3- Methylhexane dehydrocyclization

Methyl groups, as hydrocarbon surface species, vibrational spectra, 42 214—219 Methylheptane, ring closure, 25 154 3-Methylhexane dehydrocyclization, 30 13 isomerization, 30 7, 14, 39-40 Methylhexane, ring closure, 25 155 Methyl hydroperoxide, catalytic decomposition, 35 161... 

In some cases, a complete estimation of the relative contributions of the various pathways of cyclic and bond shift types requires the simultaneous use of a number of C-labeled molecules. Thus far the most complicated example is the isomerization of 3-methylhexane. This molecule, which dehydrocyclizes in three different ways, to 1,2-dimethylcyclopentane, 1,3-dimethyIcyclo-pentane, and ethylcyclopentane (Scheme 17), may isomerize by 23 different pathways, consisting of both cyclic and bond shift types. In particular, four parallel pathways account for n-heptane, and five for self-isomerized 3-methylhexane (4J). Therefore, even when using all the possible labeled molecules, one cannot distinguish between all the isomerization pathways, since the complete location of C in 3-methylhexane cannot be completely achieved. 

Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C7-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is... 

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

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