Neohexane hydrogenolysis

It is both interesting and important to note that, judged from the behavior of neohexane, hydrogenolysis over platinum, in contrast to nickel, strongly favors rupture of C—CqUart bonds. The mechanistic implications of this will be discussed subsequently. 

The study of archetype molecules. This method has been proposed and widely used by Rooney, Burwell, Anderson, and others (see, for review, 155,156,160). In this method a molecule is used which can form an archetype of chemisorbed complex ( caged molecules as derivatives of ada-mantane or ethane in its hydrogenolysis, neopentane in exchange with D2 or in reforming reactions, etc.) or which can form several complexes, but the contribution of these complexes to the overall mechanism is easily derived from the product spectrum [as is the case, for example, with neohexane (167, 168). ... 

Fig. 4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known ( ) and the speculative aspects ( ) of the 3C complex formation are indicated. 

A study on neohexane reactions revealed (167, 168) that the inactive additives suppress hydrogenolysis of neohexane in a way similar to that which suppresses hydrogenolysis of hexane. It can be further seen (see Fig. 16) that these additives do not substantially alter the regularly observed preference of Pt for the reactions induced by 3Cay complexes, as opposed to those which are induced by 2Ca/J complexes. Alloying with Au or Ag only strengthens this preference. Rather surprisingly, the addition of Cu has quite different consequences (see below). 

Fig. 16. Selectivity in the formation of various adsorbed 3C and 2C complexes upon reactions of neohexane with hydrogen for Pt/Si02, Pt-Ag/Si02, and Pt-Au/Si02 with compositions and temperatures of reactions as indicated (H-hydrogenolysis).

As mentioned before, nickel usually catalyzes demethylation. For example, besides methane, mainly neopentane (as well as much less ethane and isobutane) is formed when neohexane undergoes hydrogenolysis on nickel.251 264 In contrast,... 

Since in C2-unit hydrogenolysis both carbon atoms of the C—C bond to be broken must be primary or secondary (isobutane cannot cleave in C2-unit mode through adsorption of its tertiary carbon atom), Anderson formulated the cleavage of neohexane according to Eq. (11.80) involving carbon-metal double bonds (1,2-dicarbene mechanism) 267... 

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