Reactions of carbenium ions

Once formed, carbenium ions can form a number of different reactions. The nature and strength of the catalyst acid sites influence the extent to which each of these reactions occur. The three dominant reactions of carbenium ions are ... 

Other typical reactions of carbenium ions are alkene loss, provided sufficient chain length is available (Chap. 6.6.1), and dehydrogenation in case of the smaller ions such as ethyl, propyl, or butyl ion (Chap. 6.2.4.). 

The reverse reaction of carbenium ions with molecular hydrogen, can be considered as alkylation of H2 through the same pentacoordinate carbonium ions that are involved in C—H bond protolysis. Indeed, this reaction is responsible for the long used (but not explained) role of H2 in suppressing hydrocracking in acid-catalyzed... 

In addition to covalent species and carbenium ions, the equilibria may involve onium ions, which are formed by reaction of carbenium ions with noncharged nucleophiles [Eq. (46a)]. This decreases the carbenium ions lifetime, and therefore the time available for isomerization to more stable and less reactive carbenium ions via hydride and alkyl anion shifts [Eq. (46b)]. Decreasing the probability of rearrangements by decreasing the carbenium ions lifetime is especially useful because such rearrangements can not be prevented by decreasing the polymerization temperature. 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

The selectivity for reaction of carbenium ions with unsaturated oligomers increases not only in the absence of monomer, but also with decreasing temperature [182]. However, unsaturated macromonomers of styrene may dimerize rather than homopolymerize [cf., Eq. (96)]. That is, the molecular weight at complete conversion only doubles, rather than increasing further, because no copolymerization is possible in the absence of monomer. Because molecular weight only doubles [182], dimerization apparently dominates over Friedel-Crafts alkylation. The alkylation may... 

Similarly, isomerized carbenium ions are formed in polymerizations of a-methylstyrene derivatives by either a 1,3-intramolecular or bimolecular methide anion shift, or by reaction of carbenium ion with an exo-unsatu-rated oligomer [cf., Eq. (10)] [13]. 

The book is divided into eight chapters. The Introduction is a primer for both synthetic polymer chemistry in general, and cationic polymerizations in particular. More advanced readers may go directly to the following chapters. The second chapter covers the reactions of carbenium ions with various nucleophiles and focuses on the ionization of covalent species and the addition of carbenium ions to alkenes, arenes, and other ir-nucleo-... 

The reverse reaction of protolytic ionization of hydrocarbons to carbenium ions is the reaction of carbenium ions with molecular hydrogen giving their parent hydrocarbons [Eq. (6.6)]. It can be considered as alkylation of H2 by the electrophilic carbenium ion through a pentacordinate carbonium ion. Indeed, Hogeveen and Bickel have experimentally reduced stable alkyl cations in superacids to hydrocarbons with molecular hydrogen. 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

A self-consistent explanation of all the chemistry can be developed on the basis of Rri6nsted acidity of the zeolite, proton transfer and electrophilic methylation reactions, and the well known rearrangement, oligomerization and cracking reactions of carbenium ions. 

The formation and reactions of carbenium ions has been discussed in Section 2.10.4.2.2. 

In concluding this section, we note a number of other investigations. The reaction of carbenium ions with organosilylhydrides and organogermylhydrides in dichloromethane proceeds in a stepwise fashion, with at least one intermediate. The mechanism is thought to involve a single electron transfer at a carbon... 

Table 8.1 Selected reactions of carbenium ions relevant to acid catalysis over solid acids.

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

In the case of the butene isomers, the addition will lead to different isooctyl cations, depending on the isomer and the type of carbenium ion. The reactions involving s-butyl ions are likely to be negligible for liquid acid catalysts and of minor importance for zeolites. 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

As mentioned in Section 2.2, the complexing of carbenium ions with monomers is a well-accepted feature of the theory of cationic polymerisations, but it has not been realised clearly until recently that this implies the coexistence of first-order and second-order propagation reactions in certain systems over certain concentration ranges, i.e., the existence of (at least) dieidic polymerisations. 

Explanation The increase of both Y and DP with time implies a growing species of long life, which is characteristic of esters but not of carbenium ions, and an absence of transfer reactions. The suppression of all kinds of transfer reactions seems to require some very special features in the anionoid moiety of the ester which are not yet understood fully but which, by hypothesis, cannot affect the reaction pattern of an isolated, unpaired cation. The fact that living polymerisations can occur in toluene is a convincing demonstration that these reactions cannot involve carbenium ions, because growing cations alkylate toluene [33-37], a process which produces low DPs, independent of Y. 

In a review on the addition of carbenium ions to alkenes (equation 19) as a general procedure for carbon-carbon bond formation50, Mayr reported on investigations which also include the reactions of a variety of 1,3-dienes toward electrophilic carbon species generated by Lewis acid-promoted heterolysis of alkyl chlorides. 

The Ritter reaction [6] proceeds by the electrooxidation of alkyl iodides (56) in an MeCN-(Pt) system to form Ai-alkyl acetamides (58) (Scheme 21). Attack of carbenium ion intermediate - from dissociation of the initially formed alkyl cation radical - to acetonitrile would give the iminium cation (57). However, a different mechanism is proposed, whereby the alkyl iodide reacts with the electrogenerated iodo cation [I]" " [73]. 

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