subtle hybridization changes and
stereoelectronic interaction An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the ...
s. The table below shows how the average bond length varies in different bonding environments (carbon atoms are sp3-hybridized unless otherwise indicated for sp2 or aromatic carbon). : The variability in bond lengths and the shortening of bonds to fluorine due to their partial ionic character are also observed for bonds between fluorine and other elements, and have been a source of difficulties with the selection of an appropriate value for the
covalent radius of fluorine The covalent radius of fluorine is a measure of the size of a fluorine atom; it is approximated at about 60 picometres. Since fluorine is a relatively small atom with a large electronegativity, its covalent radius is difficult to evaluate. The co ...
.
Linus Pauling Linus Carl Pauling (; February 28, 1901August 19, 1994) was an American chemist, biochemist, chemical engineer, peace activist, author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific top ...
originally suggested 64 pm, but that value was eventually replaced by 72 pm, which is half of the fluorine–fluorine bond length. However, 72 pm is too long to be representative of the lengths of the bonds between fluorine and other elements, so values between 54 pm and 60 pm have been suggested by other authors.


Bond strength effect of geminal bonds

With increasing number of fluorine atoms on the same (
geminal In chemistry, the descriptor geminal () refers to the relationship between two atoms or functional groups that are attached to the same atom. A geminal diol, for example, is a diol (a molecule that has two alcohol functional groups) attached t ...
) carbon the other bonds become stronger and shorter. This can be seen by the changes in bond length and strength (BDE) for the fluoromethane series, as shown on the table below; also, the
partial charge A partial charge is a non-integer charge value when measured in elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+. Partial charges are c ...
s (''q''C and ''q''F) on the atoms change within the series. The partial charge on carbon becomes more positive as fluorines are added, increasing the electrostatic interactions, and ionic character, between the fluorines and carbon. :


Gauche effect

When two fluorine atoms are in vicinal (i.e., adjacent) carbons, as in
1,2-difluoroethane 1,2-Difluoroethane is a saturated hydrofluorocarbon containing an atom of fluorine attached to each of two carbons atoms. The formula can be written CH2FCH2F. It is an isomer of 1,1-difluoroethane. It has a HFC name of HFC-152 with no letter suff ...
(H2FCCFH2), the gauche conformer is more stable than the anti conformer—this is the opposite of what would normally be expected and to what is observed for most 1,2-disubstituted ethanes; this phenomenon is known as the '' gauche effect''. In 1,2-difluoroethane, the gauche conformation is more stable than the anti conformation by 2.4 to 3.4 kJ/mole in the gas phase. This effect is not unique to the
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
fluorine, however; the gauche effect is also observed for 1,2-dimethoxyethane. A related effect is the
alkene cis effect In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
. For instance, the cis isomer of 1,2-difluoroethylene is more stable than the trans isomer. There are two main explanations for the gauche effect:
hyperconjugation In organic chemistry, hyperconjugation (σ-conjugation or no-bond resonance) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electron ...
and
bent bond In organic chemistry, a bent bond, also known as a banana bond, is a type of covalent chemical bond with a geometry somewhat reminiscent of a banana. The term itself is a general representation of electron density or configuration resembling a ...
s. In the hyperconjugation model, the donation of electron density from the carbon–hydrogen σ bonding orbital to the carbon–fluorine σ* antibonding orbital is considered the source of stabilization in the gauche isomer. Due to the greater electronegativity of fluorine, the carbon–hydrogen σ orbital is a better electron donor than the carbon–fluorine σ orbital, while the carbon–fluorine σ* orbital is a better electron acceptor than the carbon–hydrogen σ* orbital. Only the gauche conformation allows good overlap between the better donor and the better acceptor. Key in the bent bond explanation of the gauche effect in difluoroethane is the increased
p orbital In atomic theory and quantum mechanics, an atomic orbital is a Function (mathematics), function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electr ...
character of both carbon–fluorine bonds due to the large electronegativity of fluorine. As a result, electron density builds up above and below to the left and right of the central carbon–carbon bond. The resulting reduced orbital overlap can be partially compensated when a gauche conformation is assumed, forming a bent bond. Of these two models, hyperconjugation is generally considered the principal cause behind the gauche effect in difluoroethane.


