Mechanisms skeletal isomerisation

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

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-...

Scheme 14.6. Skeletal isomerisation of 2-methylpentane-2- C cyclic mechanism.

Work on the remaining pairs of metals in Groups 8 to 10 can be dealt with swiftly. With PtCo/NaY, the importance of the cyclic mechanism for isomerisation of 2-methylpentane was enhanced at cobalt concentrations above 22% PdCo/NaY was more active for isopentane isomerisation than Pd/NaY, but as we have already seen Co/NaY was itself very selective for skeletal isomerisation, ... 

Double bond isomerisation of alkenes is promoted over basic catalysts, where the carbon skeleton is retained, in contrast to the action of acid catalysts which are important catalysts for skeletal isomerisation. The difference lies in the mechanism, which for basic catalysts involves carbanions, rather than the carbenium ions that form in the presence of acids (Scheme 9.20). 

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