Alkene Skeletal Isomerization

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. 

Beta scission of a carbenium ion is an elementary step that is inihated by the weakening of the bond beta to the positive charge, leading to a smaller carbenium ion and an alkene. This elementary step is further discussed in Sections 13.8.1, 13.8.3.1 and 13.8.4 within the context of alkene skeletal isomerization, isobutane-2-butene alkylation and alkane cracking, respectively. 

Alcohol Substitution. In the early period of normal thiol production, the normal alcohols were utilized as feedstocks. The use of a strong acid catalyst results in the formation of a significant amount of secondary thiol, along with other isomers resulting from skeletal isomerization of the starting material. This process has largely been replaced by uv-initiation because of the higher relative cost of alcohol vs alkene feedstock. 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

This mechanism is supported by reports of comparable reaction rates obtained during skeletal isomerization with the corresponding alkenes over metal-free zeolites [52]. 

All these results were interpreted by a free-radical mechanism with the involvement of alkenes, and smaller (C3,C4) and larger (C7,C8) ring intermediates in aromatization. Skeletal isomerization was found to occur through vinyl shift and C3,C4 cyclic intermediates.202 Transition metals with the exception of Fe and Os, as well as Re, Co, and Cu, are active in aromatization of alkanes. Platinum,... 

Dorbon M, Chodorge JA, Cosyns JA, Viltard JC, Didillon B. Method for producing high-purity isobutene through hydroisomerization reactive distillation and skeletal isomerization ofc4 alkenes. FR 2757506, Institut Frangais du Petrole, 1998. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Thermodynamical the skeletal isomerization of alkenes is fiivoured at low tenq>eratures and the rec rocal tenqrerature increases with increating carbon number. The equ rhim concentration of isobutene in the fraction of butenes decreases from ca. 50 % at 200°C to 37% at 500°C [149]. Thus, the convertion of n-butoies into isobutene at these temperatures will be limited by thermodynamic constraints. The skeletal isomerization of the alkenes with more than 4 carbon atoms is a relatively dle reaction step, vdiich is carried out at ca. 290°C over H-Ferrierite [150] or at 340 C over ZSM-5 [151]. This reaction proceeds via the skeletal rearrangement of a carbenium ion yielding a secondary carbenhun ion. The angular reaction meclumism indicates that side product formation can be minimized. Even the skeletal isomerization of C5- and C5-alkanes over Pt-Mordenite, vtiiich is thought to proceed... 

Isomerization of n-paraffin, especially normal pentane to iso-pentane is essential for making high octane gasoline with low aromatics content. Isomerization of lower paraffins has been conducted in the solid catalyzed gas-phase reaction system by using noble metal-supported solid acid under hydrogen atmosphere. The most predominant reaction mechanism for the isomerization of alkane is as follows (1) the dehydrogenation of alkane to alkene on the supported metal (2) proton addition to the alkene to form carbenium ion on the acidic component (3) skeletal isomerization of the carbenium ion on the acidic component (4) deprotonation of the isoraerized carbenium ion to form alkene on the acidic component (5) hydrogenation of the alkene to alkane on the metal [1]. 

It is generally agreed that surface dehydration, typically at 600—800 K in vacuo, is a prerequisite for activity. There is still some debate as to whether ZnO must be non-stoicheiometric or not for activity to be observed, but it will become clear that it is now generally accepted that Zn-O pair sites are the active surface centres. As expected for a weak Bronsted acid, reactions of non-cyclic alkenes are limited to non-skeletal isomerizations even at 573 K isobutene does not isomerize to n-butenes. ... 

Alkenes and alkynes exhibit skeletal isomerism in which the carbon chain is varied and positional isomerism where the position of the multiple bond is different. Functional isomers differ in the class of compounds to which they belong. For example, functional isomers of an alkyne could be a diene, cycloalkene, or bicyclic alkane. 

The present work describes catalysts for the selective synthesis of alkenes and for the skeletal isomerization. 

An alternative strategy is to catalyse the skeletal isomerization of an alkene without destroying the double bond. 

The activity for skeletal isomerization of alkenes was only achieved after a partial reduction of the yellow, WO, component of the catalyst. This reduction was made by flowing a mixture of hydrogen and water vapour in a ratio about 40 1 over the catalyst at 400°C for 16 hours. Under these conditions WOg (yellow) is partially reduced to a dark blue oxide, suggesting that W qO q (blue) and... 

The practical application of a skeletal isomerization catalyst for alkenes are numerous. There is an increasing interest in conventional petroleum refining in optimizing the use of light alkenes both to increase liquid yields and at the same time improve octane quality. 

The Puddephatt-Tipper team " have shown that reductive elimination involving the formation of cyclopropanes from platinacyclopropanes appears to involve a concerted process rather than the production of carbene-alkene intermediates (as does also the oxidative addition involving the reverse reaction, and the skeletal isomerization of platinacyclopropanes). They " have also proposed a similar concerted behavior for a reaction which could be looked upon either as a reductive elimination or a substitution, namely, the overall process in equation (46). 

The activity of OH groups on the alumina surface can be markedly enhanced by the proximity of Cl ions. In the commercial catalysts, 7-alumina is treated with HCl to make it a highly active catalyst. 7-Alumina cannot readily catalyze the skeletal isomerization of alkenes because of its weak acidity. On the other hand, chlorinated alumina is highly active for skeletal isomerization and other strong-acid catalyzed reactions which are desirable in reforming. The strength of acid sites can be controlled by the extent of chlorination. If the Cl content is too low, the reactions which occur on the acid centers slow down and the octane number of reformate drops. If excess Cl ion is present, the extent of hydrocracking increases relative to dehydrocyclization. 

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