Hexafluorobenzene
Encyclopedia
Hexafluorobenzene, HFB, , or perfluorobenzene is an organic
, aromatic
compound
. In this derivative of benzene
all hydrogen
atoms have been replaced by fluorine atoms. The technical uses of the compound are limited, although it is recommended as a solvent
in a number of Photochemical
reactions. In the laboratory hexafluorobenzene is used for several purposes:
The conclusion of the table is HFB is the only perhalobenzene being planar, the others all are buckled more or less. As a consequence in C6F6 the overlap between the p-orbitals
is optimal, while in the others it is less, also giving rise to a lower aromaticity in those compounds.
and Fluorine
is not possible. The synthetic route proceeds via the reaction of alkali
-fluorides with halogen
ated benzene:
. The carbon fluorine bond therefore is highly polarized: the carbon atom has (partially) positive charge, fluorine negative. This reasoning holds very much for the electrons in the σ-bonds
. Electrons in p-orbitals
encounter a totally different situation. The p-orbital at fluorine parallel to the one on the adjacent carbon will face an optimal interaction. Thereby fluorine, unlike the higher halogens, has no extra nodal plane in its orbitals, so size and geometry fit perfectly and no anti-bonding interactions occur. The dipole resulting from the σ-electronegativity will force a partial replacement of electric charge from fluorine towards carbon and the aromatic ring: fluorine behaves as a σ-electronegative, but as a π-electropositive element. This view is supported by the reactions C6F6 exhibits.
which for C6F6 is impossible. During the reaction a particle initially bonded to the aromatic nucleus leaves the molecule as a positive charge ion. In benzene, this is a H+-ion. In hexafluorobenzene a positive charged fluorine atom should have to leave, which does not occur. The vast number of reactions of HFB proceed with fluoride—negative charged—as leaving group. As a consequence the entering group also should be an anion.
The reaction of pentafluorophenyl derivatives has been long puzzling for its mechanism. Independent of the substituent, they all exhibit an para directing effect. The new introduced group too has no effect on the directing behaviour. In all cases, a 1,4-disubstituted-2,3,5,6-tetrafluorobenzene derivative shows up. Finally, the clue is found not in the nature of the non-fluorine substituent, but in the fluorines themselves. The π-electropositive effect introduces electrons into the aromatic ring. The non-fluorine substituent is not capable of doing so. As charge accumulates at the ortho and para positions relative to the donating group, the ortho and para-positions relative to the non-fluorine substituent receive less charge, so are less negative or more positive. Furthermore the non-fluorine substituent in general is more bulky than fluorine, so its ortho-positions are sterically shielded, leaving the para-position as the sole reaction site for anionic entering groups.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
. In this derivative of benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
all hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atoms have been replaced by fluorine atoms. The technical uses of the compound are limited, although it is recommended as a 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...
in a number of Photochemical
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
reactions. In the laboratory hexafluorobenzene is used for several purposes:
- standard in Fluor-19 NMRNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
- solvent and standard in Carbon-13 NMRNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
- solvent in Proton-NMRNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
- solvent when studying some parts in the IR-spectrumInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
- solvent in UV-spectra, as hexafluorobenzene itself hardly shows any absorbance in the UV region.
Geometry of the aromatic ring
Hexafluorobenzene in the perhalogenbenzenes stands somewhat aside. When counting for bond angles and distances it is possible to calculate the distance between two ortho fluorine atoms. Also the non bonding radius of the halogens is known. The following table presents the results:| Formula | Name | Calculated inter-halogen distance, aromatic ring assumed planar |
Twice nonbonding radius | Consequent symmetry of the benzene |
|---|---|---|---|---|
| C6F6 | perfluorobenzene | 279 | 270 | D6h |
| C6Cl6 | perchlorobenzene | 312 | 360 | D3d |
| C6Br6 | perbromobenzene | 327 | 390 | D3d |
| C6I6 | periodobenzene | 354 | 430 | D3d |
The conclusion of the table is HFB is the only perhalobenzene being planar, the others all are buckled more or less. As a consequence in C6F6 the overlap between the p-orbitals
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
is optimal, while in the others it is less, also giving rise to a lower aromaticity in those compounds.
Synthesis
The direct synthesis of hexafluorobenzene from benzeneBenzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
and 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...
is not possible. The synthetic route proceeds via the reaction of alkali
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
-fluorides with halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
ated benzene:
- C6Cl6 + 6 KF → C6F6 + 6 KCl
Fluorine and the aromatic ring
A substantial part of the chemistry of HFB is related to the position of fluorine in the periodic table. On its position at the end of the first row, fluorine is a halogen. It also is the smallest one, so taking up an electron releases the largest amount of energy of all elements, it is the strongest oxidant, it has the highest electronegativityElectronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
. The carbon fluorine bond therefore is highly polarized: the carbon atom has (partially) positive charge, fluorine negative. This reasoning holds very much for the electrons in the σ-bonds
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
. Electrons in p-orbitals
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
encounter a totally different situation. The p-orbital at fluorine parallel to the one on the adjacent carbon will face an optimal interaction. Thereby fluorine, unlike the higher halogens, has no extra nodal plane in its orbitals, so size and geometry fit perfectly and no anti-bonding interactions occur. The dipole resulting from the σ-electronegativity will force a partial replacement of electric charge from fluorine towards carbon and the aromatic ring: fluorine behaves as a σ-electronegative, but as a π-electropositive element. This view is supported by the reactions C6F6 exhibits.
Reactions
With regard to its reactions, HFB stands apart from other aromatic compounds. One of the main reactions of aromatics is electrophilic aromatic substitutionElectrophilic aromatic substitution
Electrophilic aromatic substitution EAS is an organic reaction in which an atom, usually hydrogen, appended to an aromatic system is replaced by an electrophile...
which for C6F6 is impossible. During the reaction a particle initially bonded to the aromatic nucleus leaves the molecule as a positive charge ion. In benzene, this is a H+-ion. In hexafluorobenzene a positive charged fluorine atom should have to leave, which does not occur. The vast number of reactions of HFB proceed with fluoride—negative charged—as leaving group. As a consequence the entering group also should be an anion.
The reaction of pentafluorophenyl derivatives has been long puzzling for its mechanism. Independent of the substituent, they all exhibit an para directing effect. The new introduced group too has no effect on the directing behaviour. In all cases, a 1,4-disubstituted-2,3,5,6-tetrafluorobenzene derivative shows up. Finally, the clue is found not in the nature of the non-fluorine substituent, but in the fluorines themselves. The π-electropositive effect introduces electrons into the aromatic ring. The non-fluorine substituent is not capable of doing so. As charge accumulates at the ortho and para positions relative to the donating group, the ortho and para-positions relative to the non-fluorine substituent receive less charge, so are less negative or more positive. Furthermore the non-fluorine substituent in general is more bulky than fluorine, so its ortho-positions are sterically shielded, leaving the para-position as the sole reaction site for anionic entering groups.
Literature
- Walter J. Pummer, Leo A. Wall: „Reactions of Hexafluorobenzene“, ScienceScienceScience is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
, 1958, Vol. 127, Nr. 3299, pp. 643–644; . - Harold Crosbie Fielding: „Preparation of hexafluorobenzene and fluorochlorobenzenes“, US-Patent 3277192, 4. October 1966.
- M. D. Bertolucci, R. E. Marsh: „Lattice parameters of hexafluorobenzene and 1,3,5-trifluorobenzene at −17 °C“, J. Appl. Cryst., 1974, 7, pp. 87–88; .