Isomerization cyclic mechanism

Mixtures of alkenes are formed when more than one type of (3-hydrogen is present. In acyclic compounds the product composition often approaches that expected on a statistical basis from the number of each type of hydrogen. The E-alkene usually predominates over the Z-alkene for a given isomeric pair. In cyclic structures, elimination is in the direction that the cyclic mechanism can operate most favorably. 

Different approaches to the C5 cyclic mechanism have been put forward. Comparative studies of the isomerization of 2,2,3-trimethylpentane, 2,2,4-trimethylpen-tane, and 2,2,4,4-tetramethylpentane indicated161 that the latter did not display any appreciable isomerization on palladium below 360°C. Since this compound cannot undergo dehydrogenation without rearrangement to form an alkene, the cyclic mechanism on palladium is suggested to occur via the 1,2,5 triadsorbed species (20) bonded to a single surface atom. 

In most cases, the two types of mechanisms, the bond shift and cyclic mechanisms, are not exclusive but parallel pathways. With increasing molecular weight, the contribution of the cyclic mechanism increases and may become dominant. The pure selective mechanism on iridium is a unique exception. Hydrogenolysis, however, is the characteristic transformation on this metal. The nature of possible surface intermediates in metal-catalyzed alkane reactions, the role of electronic and geometric effects in their formation, and the relation of isomerization and hydrogenolysis have been reviewed.163... 

The product distributions in the ring opening of substituted cyclopentanes show striking similarities to those in isomerization of the corresponding alkanes (n-hexane, 2- and 3-methylpentane),301 which led to the formulation of the cyclic mechanism involving adsorbed cyclopentane intermediates to interpret alkane isomerization15,251,252 (see Section 4.3.1). 

For the isomerization 2-methylpentane to 3-methylpentane the percentage cyclic mechanism remains constant at about 20% as the dispersion is increased from 5 to over 35%. At higher dispersions the cyclic mechanism increases in importance and accounts for about 80% of the 3-methylpentane formed. The bond shift (BS) mechanism, therefore, becomes progressively less important as the dispersion increases. 

For the isomerization of 2-methylpentane to n-hexane the percentage of cyclic mechanism increases steadily from 70% at 50% dispersion to 100% at 100% dispersion. 

Yacaman and Gomez342 have analysed the results obtained by Gault etal. for the isomerization of 2-methylpentane. They suggest that the cyclic mechanism proceeds over a single Pt atom, but that bond shift requires either a B2 or B3 site. They obtain a fairly good correlation between the % cyclic mechanism and the ratio of Bx to (B2 + B3) sites. 

As discussed in Section IV, Barron et al. (55, 61) found the cyclic mechanism of isomerization to be predominant, perhaps the sole route, on a highly dispersed platinum-alumina (0.2% w/w Pt). The cyclic mechanism was shown to be important also over platinum films and supported platinum of moderate dispersion (>100 A). Here, although the product distributions were very different from that found over the dispersed catalyst, the initial product distributions at 300°C were practically identical in the isomerization and in methylcyclopentane hydrogenolysis. At lower temperatures they were somewhat different as they also were at all temperatures on platinum films. It was suggested that, especially on platinum films, a bond-shift isomerization could accompany the cyclic... 

In a further study, Muller and Gault (94) reported that isomerization of 2,3-dimethylbutane on thick platinum films yielded, as well as the expected bond-shift initial products (2-methylpentane and 2,2-dimethylbutane), substantial amounts of 3-methylpentane, n-hexane, and methylcyclopentane even at 273°C. Clearly, this is another example of a multistep mechanism. On the same basis, isomerization of 2,2-dimethylbutane should give only 3-methylpentane, 2,3-dimethylbutane, and 2-methylpentane as initial products in fact, Muller et al. report that n-hexane, methylcyclopentane, and benzene represented 15% of their initial products at 275°C. Somewhat in contrast to the situation for Pt/Al203, the number of surface reactions before desorption appeared to be no greater than two or three. It turns out that in the formation of 3-methylpentane the distribution was best explained by the succession of a bond shift and cyclic mechanism. This is quite distinct from the formation of n-hexane where two consecutive bond shifts occur. Perhaps in consequence of this difference, they conclude, a marked variation with temperature of the product distributions is observed. 

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... 

R. Zaera, D. Goodbey, and G.A. Somoiiai, Methylcyclopentane Conversion Over Platinum Single Crystal Surfaces Evidence for the Cyclic Mechanism of n-Hexane Isomerization, J. Catal., 101 (1986) 73. 

