Activity of Friedel-Crafts catalysts

Table 11.1. Relative Activity of Friedel-Crafts Catalysts... 

Classification of carbenes on the basis of reactivity toward alkenes 11.1. Relative activity of Friedel-Crafts catalysts... 

Cationic polymerization of unsaturated compounds proceeds through the stage of carbanion cations, called also carbocations. Typical catalysts for this reaction are strong protic acids such as sulfuric acid, perchloric and trifluoroctane or the Lewis acids, which include halides of elements III, IV and V groups of the periodic table (Friedel-Crafts catalysts), such as boron trifluoride, aluminum trichloride, tin tetrachloride and titanium tetrachloride. The activity of Friedel-Crafts catalysts increases significantly the presence of small quantities of cocatalysts, that is, ihe compounds which most often are the source of protons. 

MSC undergoes reactions with alcohols, amines, active methylene compounds (in the presence of bases), and aromatic hydrocarbons (in the presence of Friedel-Crafts catalysts) to replace, generally, a hydrogen atom by a methanesulfonyl group (382—401). 

Niobium and tantalum compounds form adducts with virtually all types of neutral ant anionic donors. The coordination chemistry of the higher halides is widely developed, and thei activity as Friedel-Crafts catalysts is another manifestation of their Lewis acidity. The stron acceptor capacity of the high valent metal compounds tends to favor the formation of dimers and sometimes of higher condensation products, which competes with coordination with othe donor molecules. Numerous simple anionic or heteropolyanionic species, but little cationi chemistry, and no simple metal salts, are known. 

Nitration with nitrogen dioxide in the presence of sulphuric acid Nitration with nitrogen dioxide in the presence of Friedel-Crafts catalysts Nitration with nitrogen dioxide in the presence of activated silica Phoionitration with nitrogen dioxide Nitrogen Pentoxide (Nitric anhydride)... 

The interrelationships among catalyst activity, carbonium-ion stability, and positional selectivity have been studied in detail, with the use of substituted benzyl halides and a wide variety of Friedel-Crafts catalysts. These data indicate that no single mechanistic description can encompass all Friedel-Crafts alkylations. With very reactive catalysts, there is little selectivity with respect to competing aromatic substrates. In less reactive systems, substrate selectivity increases. Quantitative description of catalyst activity has not been achieved, but a number of Lewis acids have been grouped into four broad categories, based on their activity in catalyzing the benzylation of benzene. Some of the catalysts are listed in Table 7.1. 

There are two reports [15,16] on the oligomerization (up to 100%) of ethylene in the presence of catalysts based on elements of group IB. These catalysts consist of EtiAlCU or EtAlCh and CuAlCh, CuiCU, or AgAlCh. These catalysts have higher activities at moderate temperatures and pressures compared with those of Friedel-Crafts catalysts. 

The kinetics of cationic pol5merization determination was carried out in the presence of Friedel-Crafts catalysts and was specified for the cases in which the chain termination takes place. In the consideration the transfer reaction of chain activity to the monomer is ignored, because it runs with a low efficiency. 

Friedel-Crafts catalysts are electron acceptors, ie, Lewis acids. The alkylating ability of ben2yl chloride was selected to evaluate the relative catalytic activity of a large number of Lewis acid haUdes. The results of this study suggest four categories of catalyst activity (200) (Table 1). 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

Zinc chloride exchanged clay catalysts have been reported to be highly active for the Friedel-Crafts alkylation and acylation reactions these are commercially sold by Contract Catalysts under the name Envirocats. These are montmorillonite catalysts modified by ZnCU and FeCli. Some of the reported examples of Friedel-Crafts reactions are given below there are claims that some of the processes are commercially practised. 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or Friedel-Crafts catalysts to introduce formyl or acyl substituents. The active electrophiles are believed to be dications resulting from diprotonation of CO, HCN, or the nitrile.64 The general outlines of the mechanisms of these reactions are given below. 

This procedure is based upon a study 1 of the method outlined in the patent literature.2 The procedure is a general one and may be used for the condensation of succinic anhydride with naphthalene and with the mono- and dimethylnaphthalenes, although in no other case are the purification and separation of isomers so easily accomplished. In this particular type of condensation, as well as in certain other types of Friedel-Crafts reactions, nitrobenzene is far superior to the solvents which are more frequently employed. This is partly because of its great solvent power and partly because it forms a molecular compound with aluminum chloride, and so decreases the activity of the catalyst in promoting side reactions. 

The metal halide catalysts include aluminum chloride, aluminum bromide, ferric chloride, zinc chloride, stannic chloride, titanium tetrachloride and other halides of the group known as the Friedel-Crafts catalysts. Boron fluoride, a nonmetal halide, has an activity similar to that of aluminum chloride. 

The catalytic activity of certain of the Friedel-Crafts catalysts was shown to decrease over a very wide range in the series boron fluoride, aluminum bromide, titanium tetrachloride, titanium tetrabromide, boron chloride, boron bromide and stannic chloride (Fairbrother and Seymour, mentioned in Plesch al., 83). When boron fluoride is added to isobutylene at dry ice temperatures, the olefin is converted to a solid polymer within a very few seconds. The time required for complete polymerization with aluminum bromide hardly extends to a few minutes while reaction times of hours are required with titanium chloride and periods of days with stannic chloride. 

Several new effective Friedel-Crafts catalysts have been developed. These include triflate93 (trifluoromethanesulfonate) derivatives of boron, aluminum, and gallium [M(0S02CF3)3]. Trichlorolanthanides have also been proved to be active reusable catalysts in benzylation.94 Superacids as catalysts are also very efficient in many Friedel-Crafts alkylations.95... 

The mechanism of Friedel-Crafts alkylation with alkyl halides involves initial formation of the active alkylating agent, which then reacts with the aromatic ring. Depending on the catalyst, the solvent, the reaction conditions, and the alkyl halide, the formation of a polarized donor-acceptor complex or real carbocations (as either an ion pair or a free entity) may take place ... 

Friedel-Crafts catalysts are more easy handling, fewer side reactions, and longer catalyst lifetime. Over the years numerous technologies applying different reactors have been developed.7 277 284-289 Because of their rapidly declining activity, zeolites have not reached commercial application in alkane-alkene alkylation.7... 

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