Halogenation, silica surface

Modification Using a Halogenated Silica Surface. Silica gel is heated in 2 m HC1, filtered off, and washed with water the dried silica gel is then reacted with pure thionyl chloride. After removal of the excess of thionyl chloride and byproducts (S02 and HC1) under vacuum, chlorinated silica gel (=SiCl) is obtained. This is unstable in water and alcohol. The chlorinated silica gel can then be alkylated by one of a number of different reactions ... 

Halides. Another treatment which can lower the hydroxyl population, or even eliminate it altogether, is halogenation of the silica surface.This removes hydroxyls, not by condensation as with CO and sulfur, but by replacement with halide, which prevents later attachment by Cr. The presence of halide probably also changes the electronic environment on the silica. Thus, fluoriding has long been used to increase activity but decrease RMip.i l Chloride also depresses RMIP, as well as the surface bromide and iodide of silica. However, these latter two have recently been studied, and it was possible to burn off most of the iodide or bromide with oxygen above 600 C, leaving a partially dehydroxylated surface. 

The reaction between silica and halogenating reagents permits the direct replacement of hydroxyl groups with halogen atoms, yielding reactive =Si-X surface groups. 

Silica gel is also the support of choice for the activation of V-halosuccinimides. The silica functions both as a proton donor which increases the electrophilic nature of the reagent, and as a support with geometrical constraints which contributes to the stereoselectivity. Alkyl and aryl sulfoxides are readily halogenated at the a position with yields of48-80%91. The reactions are carried in the solid state on the surface of TLC plates. The conversion of the optically active alkyl 4-methylphenyl sulfoxide into 1-haloalkyl 4-methylphenyl sulfoxide is accompanied by inversion of configuration at the S-atom. The stereoselectivity in these reactions is much higher than that observed in liquid-phase halogenation. 

Coal and many coal-derived liquids contain polycyclic aromatic structures, whose molecular equivalents form radical cations at anodes and radical anions at cathodes. ESR-electrolysis experiments support this (14). Chemically, radical cations form by action of H2SO4 (15,19), acidic media containing oxidizing agents (15,20,21,22), Lewis acid media (18,23-35) halogens (36), iodine and AgC104 (37,38), and metal salts (39,40). They also form by photoionization (41,42,43) and on such solid catalytic surfaces as gamma-alumina (44), silica-alumina (45), and zeolites (46). Radical anions form in the presence of active metals (76). 

In benzimidazoles, the least susceptible position to electrophilic halogenation is C-2. An exception is the use of NBS supported on silica gel which 2-brominates benzimidazole, perhaps because the support holds the 2-position close to its surface in proximity to the activated NBS <86TL1051>. Anionic benzimidazoles can be iodinated in the 2-position <90JHC673>. 

All present industrial catalyst systems are based on silver deposited on a slightly porous solid. The most widely used support is x-alumina, but silica-alumina and carborundum can also be employed. The specific surface area of the support, its porosity, and the pore size exert a considerable influence on the metal distribution at the surface, and consequently on catalytic activity. Several techniques are also available for fixing the silver, either by impregnation from a solution, or by deposition from a suspension. An initiator, usually consisting of alkaline earth or alkaline metals, can be added to the catalyst, but other metallic additions have also been recommended. Certain halogenated organic derivatives, such as dichloropropane, may increase selectivity in trace amounts (10 ppm in the feed), by reducing combustion side reactions. 

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