Fluorine atoms, from decomposition

The temperatures quoted are those at which decomposition becomes appreciably rapid. Presumably metal fluorides are also formed in these decompositions but this has not been proven. A reaction which is apparently the reverse of these pyrolyses was mentioned above, namely the addition of platinum-fluorine bonds to tetrafluoroethylene 118). The thermal decomposition of perfluoroalkyl and polyfluoroalkyl derivatives of main group elements such as boron, silicon, or tin was mentioned in earlier sections of this chapter. Transfer of fluorine atoms from the side chains on heating was also a characteristic property. However, it is interesting to compare the reaction (97),... 

Also the reaction pathways of Sarin decomposition catalyzed by selected forms of MgO were investigated [38]. In the case of the decomposition on the nonhydroxylated MgO surface, the removal of fluorine from Sarin was modeled. Fluorine was transferred from Sarin into binding distance with the Mg atom of the MgO surface (Fig. 13.9). It was revealed that such a structure provides a reliable model for the reaction mechanism. A two-step reaction mechanism was assumed. In the first step, Sarin creates a stable adsorbed complex with MgO through three chemical bonds with the MgO surface (the Al-GB model). It is expected that the transfer of the fluorine atom to the surface of MgO is accompanied by a change in the conformation of Sarin. In the second step, the bond between P and F is broken (the Al(t)-GB model) the fluorine atom is transferred to the Mg atom of the surface and the remaining part of Sarin adopts the most energetically favorable conformation (the Al(f)-GB model). 

A considerable effort was expended in attempting to prepare a compound with a perfluoroalkyl group attached to boron before success was achieved. Difficulty arises fi om the propensity of a fluorine atom for migration from carbon to boron for example, the compound CF3BF2 has been isolated in low yields [71, 72] and delightfully described as enduringly metastable , with respect to formation of BF3. It is not clear, however, whether this decomposition is intermolecular (10.24A), intramolecular (10.24B) or both (Figure 10.24). 

Introduction of a fluorine atom is achieved using the Schiemann reaction. Originally, the reaction involved gently heating the solid diazonium fluoroborate (Scheme 8.18), but improved yields result from the thermal decomposition the hexafluorophosphate, ArN2 PF, or hexa-fluoroantimonate, ArN2 SbFg, salts. 

The T s in Table I imply wide variations in rates, but not necessarily in mechanism, which could possibly be simple monomer depropagation. That this is not so, in all cases, is concluded at once from an inspection of the volatile decomposition products. Table II (18) summarizes the mass spectrometric yields of monomer, obtained by Madorsky and his colleagues at the National Bureau of Standards. Briefly, we note the high yields of monomer in the a-substituted and fluorinated vinyls. Elimination of the a-substituents or the fluorine atoms reduces considerably the monomer yield. This is illustrated by a comparison of poly-... 

Golubl l has proposed an explanation attributing the high etch rate of PVF to the ease of fluorine formation from the decomposition of this pol3mier. Fluorine promotes degradation of molecular oxygen to its active atomic form (O2 20), further reacting... 

The activating effects of the halogen atoms on the beta-hydrogen coupled with the reluctance of fluorine to depart as an anion from saturated carbon, especially in the presence of other alpha-fluorine atoms - , make the intermediacy of carbanions in these eliminations highly probable. Unfortunately the olefinic product adds alcohol too rapidly to be isolated and this necessitated careful considerations of alternative mechanisms. A minor fraction of the decomposition may follow an alpha-elimination, viz. 

XPS is often used to detect the changes of surface composition brought by the treatment. In many cases, each step of the modification procedure is monitored by XPS. The useful information may be limited to the appearance or the disappearance of given elements from the survey spectra. More often, careful peak decomposition allows the quantification of new chemical functions brought at the surface. In some cases, the molecules used for the surface treatment bear a tag (e.g. fluorine atoms), which facilitates their detection. In some advanced studies, the results are modeled, taking into account the formation of successive layers with different properties. The two following examples illustrate the typical approach that is used when analyzing surface-modified materials by XPS. 

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