Reactions carbonium-ion type

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

The fundamental carbonium ion-type reactions of olefins— including double bond and carbon skeleton isomerization, polymerization, isotopic exchange, and hydrogen transfer—have been reviewed earlier (62). The importance of a thorough understanding of the nature of olefin transformations over zeolite catalysts cannot be underestimated. Probably the most important and frequently recurring pattern is the transfer or redistribution of hydrogen that is observed with olefins over acidic crystalline aluminosilicate catalysts. 

In the presence of strong acids such as aqueous H2SO4, carbonyl compounds may react with olefins to form unsaturated alcohols and other products, depending on the reaction conditions. Using H-mordenite as catalyst in a continuous-flow system, 10% conversion of formaldehyde to isoprene was observed at 300° using an isobutylene-to-HCHO (molar) ratio of 3.7. A carbonium ion-type reaction scheme, involving a Prins reaction (1,2) and a subsequent dehydration-rearrangement step... 

Carbonium ion-type reactions initiated by these acidic oxides are shown in Table 4.10. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

The Nature of the Intermediate in Carbonium-Ion Type Interconversion Reactions of Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives. J. Amer. chem. Soc. 73, 3542 (1951). 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

Copolymerization between an oxonium ion type monomer and a carbonium ion type monomer has never been carried out successfully. Styrene (St) does not form a copolymer with THF (1), BCMO (1), or /3-PL (2, 16). The formation of a homopolymer mixture was confirmed for the St-/ -PL system (18,19, 26). The reason for the absence of cross propagation was discussed elsewhere (6), but the reaction of the trityl cation with fi-PL and the reaction of the triethyloxonium ion with 1,1-diphenylethylene did show the absence of the bonding reaction (6). 

To obtain a high molecular weight block or random copolymer of the oxonium ion type monomer and carbonium ion type monomer, experimental conditions must be such that termination or transfer reactions are minimized. The living nature of the cationic polymerization of THF (7) is well established, but it has been difficult to obtain a high polymer of styrene or DOL by cationic mechanism. In this paper we demonstrate the living nature of the polymerization of DOL and the high polymer of St-DOL copolymer. Using this technique, we were able to obtain a block copolymer of vinyl monomer and cyclic monomer. 

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

The recent studies on the relationship between activation temperature and carbonium ion type catalytic activity of both decationized and cation exchanged zeolites show that at arid above the temperature required for the removal of all observable hydroxyls with vibrational frequencies between 3700-3500 cm" the activity sharply declines. The lowest concentration of acidic lattice hydroxyl required for carbonium ion activity seems to depend on the reaction involved. For example, dehydroxylation of La-exchanged Y to a level at which hydroxyl content was unobservable by currently-used infrared techniques led to total loss of activity to crack n-butane, but only partial loss of activity to crack cumene (vide infra) and to alkylate toluene with propylene (74). The activity and hydroxyl content lost on dehydroxylation can be restored upon subsequent treatment with water (11). Furthermore, alkali metal zeolites, which have little or no carbonium ion type activity can be made to show strong activity by the addition of a proton source, such as alkyl chlorides (51, 58). The similarity of the products obtained with the... 

Ri and R2 could be alkyl groups. The electron-deficient carbon atom of a C—H bond so polarized could then serve as the active center for reactions of the carbonium ion type complete cleavage of the C—H bond is not required. The Linde workers were unable to correlate zeolite hydrogen content with hexane isomerization activity (40) nor did they attribute the great rise in cumene cracking activity (48) obtained by replacing univalent cations with bivalent cations in zeolite Y as arising... 

Evidence for a carbonium ion type of isomerization-polymerization reaction involving proton transfer was shown for the reaction of 1-hexene over a deuterated REX catalyst (44). The catalyst was prepared by adding D2O (1.4 eq of D per gram atom of Al) to a BEX sample that had been precalcined at 500°, and showed a broad envelope at 2597-2326 cm" (0—D stretching vibrations) in its IR spectrum. The origin of protonic acidity in such REX catalysts was discussed earlier. 

