Methylpentanes skeletal isomerization

Much work has also been carried out on the reactions in the presence of H2 of branched alkanes, and on the mechanism of their skeletal isomerization [6, 7], The use of 13C-labeled molecules permits alternative reaction pathways to be distinguished. Thus, for example, most of the 3-methylpentane formed from 2-methylpentane has followed the bond-shift route, but most of the n-hexane has resulted from the cyclic mechanism. Labeled molecules also allow mechanisms of aromatization of C7 and Cg alkanes to be followed... 

C=C—C—C—C—C. Isomerization to a methylpentane would result in localizing the carbonium ion on the tertiary position. This structure can no longer ring close to a cyclopentane but only to a highly strained cyclobutane. Since skeletal isomerization occurs readily at reforming conditions, most of the n-hexane would be converted to isohexane or cracked products rather than to cyclopentane. 

The composition of a sample of commercial propene-oligomer gasoline (after hydrogenation) produced over solid phosphoric acid is illustrated in Fig. 4.16. The main products are liquid oligomers (di-, tri-, and tetramers) indicative of the high rate of the chain-transfer. The C9-carbons are predominandy doubly branched, consistent with cationic oligomerization. The structure of oligomers does not correspond exacdy to a simple reaction scheme due to skeletal isomerization. Thus, the dimers do not have exclusively the 2-methylpentyl skeleton, which is expected from the addition of the 2-propyl cation to propene. 3-Methylpentane and 2,3-dimethylbutane are also formed. The skeletons of these molecules may result from methyl shift in the 2-methyl — 3-pentyl cation. 

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

It was shown that the SbF5-intercalated graphite efficiently promotes disproportionation of various alkylbenzenes by simple mixing at room temperature (41). The isomerization of methylpentanes was carried out over the catalyst at room temperature, -30, and - I7°C in a continuous flow system a careful study of the kinetically controlled product distribution was performed to obtain information for the reaction path (42, 43). The skeletal rearrangements of l3C- abeled 2-methyl, 3-methylpentane and... 

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