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 major industrial use is as carbon source in the manufacture of steel.

PhF is a relatively inert compound because the C-F bond is very strong. PhF is generally not considered to be a versatile reagent. Its melting point is 44 °C lower than that of benzene, indicative of the remarkable low-temperature properties of fluorocarbons. In contrast, the boiling point of PhF and benzene differ by only 4 °C.

Preparation
On the laboratory scale, PhF is conveniently prepared by the thermal decomposition of the tetrafluoroborate salt of phenyl diazonium:
 * PhN2BF4 &rarr;  PhF  +  BF3   +  N2

According to the procedure, solid [PhN2]BF4 is heated with a flame to initiate an exothermic reaction that affords two volatile products, PhF and BF3, which are readily separated because of their differing boiling points.

PhF was first reported in 1886 by O. Wallach at the University of Bonn, who prepared the compound in two steps, starting also with a phenyldiazonium salt. The diazonium chloride was first converted to its piperidinide, which in turn was cleaved using hydrofluoric acid.
 * PhN2Cl +  2 C5H10NH  &rarr;  PhN=N-NC5H10  +  [C5H10NH2]Cl
 * PhN=N-NC5H10 +  2 HF  &rarr;  PhF  +  N2  +  [C5H10NH2]F

An interesting historical note: in Wallach’s era, the element fluorine was symbolized with “Fl”. Thus, his procedure is subtitled “Fluorbenzol, C6H5Fl”.

The technical synthesis is by the reaction of cyclopentadiene with difluorocarbene. The cyclopropane that is formed initially undergoes a ring expansion and subsequent elimination of hydrogen fluoride.