In tlie hydrogenolysis of methylcyclopentane, and the isomerization of hexanes, striking differences in initial product distributions on dispersed (0.2% 14-AI2O3) and concentrated (10% Pt-Al203) catalysts were ascribed to the occurrence of 2.0 and 20.0 nm crystallites, which promoted the cyclic mechanism or the bond shift + cyclic mechanisms, respectively. The possible roles of support acidity or secondary reactions were debated, for example ref. 182, which also lists earlier references. 

The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (52, 55). 

The cyclic mechanism was demonstrated by comparing the initial product distributions in the hydrogenolysis of methylcyclopentane and in isomerization of methylpentanes and -hexane. For instance, the ratios 3-methyl-pentane/n-hexane, extrapolated to zero conversion, are the same in hydrogenolysis of methylcyclopentane and in isomerization of 2-methylpentane. Since cyclic type isomerization involves first carbon-carbon bond formation and then carbon-carbon bond rupture, one does not expect hydrocracking of alkanes to occur by this mechanism. In contrast, as suggested early on (55), if bond shift isomerization involves first carbon-carbon bond rupture and then carbon-carbon bond recombination, a common intermediate should exist, leading to both the isomerization and the hydrocracking products. 

More generally, when more than two parallel pathways to the same molecule are possible, several reactant hydrocarbons labeled in various positions have to be used simultaneously. For instance, the isomerization of 2-methylpentane to n-hexane may happen in three different ways either by a cyclic mechanism, or by a methyl shift (A), or by a propyl shift (B) (55). In this case, 2-methylpentane-2- C allows one to estimate only the contribution of the bond shift (A) another molecule, 2-methylpentane-4- C, is required to estimate the contribution of the cyclic mechanism (56, 57) (Scheme 9). 

During the past two decades, the isomerization of pentanes and hexanes has very often been used as a test reaction in the study of particle size or alloying effects. In these studies, the product distribution, and especially the percentage of cyclic molecules (cyclopentane or methylcyclopentane), have often been used for estimating the contribution of the cyclic mechanism. 

The label has been located in the isomerization products obtained from 2-methylpentane-2- C, 2-methylpentane-4- C and 3-methylpentane-3- C on 10% platinum-alumina and single crystals under 20 Torr hydrogen pressure (54, 60). Under these conditions, the scrambling of the label was found to be extremely limited less than 10% of abnormal varieties are obtained. However, for these alkanes, on such catalysts and in these conditions, the selective cyclic mechanism is widely predominant and yields... 

Moreover, the comparison of the respective contributions of bond shift and cyclic mechanism in the isomerization of several labeled and C, hydrocarbons on 10% Pt/Al2O3 catalysts (Table III) shows that the contribution of bond shift to the overall isomerization process decreases with an increase in the number of carbon atoms, but not to the same extent for methyl shift and chain lengthening. Thus, on going from methylpentanes to... 

In a formal way, the isomerization of acyclic hydrocarbons according to a cyclic mechanism may be represented as the succession of three consecutive steps ... 

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). 

The metallocyclobutane mechanism, already invoked to account for some aspects of the bond shift and cyclic mechanisms, allows one to rationalize the mechanism of 1-5 ring closure-ring enlargement. This mechanism is best represented by Schemes 61 and 62 for the aromatization of 1,1,3-trimethylcyclopentane and 2,2,4-trimethylpentane, respectively. It is emphasized that in the former case carbene-olefin recombination must be favored over carbene isomerization to di-Tt-adsorbed olefin, since xylenes are the major reaction products while 2,4-dimethylhexane is not detected (69). 

Since 1-5 ring closure provides a route for the skeletal isomerization of alkanes, isomerization of substituted benzenes by a cyclic mechanism should also be possible. That was verified by Shephard and Rooney (95), who found that, on 0.5% Pt/Al2O3, interconversion of o-ethyltoluene and n-propyl-benzene accompanied dehydrocyclization to indane (Scheme 80). In these... 

In the case of n-pentylbenzene and 2-phenylpentane (759), the same types of isomerization, via bond shift and cyclic mechanisms, proceed, but the situation seems complicated by consecutive reactions occurring without... 

Fig. 8. Particle size effect in isomerization of 2-methylpentane-2- C to 3-methylpentanes contribution of cyclic mechanism.

However, the electronic factor seems to be of key importance on iridium. Here, isomerization of 2-methylpentane to 3-methylpentane occurs exclusively according to a selective cyclic mechanism, whatever the metal particle size (from 10 to 60 A) (S8, /02), which shows that the presence of two contiguous edge atoms is not really critical in this case. Moreover, even in... 

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