Methylpentanes interconversion

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. 

A silicon version of the well-studied electrocyclic interconversion of C4H6 (cyclobutene, bicyclobutane, butadiene, etc.) has been investigated both experimentally23,101 and theoretically.102,103 Irradiation of 51 in 3-methylpentane (X >420nm) gives a 1 9 mixture of 51 and bicyclo[1.1.0]tetrasilane 88 at the photostationary state as determined by UV-vis spectroscopy.23 When the mixture is left for 12 h in the dark at rt, 51 is produced quantitatively [Eq. (58)]. The photochemical conversion of 51 to 88 and the thermal reversion can be repeated more than 10 times without any appreciable side reactions. 

FIG. 14. Nonselective ring opening of methylcyclopentane in interconversion ofn-hexane, 2-methylpentane, and 3-methylpentane (62). 

The rates at which the above conversions take place vary considerably as shown in Table II. The interconversion of 2-methylpentane and 3-methylpentane is extremely rapid while the isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane is very slow. Thus, the ratedetermining step in the formation of 2,2-dimethylbutane (neohexane) from n-hexane is the slow isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane. In the isomerization of w-hexane to 2,3-dimethylbutane, the rate-determining step is the isomerization of n-hexane to the methylpentanes. 

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

It was suggested that the interconversion between 2-methylpentane and 3-methylpentane occurs mainly by the 1,2- and 1,3-alkyl shifts in the hexyl cations which accompany rapid hydride shift. The rate of alkyl shift is in the order,... 

Interconversions of 2-methylpentane 2,3-dimethylpenttuie as well as 3-methylpentane 3= 2,3-dimethylpentane proceed probably via protonated cyclopropane-type intermediates (IV) ... 

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