Skeletal isomerisation

On solid acid—base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 

Scheme 1. Skeletal isomerisation of tetrabydrodicyclopentadiene to form adamantane.

However, in general, the free radical process is not amenable to controlling the degree of polymerisation and gives low product yields. The products of free radical oligomerisation tend to have poor viscosity/temperature characteristics due to skeletal isomerisation during tlie process. 

There are a few other references in the scientific literature (as well as a number in the patent literature) to chemically fixed supported Lewis acids. Iron(III) chloride should be reactive enough to form surface OFeCk bonds, for example, and a stable form of supported FeCl3 has been reported.121 The solid acid has been used to catalyse liquid phase Friedel-Crafts benzoylations, although the surface structure and activity on reuse of the catalyst have not been described. At least one commercial form of supported iron(III) chloride is available." Supported SbFs has also been extensively studied, although mostly for gas-phase reactions such as the skeletal isomerisation of alkanes where it exhibits high activity indicative of its strong acidity (H0 = ca. —14).122... 

In Fig. 9 we sketch two process routes to isobutene. Starting from n-butane, isomerisation to isobutane is followed by dehydrogenation to isobutene. Both process steps have been widely operated commercially. The second step involves a capital intensive dehydrogenation reaction. Because of the intensity large plants are called for (economy of scale). An alternative and in principle less capital-intensive route is the skeletal isomerisation of n-butenes to isobutene. Previously this was not economically attractive because of the limited lifetime of the catalyst and the limited yield of isobutene. The latter drawbacks were due to the high temperatures applied previously (>400°C) and the related fast coke formation. 

Figure 10. Skeletal isomerisation of n-butenes effect of temperature (catalyst ferrierite, feedstock 1-butene).

MTBE manufacture from FCC-produced butenes can at least be doubled by skeletal isomerisation of normal butenes to isobutene. Addition of ZSM-5 to the FCC catalyst inventory may be applied to quadruple the MTBE output. 

On pure zirconium oxide, 4-methylpent-2-ene is favoured compared to the alkene with the double bond in terminal position, which is the desired product. The skeletal isomerisation reaction occurs to a rather low extent and the dehydrogenation is almost negligible. 

Steric interactions between the alkyl substituents and the catalyst surface cause trans-o tfin formation to be less favourable than cis. Dehydration of 2-methyl-1-propanol over a variety of alumina catalysts gives isobutylene (77-88%), similar amounts of 1-butene and r/i-2-butene (4-10%) and smaller yields of tra 5-2-butene (2-4%). Non-classical rather than classical intermediates were suggested, as the least acidic alumina, which possessed the lowest dehydration activity, caused the greatest skeletal isomerisation (173). 

We have met the breaking of C—C bonds by hydrogen already in Chapter 11, but the molecules considered there (cyclopropane and cyclobutane) had some degree of alkene-like character and reacted easily (especially the former). In this chapter we shall be involved with linear and branched alkanes having two, three or four carbon atoms. C—C bond fission is the principal process, but with the butanes skeletal isomerisation is also possible, and dehydrogenation sometimes happens at the same time. Reactions of acyclic and cyclic alkanes having five or more carbon atoms feature in the following chapter, where isomerisation and dehydrocyclisation are the important reactions. Some limited overlap between this chapters and the next is unavoidable. 

Scheme 14.1A. Schematic representation of the mechanism of skeletal isomerisation on a bifunctional catalyst.

Scheme 14.4. Cyclic intermediates in the skeletal isomerisation of C7 alkanes. Routes only feasible by bond-shift are shown by dark arrows.

What if anything can we then be certain of The routes whereby skeletal isomerisations proceed are very clearly indicated by isotopic labeUing experiments (Section 14.3.2), but even here the structures devised to explain them have relied heavily on organic chemical intuition, and have often involved multiple carbon-metal bonds (i.e. carbenes, C=M, and carbynes, C M) that stretch the imagination to near-breaking point. Formal multiple C—M bonds of this type, i.e. having a tt-component, are now thought unlikely, and representation of, for example, C=M... 

There are two separate and distinct mechanisms by which skeletal isomerisation can occur (i) the bond shift mechanism, and (ii) the C5 cyclic mechanism. The first is clearly the only possibility when there are less than five carbon atoms in the chain so the way of isomerisation of n-to isobutane has to be by bond-shift. Two somewhat different mechanisms with a number of minor variations have been proposed. The first involves an actual or virtual cyclopropanoid species formed... 

Scheme 14.5. Skeletal isomerisation of 2-methylbutane-2- C the bond-shift mechanism via a cyclo-...

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