Fluorobenzene is an
aryl fluoride and the simplest of the
fluorobenzenes, with the formula C
6H
5F, often abbreviated
PhF. A colorless liquid, it is a precursor to many fluorophenyl compounds.
Preparation
PhF was first reported in 1886 by O. Wallach at the
University of Bonn
The University of Bonn, officially the Rhenish Friedrich Wilhelm University of Bonn (), is a public research university in Bonn, North Rhine-Westphalia, Germany. It was founded in its present form as the () on 18 October 1818 by Frederick Willi ...
, who prepared the compound in two steps.
Phenyldiazonium
Benzenediazonium tetrafluoroborate is an organic compound with the chemical formula, formula 6H5N2F4. It is a salt of a diazonium compound, diazonium cation and tetrafluoroborate. It exists as a colourless solid that is soluble in polar solve ...
chloride was first converted to a
triazene using piperidine:
:
hN2l + 2 (CH
2)
5NH → PhN=N-N(CH
2)
5 +
CH2)5NH2l
The triazine was then cleaved with
hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling p ...
:
:PhN=N-N(CH
2)
5 + 2 HF → PhF + N
2 +
CH2)5NH2
Historical note: in Wallach's era, the element fluorine was symbolized with "Fl". Thus, his procedure is subtitled "Fluorbenzol, C
6H
5Fl".
On the laboratory scale, PhF is prepared by the
thermal decomposition of the
benzenediazonium tetrafluoroborate:
:PhN
2BF
4 → PhF + BF
3 + N
2
According to the procedure, solid
hN2F
4 is heated with a flame to initiate an
exothermic reaction
In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC define ...
, which also affords
boron trifluoride and nitrogen gas. Product PhF and BF
3 are readily separated because of their differing
boiling point
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid varies depending upon the surrounding envi ...
s.
The technical synthesis is by the reaction of
cyclopentadiene
Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−.
This colorless liquid has a strong and unpleasant odor. At room temperature, ...
with
difluorocarbene
Difluorocarbene is the chemical compound with formula CF2. It has a short half-life, 0.5 and 20 ms, in solution and in the gas phase, respectively.Douglas A Jean Osteraas "Difluorocarbene Modification of Polymer and Fiber Surfaces," ''Journal ...
. The initially formed
cyclopropane
Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a triangular ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane ...
undergoes a ring expansion and subsequent elimination of
hydrogen fluoride
Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
.
Reactions
PhF behaves rather differently from other halobenzene derivatives owing to the pi-donor properties of fluoride. For example, the para position is more activated than benzene toward electrophiles. For this reason, it can be converted to 1-bromo-4-fluorobenzene with relatively high efficiency.
Solvent properties
:
PhF is a useful solvent for highly reactive species. Its melting point at −44 °C is lower than that of benzene. In contrast, the boiling points of PhF and benzene are very similar, differing by only 4 °C. It is considerably more polar than benzene, with a dielectric constant of 5.42 compared to 2.28 for benzene at 298 K. Fluorobenzene is a relatively inert compound reflecting the strength of the C–F bond.
Although it is usually considered a non-coordinating solvent, a metal complex of PhF has been crystallized.
See also
*
Bromobenzene
*
Chlorobenzene
*
Iodobenzene
*
Pentafluoromethylbenzene
References
{{fluorine compounds
Fluorobenzenes
Phenyl compounds