Metal halide catalyst

In contrast to bulk polymerization, solution polymerization provided soluble polymers with high molecular weights using low FeCl3 concentration at 120-140 C.31 A major disadvantage of the above approaches is that all the metal-halide catalysts need to be removed, since the catalyst residue will deteriorate die thermal stability and electrical and other properties. 

It is useful to note here a fundamental distinction between cationic and anionic polymerizations (including Ziegler-Natta systems). In the latter, residual water merely inactivates an equivalent quantity of catalyst, whereas in the former water may be a cocatalyst to the metal halide catalyst in excess it may decrease the rate by forming catalytically inactive higher hydrates and in very many systems it, or its reaction product(s) with a metal halide, act as extremely efficient chain-breakers, thus reducing the molecular weight of the polymers (see sub-section 5.4). 

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 metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

It seems questionable, however, that the geometry of all metal halide catalysts would be such as to permit the formation of the carbon-halogen coordinate bond. 

Fluorinated and Chlorfluorinated Sulfonic Acids. The synthesis of chlorinated and fluorinated sulfonic acids has been extensively reviewed (91,92). The literature discusses the reaction of dialkyl sulfides and disulfides, sulfoxides and sulfones, alkanesulfonyl halides, alkanesulfonic acids and alkanethiols with oxygen, hydrogen chloride, hydrogen fluoride, and oxygen—chloride—hydrogen fluoride mixtures over metal halide catalysts, such as... 

It is usually considered that the carbocation formed in the interaction between the alkyl halide and the metal halide catalyst attacks the aromatic ring and forms the... 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

This class of reaction is called Friedel-Crafts alkylation in honor of its discoverers, C. Friedel (a French chemist) and J. M. Crafts (an American chemist). The metal-halide catalyst functions much as it does in halogenation reactions to provide a source of a positive substituting agent, which in this case is a carbocation ... 

Exercise 22-21 a. Substitution of a chloromethyl group, —CH2CI, on an aromatic ring is called chloromethylation and is accomplished using methanal, HCl, and a metal-halide catalyst (ZnCI2). Write reasonable mechanistic steps that could be involved in this reaction ... 

Koch aldehyde synthesis) introduces the H-—C=0 function into reactive aromatic compounds such as 2-naphthalenol. The necessary reagents are HCN, HCl, and a metal-halide catalyst (ZnCI2 or AICI3), and the initial product must be treated with... 

The Friedel-Crafts acylation reaction, rapid under the influence of a powerful metal halide catalyst, is very selective. The data gathered in recent studies of the acetylation reaction in ethylene dichloride are presented graphically in Fig. 13. The p-phenyl substituent again deviates from the correlation line. Certain apparently real accelerations exist for the large ra-alkyl groups in this reaction. 

The intrinsic reactivity of strained cycloalkenes such as norbornene and cyclobutene ensures that they react as desired, and simple homogeneous metal halide catalysts are often effective for this transformation. However for less strained cyclic substrates, manipulation of catalyst activity/selectivity by means of modifying ligands is required. This is where the well-defined alkylidene catalysts pioneered by Grubbs and Schrock have come to the fore. An interesting example illustrating the range of catalyst reactivity is provided by the... 

The dual characteristics of sequential insertion and lack of metallacyclopentadiene formation appear to apply to other high-valent metal halide catalysts as well, e.g. NbCls. Regioselectivity varies from one system to the next, however. The related Ziegler-type catalysts typically give l,3,S-systems in comparable or greater amounts relative to their 1,2,4-isomers. For example, TiCU/AlEt2Cl converts... 

To accomplish this either ethylene or propene is combined with isobutane at 50-20°C (125-450°F) and 300-1000 psi (20-68 atmospheres) in the presence of metal halide catalysts such as aluminum chloride. Conditions are less stringent in catal d ic alkylation olefins ( propene, butene, or pentene) are combined with iso-butane in the presence of an acid catalyst (sulfuric acid or hydrofluoric acid) at low temperatures and pressures (1-40°C, 30-105°F, and 14.8-150 psi, 1-10 atmospheres). 

