Fluorobenzene

PhF was first reported in 1886 by O. Wallach at the University of Bonn, who prepared the compound in two steps. Phenyldiazonium chloride was first converted to a triazene using piperidine:

[PhN₂]Cl + 2 (CH₂)₅NH → PhN=N-N(CH₂)₅ + [(CH₂)₅NH₂]Cl

The triazine was then cleaved with hydrofluoric acid:

PhN=N-N(CH₂)₅ + 2 HF → PhF + N₂ + [(CH₂)₅NH₂]F

Historical note: in Wallach’s era, the element fluorine was symbolized with “Fl”. Thus, his procedure is subtitled “Fluorbenzol, C₆H₅Fl”.[1]

On the laboratory scale, PhF is prepared by the thermal decomposition of the benzenediazonium tetrafluoroborate:

PhN₂BF₄ → PhF + BF₃ + N₂

According to the procedure, solid [PhN₂]BF₄ is heated with a flame to initiate an exothermic reaction, which also affords boron trifluoride and nitrogen gas. Product PhF and BF₃ are readily separated because of their differing boiling points.[2]

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

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.[3]

Structure of [(C₅Me₅)₂Ti(FC₆H₅)]⁺, a coordination complex of fluorobenzene.

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.[4] 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.[5]

• Bromobenzene
• Chlorobenzene
• Iodobenzene