Carbenium ion, isomerization

The isomerized structure dominates even at - 100° C, but accounts for only 70% of the repeat units at - 130° C. Similar but more complicated structures are formed in 4-methyl-1-butene polymerizations by competing hydride and methide shifts [298]. Other monomers whose propagating carbenium ions isomerize include 5-methyl-l-hexene, 4,4-dimethyl-1-pen-tene and some terpenes [299]. 

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. 

Catalytic cracking proceeds mainly via carbenium ion intermediates. The three dominant reactions are cracking, isomerization, and hydrogen... 

Decomposition of the adsorbed carbenium ions is the main reaction charmel. However, isomerization (aromatization) and oligomerization reactions also proceed, and are the route to coke formation. 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

A non-acidic isomerization catalyst system has unexpectedly emerged from recent studies by French workers [4] in the area of Mo-oxycarbides. Although at an early stage of development, these new materials exhibit high selectivities for the isomerization of paraffins such as n-heptane. An alternative non-carbenium ion mechanistic route to achieve isomerization of higher alkanes could potentially overcome some of the limitations of conventional solid acid based catalyst systems. 

The classical HCK mechanism on bifunctional catalysts separates the metallic action from that of the acid by assigning the metallic function to the creation of an olefin from paraffin and the isomerization and cracking of the olefins to the acid function. Both reactions are occurring through carbenium ions [102],... 

When -butenes are used, the initiation produces a secondary carbenium ion/butoxide. This species may isomerize via a methyl shift (Reaction (2)) or accept a hydride from isobutane to form the tertiary butyl cation (Reaction (3)). Isobutylene forms the tertiary cation directly. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

However, the behaviour near m = raB needs some other explanation. My proposal involves the specific solvation of the backside of the carbenium ion by the strong dipole of the solvent this displaces the monomer molecule which is located there in the absence of the solvent, so that the 7t-bond to the monomer at the front is weakened and the unimolecular isomerization-propagation becomes accelerated, despite the statistical factor which, alone, would produce a deceleration, as explained at the end of Section 3a. As the dilution proceeds from m = raB downwards, the polymerization goes through a dieidic phase, in... 

To explain the very varied behaviour patterns shown by the various monomers in various solvents, use has been made of a further, hitherto unrealized, implication of the model, namely that the rate of the isomerization-propagation must depend upon the electrochemical environment of the complex. This vague idea has been given precision by concentrating attention on the species which occupies the site at the back-side of the near-planar carbenium ion, the front-side of which is 7t-bonded to the double bond of the monomer. The idea is that the stronger the dipole at the back, the weaker is the Jt-bond, and the lower is the energy of the transition state, and therefore the greater is the rate. 

The formation of both isomeric chlorides 147 and 148 and the corresponding methoxy adducts 149 and 150 in methanol is at variance with the behavior observed in AcOH/ LiC104, where only the acetoxy species 140 is formed. This has been interpreted by taking into account the possible role of a specifically solvated carbenium ion pair, such as 145, prior to the formation of a free carbenium ion of type 144. 

However, acid-catalyzed isomerization attracts more attention, probably due to its connection with the recent intensive development of carbenium ion chemistry. It is common knowledge that effective methods for stabilization of reactive carbocations have been known since 1962 while base-catalyzed processes with the participation of carbanions were developed more than 100 years ago. 

Example The effects of isomerization upon thermal stability of butyl ions, C4H9 are impressing. This carbenium ion can exist in four isomers with heats of formation that range from 837 kJ mol in case of -butyl over 828 kJ mol for wu-butyl (also primary) to 766 kJ mol for 5cc-butyl to 699 kJ mol in case of t-butyl, meaning an overall increase in thermodynamic stability of 138 kJ mol (Chap. 6.6.2) [36]. 

Example The mass spectra of both acetone and butanone show typical acyiium ion peaks at m/z 43, whereas the signals in the spectra of isopropyl ethyl thioether (Fig. 6.9), of 1-bromo-octane, (Fig. 6.10), and of isomeric decanes (Fig. 6.18) may serve as examples for carbenium ion signals. The superimposition of both classes of ions causes signals representing an average pattern. The properties of larger carbenium ions are discussed in the section on alkanes (Chaps. 6.6.1 and 6.6.3). 

As pointed out, carbenium ions make up the largest portion of alkane mass spectra and dissociate further by alkene loss. This type of fragmentation is rather specific, nevertheless isomerizations of the carbenium ion can happen before. Generally, such processes will yield a more stable isomer, e.g., the tcrt-butyl ion instead of other [C4H9] isomers (Fig. 6.19). 

Hydride shift is a fast elementary step that enables the positive charge to move around the carbocation ion to ultimately achieve thermodynamically the most favorable configuration. 1,2 hydride shift in the secondary butyl carbenium ion is illustrated in Figure 13.23. The use of this common step during alkene skeletal isomerization is further discussed in Section 13.8.1. 

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