Most reactions involving the enzymes listed in Section II include the combination of a carbanion center with an electrophilic center that is part of an un-symmetrical double bond, such as a carbonyl or imino function. The cases in which carbonium ion-type intermediates form involve a completely different structure. This is not a case of competing mechanisms among common functional groups, but rather a specific relationship between functional group structure and mechanism. 

An early process in the cure reaction is protonation of a methylolphenol, followed by loss of a molecule of water to produce a benzylic carbonium ion (see reaction 4.3). This may be followed by reaction with a second phenol to generate a bridged structure, as illustrated in Reaction 4.4. Alternatively the benzylic carbonium atom may react with another methylol group, thus generating a bridge based on an ether-type structure (see Reaction 4.5). 

In aqueous and related media, most of the chemistry of organic electrode reactions is concerned with species resulting from the eventual uptake of one electron and one proton or, more usually, two electrons and two protons (in cathodic reactions) and the reverse processes in some anodic reactions. However, in the latter reactions, the formation of radical intermediates as a result of de-electronation may lead to other chemical changes, e.g., in the total oxidation of hydrocarbons to carbon dioxide in aqueous media and in other ways in the Kolbe, Hofer-Moest and Crum Brown-Walker reactions, and some other carbonium ion type rearrangements. 

The CH5 group, having a carbon atom in five-fold coordination, is called a carbonium ion. Carbonium-ion formation is an easier process in larger hydrocarbons, where the reaction is between the proton and the secondary or tertiary carbon atom, but is significantly more activated in the case of methane. The carbonium ion is the intermediate toward carbenium ions (3.29), where the positively charged carbon atom has a threefold coordination. Once formed, carbenium ions can be converted to hydrocarbons and new carbonium ions, in hydride transfer reactions, as in (3.31). Because of the lower activation energies of such reactions, the route via carbenium ions usually dominates over mechanisms involving carbonium ions. Reactions of the carbonium-ion type are terminated by a step in which a proton is transferred to the zeolite lattice. 

Reactions in which the a carbon possesses a carbonium ion type structure in the transition state are greatly accelerated by complexation. Ionization reactions of benzylic derivatives, i.e., the solvolysis of chlorides (Holmes et al, 1965) or the isomerization of thiocyanates or isothiocyanates (Ceccon, 1971), are accelerated by a factor of 10 -10 . This extraordinary donating effect of the Cr(CO)3 group has been interpreted either as a direct interaction of a filled metal d orbital with the empty carbon p orbital, or as metal-... 

This type of polymerization takes place by the addition of monomer molecules to a positively charged growing chain, known as a carbonium ion. The reaction is initiated usually by the donation of a proton (H ) to a monomer molecule by a strong acid. For example an ordinary mineral acid such as perchloric acid will initiate the polymerization of styrene in ethylene dichloride solution... 

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

The susceptibihty of dialkyl peroxides to acids and bases depends on peroxide stmcture and the type and strength of the acid or base. In dilute aqueous sulfuric acid (<50%) di-Z fZ-butyl peroxide is resistant to reaction whereas in concentrated sulfuric acid this peroxide gradually forms polyisobutylene. In 50 wt % methanolic sulfuric acid, Z fZ-butyl methyl ether is produced in high yield (66). In acidic environments, unsymmetrical acychc alkyl aralkyl peroxides undergo carbon—oxygen fission, forming acychc alkyl hydroperoxides and aralkyl carbonium ions. The latter react with nucleophiles,... 

Although in general azoles do not undergo Friedel-Crafts type alkylation or acylation, several isolated reactions of this general type are known. 3-Phenylsydnone (120) undergoes Friedel-Crafts acetylation and Vilsmeier formylation at the 4-position, and the 5-alkylation of thiazoles by carbonium ions is known. 

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