C-F bonds

The reaction proceeds in the direction indicated because a C—F bond is much stronger than a C—Br bond... 

The large C-F bond momenf in fluoroefhylene, shown in Figure 4. f 8(g), dominates fhe dipole momenf, which musf lie in fhe molecular plane, buf ifs direction in fhaf plane will depend on fhe effecfs of fhe ofher parts of fhe molecule wifh fhe resulf fhaf if will nof lie exacfly along fhe C-F bond. 

Just as group vibration wavenumbers are fairly constant from one molecule to another, so are their intensities. For example, if a molecule were being tested for the presence of a C—F bond there must be not only an infrared absorption band due to bond-stretching at about 1100 cm but also it must be intense. A weak band in this region might be attributable to another normal mode. 

Fluorochloro, fluorobromo, and fluoroiodoalkanes react selectively with aromatics under boron trifluoride catalysis to provide chloro-, bromo- and iodoalkylated products (48). The higher reactivity of the C—F bond over C—Cl, C—Br, and C—I bonds under Lewis acid catalysis results in the observed products. 

Polyimides containing C—F bonds have been receiving strong attention (96—98). Fluorine-containing polyimides possess lower dielectric constant and dielectric loss because of reduced water absorption and lower electronic polarization of C—F bonds vs the corresponding C—H bonds. Fluorine-containing polyimides are often more soluble and readily processible without sacrificing thermal stabilities. The materials are appHed primarily iu... 

Polytetrafluoroethylene contains only C—C and C—F bonds. These are both very stable and the polymer is exceptionally inert. A number of other fluorine-containing polymers cU e available which may contain in addition C—H and C—Cl bonds. These are somewhat more reactive and those containing C—H bonds may be cross-linked by peroxides and certain diamines and di-isocyanates. 

In addition to the presence of stable C—F bonds, the PTFE molecule possesses other features which lead to materials of outstanding heat resistance, chemical resistance and electrical insulation characteristics and with a low coefficient of friction. It is today produced by a number of chemical manufacturers such as Du Pont (Teflon), ICI (Fluon), Hoechst (Hostaflon TF), Rhone-Poulenc (Soreflon), Montecatini (Algoflan), Nitto Chemical-Japan (Tetraflon) and Daikin Kogyo-Japan (Polyflon). 

The carbon-fluorine bond is very stable. Further, where two fluorine atoms are attached to a single carbon atom there is a reduction in the C—F bond distance from 1.42 A to 1.35 A. As a result bond strengths may be as high as 504 kJ/mole. Since the only other bond present is the stable C—C bond, PTFE has a very high heat stability, even when heated above its crystalline melting point of 32TC. 

Fig. 2.10. Chemical state images obtained with the Escascope in Fig. 2.9, from a contaminated fluoropolymer [2.29]. (A) image in contribution to C Is from C-F bonding, (B) image in contribution to C Is from C-C bonding.

A similar mechanism operates in the reaction of 3,3,3-trifluoropropene with benzene and aluminum chloride [12 13] Perfluorophenylpropene undergoes intramolecular electrophilic attack m a rare example of ring closure at a C -F bond [14] (equation 11)... 

The C-F bonds in 1 fluoroalkenes and fluorobenzenes also are very strong (Table 17), but alkene k bond strengths vary with the level of fluonnabon (Table 18) Both CHF=CF2 and CF2=Cp2 have significantly weaker 7t bonds than CH2= CH2, CH7=CHF, and CH2=Cp2, consistent with other data indicatuig that tn- and tetrafluonnation thermodynamically destabilize double bonds [75] The low n bond energy in Cp2=CF2 underlies its propensity to undergo thermal biradical [2-1-2] cycloaddibons [103] (see p 767 )... 

With the publication of several books on this subject [2-9] during the past 20 years, the pace of research on the biochemistry of the C-F bond has quickened, and many new investigators have joined the field This sechon presents an overview so that the reader can sense the flavor and excitement of research in this area and observe the directions of some of the significant invesngauons TTie most thorough and up-to-date coverage will be found in reference 9, from which much of the subsequent discussion has been drawn... 

Carbon-fluorine bonds are quite strong (slightly stronger than C—FI bonds), and like polyethylene, Teflon is a very stable, inert material. We are all faniliar- with the most characteristic property of Teflon, its nonstick surface. This can be understood by comparing Teflon and polyethylene. The high electronegativity of fluorine makes C—F bonds less polarizable than C—FI bonds, causing the dispersion forces in Teflon to be less than those in polyethylene. Thus, the surface of Teflon is even less sticky than the already slick surface of polyethylene. 

Not all kinds of bonding are represented. For example, there are no ring systems within the molecule set, and there are no C-F bonds. 

The IRC calculation confirms that the preceding transition structure does indeed connect these two minima. The C-Cl bond length increases as it proceeds in the forward direction along the reaction path, and this bond decreases in length in the reverse direction (naturally, the C-F bond length changes in the complementary manner). 

A very stable bond involving fluorine is the carbon-fluorine bond. The strength of this C—F bond is comparable to the C—H bond, and has led to the existence of a series of compounds known as the fluorocarbons. These are analogous to the hydrocarbons and can be imagined as being derived from them by substituting F atoms for H atoms. For example,... 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

A variety of electronic effects control the relative energies of the disubstituted benzenes. For the difluorobenzenes, the meta and para isomers are of comparable energy the ortho isomer is clearly less stable. This is most likely due to the electrostatic repulsions of the two C-F bond dipoles which are aligned in the same direction. This is just like the difluoroethylenes where the 1,2-cis isomer is much less stable than the 1,1-isomer.poj. he dihydroxybenzenes. 

---