Isomerization saturated hydrocarbons, mechanism

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

Indications of the mechanism of isomerization of saturated hydrocarbons were obtained by Ciapetta (C3), who observed that olefins were isomerized over nickel-silica-alumina catalyst at appreciably lower temperatures than were the corresponding saturated hydrocarbons, suggesting that olefins were intermediates in the reaction. Ciapetta also suggested that the rearrangement of the carbon skeleton took place via a carbonium... 

While mechanically mixed catalysts have been shown to be successful for the isomerization or dehydroisomerization of saturated hydrocarbons,... 

Propane and cyclopentane give isopropyl chloride and cyclopentyl chloride, respectively, whereas isobutane is transformed to ferf-butyl chloride under the same reaction conditions (yields are 69%, 74%, and 76%, respectively). Neopentane undergoes isomerization to yield 2-chloro-2-butane (88%). When saturated, hydrocarbons were allowed to react with methylene bromide and SbF5 bromoalkanes were obtained in comparable yields (64-75%). Formation of the halogenated product can be best explained by the mechanistic pathway (I) depicted in Scheme 5.55. Since SbF5 always contains some HF, mechanism (II) may also contribute to product formation (Scheme 5.55). 

This is a mechanism of polyolefin cracking. The main polymer chains are reduced by reaction with protons or other carbonium ions, followed by chain scission giving C30-C50 oligomeric hydrocarbons [7], As a result of further, secondary cracking reactions by P-scission of C30-C50 hydrocarbons, gas and lower-molecular liquid C10-C25 hydrocarbons are produced. Other reactions are double bond and saturated hydrocarbon isomerization as well as methyl group shift . 

The mechanism described above does not explain the fact that imder controlled conditions, benzene in the presence of aluminum chloride-hydrogen chloride catalyst, inhibits not only the cracking but also the isomerization reaction (Table XXV, experiment 3) while in the absence of benzene cracking is the predominant reaction. The mechanism postulated above does not take into consideration the observations made that under controlled conditions saturated hydrocarbons such as methylcyclopentane, cyclohexane, or butanes (7, 23, 35) do not undergo isomerization, unless traces of olefins are present. 

Investigations which concern the mechanisms of skeletal rearrangements of saturated hydrocarbons induced by heterogeneous transition metal catalysis are of great interest for industrial applications, e.g. for petroleum reforming processes The developments in this field were reviewed recently by Hejtmanek, and by Maire and Garin , who focussed on the probable reaction mechanisms which include bond-shift and cyclic mechanisms for the skeletal isomerization of acyclic alkanes. Scheme 1 summarizes the... 

The mechanism of skeletal isomerization of saturated hydrocarbons can be considered as a chain reaction. The initiation step is to form the first car-bocation (eq. 42) followed by its isomerization to another, usually more stable intermediate carbocation (eq. 43). The final propagation step is chain transfer through hydride ion transfer between the isomeric carbocation and the starting... 

Thus, study of the kinetics of n-pentane isomerization on H-mordenite leads to the conclusion that the mechanism of the reaction in question is different from that of isomerization on bifunctional and metal-zeolite catalysts. This difference lies in the manner of carbonium ion formation. With bifunctional catalysts, carbonium ion originates with the attachment of a proton to the olefin molecule, while with H-mordenite it originates as a result of splitting off hydride ion from the saturated molecule of the starting hydrocarbon by mordenite proton, as has been suggested by the above reaction scheme. 

The mechanism given above is merely a suggestion, based on presently available data, as to the function of the decomposition inhibitors in the isomerization of n-pentane. It is hoped that a systematic study of the kinetics of isomerization of saturated liquid hydrocarbons in the presence and absence of cracking suppressors, may furnish additional information about the mechanism of isomerization. 

The mechanism originally proposed by the writer for the cracking reaction invoked both carbonium ions and carbanions, depending on whether the hydrocarbon had unsaturated or saturated carbon-carbon bonds. With olefins or aromatics protonation to form a carbonium ion is relatively easy. However, at that time it was not obvious how a paraffin can be converted to a carbonium ion. So it was postulated that water might extract a proton from the paraffin to form in this case a carbanion. However, this concept was very soon abandoned because it was inconsistent with the observed rearrangements (isomerization) in reactants and products of cracking. An all-cationic mechanism was then proposed ( ) in which the activation of paraffins occurs by hydrogen transfer to form a carbonium ion intermediate. 

The organization is fairly classical, with some exceptions. After an introductory chapter on bonding, isomerism, and an overview of the subject (Chapter 1), the next three chapters treat saturated, unsaturated, and aromatic hydrocarbons in sequence. The concept of reaction mechanism is presented early, and examples are included in virtually all subsequent chapters. Stereoisomerism is also introduced early, briefly in Chapters 2 and 3, and then given separate attention in a fuU chapter (Chapter 5). Halogenated compounds are used in Chapter 6 as a vehicle for introducing aliphatic substitution and elimination mechanisms and dynamic stereochemistry. 

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