Bromination Using Solid Catalysts

H-beta is catalytically active in the bromination of chlorobenzene to the 4- and 2-bromo derivatives [77]. The solid catalyst is less active but more selective for the 

4 bromo isomer than H2SO4. There was no conversion in the absence of the catalyst. The conversion of NBS (A -bromosuccinimide), rate of NBS conversion and isomer ratio (4-BCB/2-BCB) over H-beta are 11.2% (vr/w), 0.61 mmol h and 7.0, respectively. 

The bromination of toluene with NBS over H-ZSM-5, H-mordenite, H-beta, H-Y, conventional catalyst H2SO4, and in the absence of any catalyst yields 

2-bromotoluene, 4-bromotoluene, and the side-chain product, a-bromotoluene 

The selective para bromination of phenyl acetate over the sodium forms of zeolites X and Y has recently been reported by Smith et al. [126], The removal of the HBr formed by the sodium ions prevents the formation of phenol. Bases, acetic anhydride, and some metal acetates also improve the selectivity. 

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The usual aromatic bromination are performed by free bromine in the presence of a catalyst, most often iron. However, liquid bromine is not easy to handle because of its volatile and toxic character. On the other hand, alumina-supported copper(II) bromide can be treated easily and safely as a solid brominating reagent for aromatic compounds. The advantages of this procedure using the solid reagent are simple workups, mild conditions, and higher selectivities. Products can be isolated in good yield by simple filtration and solvent evaporation, and no extraction steps are required. 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

Free halogens are generally inconvenient to use, owing to their toxic and corrosive nature, but can be replaced by quaternary ammonium polyhalides. Quaternary ammonium tribromides are well established [e.g. 1] as solid, readily handled and relatively non-toxic alternatives for electrophilic bromine. More recently, other quaternary ammonium polyhalides have been produced, which together with the tribromides, have wide application as catalysts or in stoichiometric quantities in electrophilic substitution and addition reactions, oxidations, etc. 

Solid-supported reagents which have found utility include Nafion-scandium Lewis acid catalyst (allyl additions to aldehydes) [62], HOBt (medium-ring lactamization) [63], EDC (preparation of active esters) [64], and thiazolium hydrotribromide (brominations) [65], A review has also appeared describing the use of supported reagents in separation science, primarily for the selective sequestration of metal ions [66],... 

It is worth noting that the palladium catalyst could successfully be recycled palladium in reduced form was allowed to precipitate on added celite over the course of the reaction, the solid was later removed by centrifugation, and the palladium reoxidized with the corresponding amount of bromine. This version of the Mizoroki-Heck reaction may not qualify as an attractive alternative to standard protocols for everyday laboratory use, but it can be advantageous especially for companies with an in-house supply of carboxylic acids and propyne gas. 

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