Spectroscopy

The carbon–fluorine bond stretching appears in the
infrared spectrum Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
between 1000 and 1360 cm−1. The wide range is due to the sensitivity of the stretching frequency to other substituents in the molecule. Monofluorinated compounds have a strong band between 1000 and 1110 cm−1; with more than one fluorine atoms, the band splits into two bands, one for the symmetric mode and one for the asymmetric. The carbon–fluorine bands are so strong that they may obscure any carbon–hydrogen bands that might be present.
Organofluorine compounds Organofluorine chemistry describes the chemistry of the organofluorines, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil and water repellents to pharmaceuticals, ref ...
can also be characterized using
NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
, using
carbon-13 Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth. Detection by mass spectrometry A mass ...
,
fluorine-19 Fluorine (9F) has 18 known isotopes ranging from to (with the exception of ) and two isomers ( and ). Only fluorine-19 is stable and naturally occurring in more than trace quantities; therefore, fluorine is a monoisotopic and mononuclidic elem ...
(the only natural fluorine isotope), or
hydrogen-1 Hydrogen (1H) has three naturally occurring isotopes, sometimes denoted , , and . and are stable, while has a half-life of years. Heavier isotopes also exist, all of which are synthetic and have a half-life of less than one zeptosecond (10 ...
(if present). The
chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...
s in 19F NMR appear over a very wide range, depending on the degree of substitution and functional group. The table below shows the ranges for some of the major classes. :


Breaking C–F bonds

Breaking C–F bonds is of interest as a way to decompose and destroy
organofluorine Organofluorine chemistry describes the chemistry of the organofluorines, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from oil and water repellents to pharmaceuticals, ref ...
" forever chemicals" such as
PFOA Perfluorooctanoic acid (PFOA; conjugate base perfluorooctanoate; also known colloquially as C8, for its 8 carbon chain structure) is a perfluorinated carboxylic acid produced and used worldwide as an industrial surfactant in chemical processes a ...
and
perfluorinated compound A perfluorinated compound (PFC) or perfluoro compound is an organofluorine compound containing only carbon-fluorines and C−C bonds, as well as potentially heteroatoms. Perfluorinated compounds have properties that result from the presence of flu ...
s (PFCs). Candidate methods include catalysts, such as platinum atoms; photocatalysts; UV, iodide, and sulfite, radicals; etc.


See also

*
Fluorocarbon Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
*
Organofluorine chemistry Organofluorine chemistry describes the chemistry of the organofluorines, organic compounds that contain the carbon–fluorine bond. Organofluorine compounds find diverse applications ranging from Lipophobicity, oil and hydrophobe, water repellents ...
*
Carbon–hydrogen bond In chemistry, the carbon-hydrogen bond ( bond) is a chemical bond between carbon and hydrogen atoms that can be found in many organic compounds. This bond is a covalent, single bond, meaning that carbon shares its outer valence electrons with up ...
*
Carbon–carbon bond A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is formed b ...
*
Carbon–nitrogen bond A carbon–nitrogen bond is a covalent bond between carbon and nitrogen and is one of the most abundant bonds in organic chemistry and biochemistry. Nitrogen has five valence electrons and in simple amines it is trivalent, with the two remaining el ...
*
Carbon–oxygen bond A carbon–oxygen bond is a polar covalent bond between atoms of carbon and oxygen. Carbon–oxygen bonds are found in many inorganic compounds such as carbon oxides and oxohalides, carbonates and metal carbonyls, and in organic compounds such as ...


References

{{DEFAULTSORT:Carbon-fluorine bond Fluorine Organic chemistry Chemical bonding