Phase halogen exchange

In addition to the liquid-phase processes discussed above, vapor-phase halogen-exchange processes have also been developed. A variety of metal... 

Starr Finger Chem. Ind. (London) 1962, 1328 Shiley Dickerson Finger J. Fluorine Chem. 1972,2. 19 Kimura Suzuki Tetrahedron Lett. 1989, 30. 1271. For the use of phase transfer catalysis in this reaction, sec Yoshida Kimura Chem. Lett. 1988, 1355. For a review of the preparation of aryl fluorides by halogen exchange, sec Dolby-Glover Chem. Ind. (London) 1986, 518-523. 

The first traceless linkers for arenes were silyl linkers. This linker type was used in the synthesis of a benzodiazepine library, a milestone in the solid-phase synthesis of small organic molecules (Scheme 6.1.9). Synthesis of the linker involves lithium-halogen exchange and, after chlorosilane attachment, coupling with an... 

At first sight, the catalytic behaviour and the surface properties of pyrochlore Al(OH,F)3 does not fit this model since the pyrochlore structure (cf Fig. 9(c)) is a more open one than the HTB-A1F3 structure. However, since aluminium hydroxofluoride is susceptible to thermal decomposition, it is in fact no longer pyrochlore Al(OH,F)3 under the temperature conditions employed for the catalytic reactions. Thus, the behaviour of this phase in heterogeneous catalytic halogen exchange can be explained by the presence of amorphous alumina which determines the surface characteristics at the initial stage. Consequently, this phase acts in a manner similar to alumina and not until the surface becomes completely fluorinated does it reach its full catalytic activity. 

AUcoxycarbonylsulfenyl chlorides react at 20 ° with aqueous potassium iodide primarily with halogen exchange. The sulfenyl iodides, R8I, are unstable and combine in a bimolecular reaction to give the disulfane and elemental iodine (equation 19), which is removed by washing the organic phase with sodium thiosulfate (equation 20). " " ... 

The introduction of chromium leads to a progressive increase of the seleetivity to the halogen exchange product (HFC -125) and, regarding the support, it is obvious that it is reduced towards the CFC-1113 product. It can be assumed that the active phase containing chromium, introduced on the sur ce of the support, is characterized by sites of different nature, only selective for the reaction of halogen exchange it is therefore necessary to cover eonq)letely the surfoce of the support, evidently not inert. The increased activity leads to an increment of the yield of "heavy products" adsorbed on the surface of the catalyst. 

The use of a pulse microreactor, without continuous use of HF, has allowed the study of catalysts for the halogen exchange reaction in gas phase. 

The chloromethanes are reacted with HF to form the chlorofluoromethanes and HC1. The reaction takes place in the gas phase at 150°C on fixed bed catalysts containing AlF3, CrF3, or CrOF. Halogen exchange also can take place in the liquid phase. Pressures of 2—5 bar and temperatures about 100°C are used [162]. 

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