Fluorocarbon systems

Fluorocarbon systems, in general, present no peculiar handling difficulties and the familiar and powerful techniques of isolation, purification and identification in organic... 

The electronic properties and size of fluorine relative to hydrogen and chlorine are set out in Table 1.2 at this point it is worthwhile to examine some of the possible consequences of these differences for the chemistry of fluorocarbon systems. In this way it can be emphasised, at the outset, how far-reaching these effects will be and, at the same time, it sets the scene for a rational approach to the chemistry. 

We can now outline, in a collective fashion, some electronic effects of fluorine that act, or have been suggested to act, in a fluorocarbon system but no attempt is made to discuss the detail of these effects at this point. 

Part of the interest in fluorocarbon systems lies in a comparison of the chemistry, and particularly reaction mechanisms, of fluorocarbon derivatives with those of the corresponding hydrocarbon compounds. Indeed, such comparisons pose quite a strenuous test on our theories of organic chemistry. As will be seen, our understanding of the influence of carbon-fluorine bonds on reaction mechanisms has made considerable progress. Nevertheless, it must be emphasised that fluorocarbon derivatives present much more complicated systems than their corresponding hydrocarbon compounds because, in addition to effects arising from different electronegativities, the effect of the lone pairs of electrons of fluorine that are not involved in o-bonds must be taken into consideration. Furthermore, the relative importance of these effects seems to be very dependent on the centre to which the fluorine is attached. 

Nucleophilic Displacement of Halogen from Fluorocarbon Systems... 

Fluoromethylene and polyfluoroalkylmethylene units can be introduced into various molecules by a variety of processes that, overall, involve a-elimination from the original fluorocarbon system. The processes themselves are carbenoid but may not necessarily involve carbene intermediates. There are many carbenoid procedures available for the insertion of fluorine-containing imits and they can be roughly divided into the following four types [40, 41]. 

Either fusion with alkali metals or reaction with aUcali-metal complexes with aromatic hydrocarbons will break down most fluorocarbon systems, due to the high electron affinities of these systems. Such reactions form the basis of some methods of elemental analysis [13], the fluorine being estimated as hydrogen fluoride after ion exchange. Surface defluorination of PTFE occurs with alkali metals and using other techniques [14]. Per-fluorocycloalkanes give aromatic compounds by passage over hot iron and this provides a potential route to a variety of perfluoroaromatic systems (Chapter 9, Section IB). 

The postulated primary chemical steps can be examined in the light of the mass spectroscopic fragmentation pattern of F-cyclobutane (14), which shows C2F/ as the parent peak (abundance 100 arbitrary units). A 1-3 split is also favorable C3Fr,+ has an intensity of 87 units, and CF3 25 units. These data are consistent with Steps 5b and 5c. Rupture of a C—F bond (Reaction 5a) must be more important in the radiolysis mechanism than indicated by the low abundance of 0.1 unit for the C4F7+ ion in the mass spectrum. However, this anomaly occurs not only in other fluorocarbon systems (1, 5, 10, 11, 22) but in most hydrocarbon systems studied to date. The intensities of CFL>+ and CF+ are also substantial in the mass spectrum, being 13 and 54 units respectively. These results, and the fact that CFL> has often been found under pyrolytic conditions (2,12), suggest the possibility that difluorocarbene plays a role in the mechanism, perhaps leading to a portion of the odd-carbon products. We have no evidence on this point, however. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

Cells of the lithium-fluorocarbon system are very close by their characteristics to the cells of the lithium-manganese dioxide system. Though the working voltage of fluorocarbon-lithium cells is lower than the voltage of manganese dioxide-lithium cells, such cells can in many cases be interchangeable. 

The fluorocarbons are generally compatible with most of the common metals except at high temperatures. At elevated temperatures, the following metals resist fluorocarbon corrosion (and are named in decreasing order of their corrosive resistance) Inconel, stainless steel, nickel, steel, and bronze. Water or water vapor in fluorocarbon systems will corrode magnesium alloys or aluminum containing over 2 percent magnesium. These metals are not recommended for use with fluorocarbon systems in which water may be present. 

Significant liquid leaks in fluorocarbon systems may be detected visually. As the material escapes, moisture in the air surrounding the leak condenses and then freezes around the leak due to the refrigerating effect of the vaporizing fluorocarbons. The frost thus formed is readily apparent. 

I 5 ReactivitY and Thermochemistty of Selected Metal/Fluorocarbon Systems Table 5.8 Species at 10 MPa (moles/mole). 

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