Dimethylcyclopentanes isomerization

A cis/trans mixture of 1,2-dimethylgermacyclopentanes 56 and 57 was prepared as outlined in Scheme 1632. The isomeric germanes are stable and can be separated by spinning band distillation. Stereochemical assignments were made by comparison of the chemical shifts of the methyl carbons in the 13C NMR spectra with those of the analogous dimethylcyclopentanes and silacyclopentanes. 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

Dimethylcyclobutane, isomerization, 30 31 Dimethylcyclohexane conformational analysis of, 18 14 experimental equilibrium, 18 17 epimerization of, 25 136 vibrational spectra, 42 239 Dimethylcyclohexylbenzene, 42 432 Dimethylcyclopentanes aromatization, 30 54 isomerization, 30 34... 

A fourth type of petroleum isomerization, which was commercialized on a small scale, involves the rearrangement of naphthenes. In the manufacture of toluene by dehydrogenation of methylcyclohexane, the toluene yield can be increased by isomerizing to methylcyclohexane the dimethylcyclopentanes also present in the naphtha feed. This type of isomerization is also of interest in connection with the manufacture of benzene from petroleum sources. 

Naphthene Isomerization. In addition to the paraffin isomerization processes, naphthene isomerization also proved useful during the war in connection with the manufacture of toluene. In the Shell dehydrogenation process for the manufacture of toluene, good yields depend upon increasing the methylcyclohexane content of the feed by isomerization of dimethylcyclopentanes. This process was employed commercially at one refinery in the Midwest and one on the Pacific Coast. 

Methylcyclohexane undergoes little isomerization when treated with aluminum halides because the equilibrium between the dimethylcyclopentanes and methylcyclohexane greatly favors the latter. 

Cis-trans isomerism is also possible when there is a ring present. Cis- and trans-1,2-dimethylcyclopentane are geometric isomers, and they are also diastereomers. The trans diastereomer has an enantiomer, but the cis diastereomer has an internal mirror plane of symmetry, so it is achiral. 

Essential features of the Shell naphthene isomerization process (24) are outlined in Figure 26. Although the contactor principle employed in the other liquid-phase Shell processes is used, the catalyst is handled in the form of hydrocarbon complex. A carefully fractionated and dried concentrate of dimethylcyclopentanes is preheated to 200°F., and about 0.1% of anhydrous hydrogen chloride is added. The feed is joined by a stream of catalyst complex and charged to the reactor under a gauge... 

Still larger quantities of aromatic hydrocarbons are needed, and these are synthesized from alkanes through the process of catalytic reforming (Sec. 9.3). This can bring about not only dehydrogenation as in the formation of toluene from methylcyclohexane, but also cyclization and isomerization as in the formation of toluene from //-heptane or 1,2-dimethylcyclopentane. In an analogous way, benzene is obtained from cyclohexane and methylcyclopentanc, as well as from the hydrodealkylation of toluene. 

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. 

Further information was then provided by the study of the hydrogenolysis of cis- and trans-dimethylcyclobutanes on metal films (S9). In contrast to the rapid cis-trans interconversion of 1,2-dimethylcyclopentanes under any condition, the cis-trans isomerization of 1,2-dimethylcyclobutanes is slow on platinum films and negligible on nickel and rhodium films. This allows the initial product distributions to be determined. 

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). 

The situation is more complicated in the case of the pinacolic rearrangement of the isomeric cyclopentane-1,2-diols (Bunton and Carr, 1963b). In aqueous perchloric acid, cis-1, 2-dimethylcyclopentane-l,2-diol (19) is converted into a mixture of the polymeric cyclopentadiene). No O18 is found in the unrearranged cis-diol. The trans-1,2-dimethylcyclopentane-1,2-diol (20), on the other... 

The combined liquid effluent from the fluidized-bed run was collected, and a complete analysis was made. The data are shown in Table V. The extensive isomerization taking place is of some interest all of the C6 paraffins except diisopropyl are present in the effluent, along with cyclohexane-methylcyclopentane, MCH-dimethylcyclopentane, etc. Nickel is, of course, known to have isomerizing activity, but in addition the catalyst may be functioning as a dual-function isomerization catalyst. 

Ruthenium deposited on aluminum oxide is very active with respect to the isomerization of ethylcyclopentane to 1,2- and 1,3-dimethylcyclopentanes. 

Mention should be made of the high activity of ruthenium deposited on alumina in the isomerization reaction both to methylcyclohexane and especially to the mixture of trans- and cis-1,2- and 1,3-dimethylcyclo-pentanes, the dimethylcyclopentane content in the cyclane-alkane portion of the catalysate obtained in the presence of this catalyst being 60%, of which 85-90% constituted frans-1,2Kiimethylcyclopentane. The cyclane-alkane portion of the catalysate obtained on contact with Pd-Si02 also contained 10 % dimethylcyclopentanes. 

Investigating the behavior of methylcyclohexane in contact with the carriers alone (AljOg and SiOa) under the given conditions, it was foimd that at pressures of 20, 35, and 50 atmospheres silica gel brings about a slight isomerization to irons- and cis-l,2-dimethylcyclopentanes, but that aluminum oxide has the same effect only at a pressure of 50 atmospheres. 

The lack of free rotation around C—C bonds in disubstituted cycloalkanes leads to an extremely important kind of isomerism called stereoisomerism. Two different compounds that have the same molecular formula and the same structural formula but different spatial arrangements of atoms are called stereoisomers. For example, consider a molecule of 1,2-dimethylcyclopentane. The cyclopentane ring is drawn in I Figure 1.17 as a planar pentagon with the heavy lines indicating that two of the carbons are in front as one views... 

This result confirmed the earlier work of Whitmore, F. C. Johnston, F. /. Am. Chem. Soc. 1933, 55,5020, who had found that the addition of HCl to 3-methyl-l-butene without solvent, in a sealed reaction tube for 7 weeks, gave both 2-chloro-3-methylbutane and 2-chloro-2-methylbutane. This result contradicted the suggestion of earlier investigators that the 3° alkyl halide was formed by rearrangement of the 2° alkyl halide formed from the addition reaction. Hammond, G. S. Collins, C. H. /. Am. Chem. Soc. 1960, 82, 4323 found that addition of HCl to 1,2-dimethylcyclopentene produced Tchloro-frans-l,2-dimethylcyclopentane as the major (perhaps only) addition product, but it isomerized to l-chloro-c/s-l,2-dimethylcyclopentane. 

Here s a difficult one. There are two different substances named trans-1,2-dimethylcyclopentane. What is the relationship between them (We ll explore this kind of isomerism in the next chapter.)... 

Make a model of 1,2-dimethylcyclopentane. How many stereoisomers are possible for this compound Identify each of the possible structines as either cis or trans. Is it apparent that cis-trans isomerism is possible in this compound because of restricted rotation Are any of the stereoisomers chiral What are the relationships of the 1,2-dimethylcyclopentane stereoisomers ... 

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