3- Methylhexane isomerization

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... 

A special case of isomerization is the racemization of optically active compounds catalyzed, for example, by sulfuric acid or promoted A1C13. Thus, treatment of (+)-(5)-3-methylhexane at 60°C with 96% sulfuric acid yields a mixture of racemic 2- and 3-methylhexane68 (Scheme 4.4). At lower temperature (0 or 30°C), racemization occurs, but shift of the methyl group does not take place. It can be concluded that at 60°C methyl migration is faster than hydride abstraction to yield isomeric alkanes. At 0 or 30°C, hydride transfer occurs before methyl... 

The isomerization of the olefin prior to its hydroformylation has been the explanation of this question (3) and the formation of isomeric aldehydes was related to the presence of isomeric free olefins during the hydroformylation. This explanation, however, is being questioned in the literature. The formation of (+) (S) -4-methylhexanal with an optical yield of more than 98% by hydroformylation of (+) (S)-3-methyl-l-pentene (2, 6) is inconsistent with the olefin isomerization explanation. Another inconsistency has been the constance of the hydroformylation product composition and the contemporary absence of isomeric olefins throughout the whole reaction in hydroformylation experiments carried out with 4-methyl-1-pentene and 1-pentene under high carbon monoxide partial pressure. The data reported in Ref. 4 on the isomeric composition of the hydroformylation products of 1-pentene under high carbon monoxide pressure at different olefin conversions have recently been checked. The ratio of n-hexanal 2-methylpentanal 2-ethylbutanal was constant throughout the reaction and equal to 82 15.5 2.5 at 100°C and 90 atm carbon monoxide. 

The results for the isomerization of n-heptane are presented in Table 20.5. Over all the catalysts the main products are again the 2-methyl (M2H) and 3-methylhexanes (M3H), with a significant contribution from the dimethylpentanes of 12-13% over the platinum and Mo2C-oxygen-modified catalysts, and 21% over the Mo03-carbon-modified catalyst. 3-Ethylpentane always contributes around 3% to the isomer distribution and almost no cyclic products are observed. Increasing the pressure over the... 

Figure 20.10 Comparison of 3-methylhexane/n-heptane (M3Hex/Hep) ratios for the isomerization of 2-methylhexane over Pt// -zeolite and molybdenum oxycarbide...

Formation of the 2-olefin was then conveniently measured by the percent racemization. Under conventional Oxo conditions the products were 4-methylhexanal (V), 3-ethylpentanal (VI), and 2,3-dimethyl-valeraldehyde (VII) in the ratio 93 4 3 and there was only 1.8% racemization of (V). At higher temperature the racemization of (V) increased to 32.2% at 145° and 93.9% at 180° C. The yield of (V) also dropped off to 61.3% at 180° C. The carbon monoxide pressures in these cases were in the range 71-95 atm. Decreasing the carbon monoxide pressure from 95 to 16 atm at 90° C increased the racemization from 1.8% to 8.7%. These results clearly show that under normal Oxo conditions (V) was mainly formed by a mechanism which did not change the stereochemistry at C3. The degree of racemization can be explained by the reversible formation of a 2-olefin intermediate. Pino et al. concluded that under conventional Oxo conditions, little isomerization occurred. 

A further difference with Johnson s work, which, however, received no comment, was the fact that under these conditions, 3-methylhexanal appeared as a product to the extent of 2.9% of the total aldehydes when only 1% of the olefin present had isomerized. Johnson found no 3-methylhexanal... 

Reactions of 3-methylhexane were found to occur on the support alone which complicated the above studies. Besides aromatization to toluene (A), isomerization to cyclopentane derivatives (B), isomerization to heptane and other branched hexanes (C) and cracking to Ci to C6 hydrocarbons (D) were all observed as depicted in Eq. 5.23... 

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 evidence for the existence of two bond shift mechanisms and also for the metallocyclobutane mechanism are provided by the isomerization of C labeled C7 hydrocarbons 2,3-dimethylpentane (3S), 2-methyl-hexane (45), and 3-methylhexane (47) on Pt/AljOj catalysts. 

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 formation of wetn-labeled toluene can be explained neither by direct 1-6 ring closure, nor by cyclic-type isomerization of n-heptane to 3-methylhexane followed by 1-6 ring closure of the latter (94). We suggest that the abnormal aromatization process responsible for the formation of meta-labeled toluene is initiated by a dicarbene as in the nonselective mechanism A (see Section IV, Scheme 47). Aromatization is not influenced by the dispersion of the platinum on the support (758), so that it may be assumed that aromatization involves a single metal atom. Isomerization of the dicarbenes (7) to the dicarbenes (8) via rt-adsorbed cyclopentanes, followed by isomerization to the suitable carbene-olefin species (9), would result in 1-6 ring closure and aromatization (Scheme 69). 

One can notice that the easiest route on all catalysts is the isomerization of 2-methylbutane-2- C to 2-methylbutane-3- C, involving certainly a symmetrical intermediate. This result, and the earlier mentioned one, fast isomerization of 3-methylhexane-3- C to 3-methylhexanes-2- C and -4- C (see Section III, p. 27), support the view that, irrespective of the exact nature of the mechanism, bond shift reactions involving symmetrical intermediates are favored over the other reaction pathways. 

Skeletal rearrangements of saturated hydrocarbons on CePd3 were studied by Le Normand et al. (1984). Hydrogenolysis of methylcyclopentane, isomerization of 2-methyl-pentane and aromatization of 3-methylhexane were performed at 350 or 360°C. The results were compared with those of classical Pd/Al203, Pd/Si02 or Pd/Ce02 catalysts. The activity of CePd3 itself was very low and palladium atoms seemed to play a minor role. For example, the initial reaction of methylcyclopentane was the selective formation of isopentane, which is not observed on classical palladium catalysts. Air treatment at... 

Could acidic minerals catalyze the isomerization of methylhexanes to dimethylpentanes The answer is yes under certain conditions. As extensive thermal cracking generates mixtures of lighter n-alkanes and a-olefms, conversion of a-olefms into mixtures of methyl-substituted alkanes is typical for olefin transformation in the presence of acidic catalysts. The acid-catalyzed transformation of hydrocarbon such as isomerization, alkylation, and cracking play an important role in the industrial processes of petroleum industry (Brouwer and Hogeveen 1972 Poutsma 1976 Corma and Wojciechowski 1985 Boronat et al. 1996). 

Increasing the chain length of the alkanes, conformational analysis of a series of methyl substituted hexanes indicates that the important potential energy profiles can be identified with those bonds which contain the methyl substituent. This has important consequences for polymers. The theoretical rotational isomeric potential profile for 2-methylhexane is shown in Figure 3.8. 

Any carbon atom with four different substituents in a tetrahedral arrangement exhibits optical isomerism. Consider 3-methylhexane ... 

The modification of this catalyst with zirconium introduced in the MCM-41 structure [20] resulted in a further improvement of the isomerization activity. Thus, the catalytic properties of the l%Pt/25%H3PWj20 /Zr-MCM-41 in the hydroisomerization of n-heptane at atmospheric pressure are characterized by the enhanced selectivity to multibranched isoheptanes. 2-Methylhexane (2-MH) was predominant in the monobranched isoheptanes, and 2,3-dimethylpentane (2,3-DMP) was the prevailing compound in the multibranched hydrocarbons. Among the cracking products, butanes and propane were formed. The formation of the multibranched isoheptanes showed a correlation with the mesopore diameters of the catalysts. 

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