Hydrocarbons undergo metal or non-metal halide-catalyzed C—H bond exchange with strong deuterio acids. Reactions are accelerated by metal-halide catalysts such as AICI3, AlBtj, FeBrj or TiBr, ... 

Both of the above processes have normally been carried out over supported metal halide catalysts at elevated temperature and pressure. One of the most diflBcult problems has been removing the large quantity of heat generated at the surface of the catalyst by the reaction. If temperature is not adequately controlled in oxychlorination, a serious loss in selectivity will result, and catalyst volatilization will occur. In some cases fluidized solids or moving-bed techniques have been used, but these have generally met with difficulties owing to the volatile nature of the more active metal halide catalysts, such as copper chloride and the corrosive nature of the system. 

Reactions that are catalyzed by metal halide catalysts are also catalyzed by proton acids. The most commonly used Brpnsted acids are H2SO4, H3PO4 and HF. 

Thus, treatment of phosgene with an excess of SOFj at 400 "C (in the presence of a metal halide catalyst, such as FeClj, AICI3, UCl, LaFj, WOF or SbCl ) gives carbonyl difluoride in 96% conversion based upon phosgene [704],... 

The use of metal halide catalysts is restricted to mono-hydrosilanes because disproportionation of polyhydrosilanes is also promoted by these catalysts . 

In general, the use of metal halide catalysts is restricted for the reactions of mono-hydrosilanes. This is mainly due to the fact that disproportionation of poly-hydrosilanes is also promoted by these catalysts112. However, it has recently been shown that some alkali metal salts and trialkylammonium fluorides, such as CsF, KF, HCOOK,... 

The use of metal halide catalysts discovered jointly by the French chemist Charles Friedel and the American chemist James Crafts more than one hundred years ago, has evolved into a broad field of chemistry that is still thriving. 

The difference between the behavior of the metal halide catalyst and that of the acid catalysts is readily explained (Schmerllng, 14d) by postulating that the reaction which occurs most rapidly when a mixture of isobutane and 1- or 2-butene comes in contact with liquid hydrogen fluoride or sulfuric acid, is the addition of the acid to the olefin to form s-butyl ester. There then exists in the reaction mass an equilibrium mixture which may be described by the following ... 

The composition of a large number of alkylates, both those prepared in the laboratory using pure hydrocarbons and those obtained on a commercial scale using olefin mixtures, has been investigated. The principal products are for the most part similar to those obtained in the presence of the metal halide catalyst, the chief difference being in the relative amounts of the various isomers. Alkylation of isobutane ivith propene at 30° in the presence of 98% sulfuric acid yielded an alkylate that was 62-66% by weight 2,3-dimethylpentane and 8-12%, 2,4-dimethylpentane propane and trimethylpentanes (11-19%) were also obtained (McAllister et al., 12). A commercial isobutane-propene alkylate obtained at 21° was shown to contain about 36% by volume of 2,3- and 26% of 2,4-dimethylpentane as well as 15% of trimethylpentanes (Glasgow et al., 40). 

Cyclobutene, cyclopentene, and norbomene also give their polyalken-amers (64). The molecular weights are all high, and the stereochemistries are largely cis. Of these, cw-polypentenamer has also been made with molybdenum, tungsten, and rhenium halide catalysts, and cii-polynor-bomenamer (of lower molecular weight) with a molybdenum chloride catalyst, but polybutenamer made with metal halide catalysts (the metals tried were Ti, Mo, W, V, Cr, and Ru) has never been found to have more than 60% of its double bonds cis (64). 

Methylbenzene has two sites where reaction with Cl 2 is possible—the aromatic ring or the alkane portion (methyl group). In the absence of a metal halide catalyst, aromatic substitution is not possible. In the absence of the catalyst and at high temperature or in the presence of light, substitution occurs at the alkane C—H bond. 

A successful ATRP reaction depends on the reversible activation of a dormant species, such as an alkyl halide (R—X), by a transition metal halide catalyst (MpY/L). The metal halide catalyst abstracts the halogen atom (X) from the dormant species to... 

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