Replacement by other halogens

The formyl group provides enough activation so that lithium fluoride will convert 5-bromofuran-2-carboxaldehyde into 5-fluorofuran-2-carboxalde-hyde, but only in dimethylformamide at 100°C. Other metal cations are ineffective. Replacement by other halogens is easy, the chloride-bromide displacement being reversible.180... 

The C—F bond energy is indeed very high (486 kJ mol-1 cf. C—H 415, and C—Cl 332 kJ mol"1), but organic fluorides are not necessarily particularly stable thermodynamically rather, the low reactivities of fluorine derivatives must be attributed to the impossibility of expansion of the octet of fluorine and the inability of, say, water to coordinate to fluorine or carbon as the first step in hydrolysis, whereas with chlorine this may be possible using outer d orbitals. Because of the small size of the F atom, H atoms can be replaced by F atoms with the least introduction of strain or distortion, as compared with replacement by other halogen atoms. The F atoms also effectively shield the carbon atoms from attack. Finally, since C bonded to F can be considered to be effectively oxidized (whereas in C—H it is reduced), there is no tendency for oxidation by oxygen. Fluorocarbons are attacked only by hot metals, e.g., molten sodium. When pyrolyzed, fluorocarbons tend to split at C—C rather than C—F bonds. 

Despite the hazards involved in the use of these compounds, the halogenated ethanes find continued widespread use and the PCBs are widely encountered, although their original roles are being satisfied through replacement by other substances and they are no longer being manufactured nor imported into the United States. 

The reason why boiling points of completely shielded compounds rise if the halogens are replaced by others with a higher atomic number will be dealt with later in Section 51. 

The replacement of halogens by other halogens and by nitro groups was discussed in Sections V,A, 1 and V,A,2. 

Since hydrazine also reacts vigorously with the halogens, fluorine is often used instead of oxygen as the oxidizer in rocket engines. Substituted hydrazines, where one or more of the hydrogen atoms are replaced by other groups, are also useful rocket fuels. For example, monomethylhydrazine,... 

When two HX molecules are paired together, it is clear that a H-bond can form between the proton of one molecule and the X atom of the other. But the situation is less clear cut if the H atom of the proton acceptor molecule is replaced by another halogen atom. In such a case, the dihalogen will not be polarized much at all, lessening the electrostatic part of the interaction. 

The replacement of other halogens by chlorine is in most cases a devaluation of the more valuable starting material. If necessary (e.g. isotopic labeling), it can be done with LiCl, ° AICI3 (equation 11) or copper(I) chloride (equation 12). ... 

Fluorescein is technically prepared on the large scale by the method given above. While phenolphthalein, in spite of the intense colour of its salts, is not a dye, in that it does not colour fibres, fluorescein is a true dye which colours animal fibres a fast yellow. But it is not manufactured as a dye, since it has been replaced by other dyes that give as beautiful colours and are cheaper. A number of its halogen-and nitro-substitution products have valuable colouring properties, and are prepared from it. The simplest dye of this kind is eosin or tetra-brom-fluoresceln, discovered in 1874 by Caro- The four bromine atoms are equally divided between the two resorcinol residues ... 

Halogens (particularly chlorine) can be replaced by other electron-attracting functions snch as trifluoromethyl or cyano groups. In the antibiotic chloramphenicol, both the chlorine atoms of the dichloroacetic moiety and of the para-nitro-phenyl group yielded productive isosteric replacements (Table 15.6). Many other examples of univalent atoms or groups replacements are found in the chapter dealing with substituent effects (Chapter 20) and with quantitative structure-activity relationships (Chapter 23). 

Cells were also used in which the potassium was replaced by other alkali metals and the chloride by other halogen ions. On solving equation (21) for win substituting in equation (2) and rearranging, the resulting expression is... 

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