F-bonding

The heats of formation of the gaseous atoms, 4, are not very different clearly, it is the change in the bond dissociation energy of HX, which falls steadily from HF to HI, which is mainly res ponsible for the changes in the heats of formation. 6. We shall see later that it is the very high H—F bond energy and thus the less easy dissoeiation of H—F into ions in water which makes HF in water a weak aeid in comparison to other hydrogen halides. 

Using GAUSSIAN for Windows, we can carry out a MNDO, AMI. or PM3 optimization, of. for example, the IIP molecule, stalling from any reasonable II-F bond distance. The input file is similar to the standard MOPAC input file... 

The symbol —> represents the direction of polarization of electrons m the H—F bond)... 

The H—F bond is polarized so that hydrogen is par tially positive (blue) and fluorine partially negative... 

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

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

In the planar molecule BF3, in Figure 4.3(b), the C3 axis through B and perpendicular to the figure is the highest-fold axis and, therefore, the three planes of symmetry, perpendicular to the figure and through each of the B-F bonds, are labelled (t . The plane of the molecule is also a plane of symmetry and is labelled u , where /z stands for horizontal with respect to C3. 

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. 

The BF3 molecule, shown in Figure 4.18(i), is now seen to have /r = 0 because it belongs to the point group for which none of the translational symmetry species is totally symmetric. Alternatively, we can show that /r = 0 by using the concept of bond moments. If the B-F bond moment is /Tgp and we resolve the three bond moments along, say, the direction of one of the B-F bonds we get... 

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. 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

The OF radical has also been detected by CO2 laser magnetic resonance (30). The O—F bond length is 0.135789 nm. 

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... 

F Bonding force kg/kg IVlb Weight fraction of upper-size particles ... 

An extreme example of hybidization is the structure proposed for sulphur hexafluoride, SFe. The six S-F bonds are dhected to the apices of a regular octahedron. An aiTangement which would satisfy this number of covalent bonds is sp d hybridization. The ground state of the sulphur atom is s p° and... 

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.

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