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... 

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 ... 

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... 

It is interesting to compare this transition state in the solid with the one calculated from the HF-SbF5 system. In the liquid superacid, the ionic character is very strong and it is easier to connect the reactivity with the unusual activity of the proton even when solvated by the HF solvent. In contrast, on the solid the theoretical calculated transition state is further away from the carbonium ion type and in line with the much higher temperatures needed to activate the alkane with weaker acids. 

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. 

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. 

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. 

Active Sites in Zeolites. Based on the literature reviewed here, the main characteristics of carbonium ion type zeolite catalysts emerge in the following manner. 

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... 

A polar-type transition state is also supported by the occurrence of Wagner-Meer-wein rearrangements accompanying the elimination of hydrogen chloride, from neo-pentyl - " - - , bornyl > andisobornylchlorides - ° In the case of neopentyl chloride the formation of 2-methylbut-l-ene and 2-methyl-but-2-ene has been explained in terms of a non-classical carbonium ion type transition state. 

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. 

The following typical carbonium ion type of mechanism is consistent with the above data ... 

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. 

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. 

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