Methylpentanes isomerization

Concentrated sulfuric acid does not catalyze the isomerization of n-alkanes neither does silica-alumina. On the other hand, each does isomerize methylpentanes and higher alkanes that possess tertiary carbon atoms, the isomerization occurring by formation of alkanes containing tertiary carbon atoms in new positions ... 

Where necessary, as in the isomeric methylpentanes (II and III), indicate by a number the carbon to which the alkyl group is attached. 

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. 

Proportions of Isotopically Labeled Products from Isomerization of 2-Methylpentane-2-13C over Thick... 

Fig. 12. Variation with average platinum particle diameter of the initial rate of reaction (isomerization plus dehydrocyclization) of n-hexane (- -) and 2-methylpentane (-O-) over ultrathin film catalysts at 275°C. Hydrogen/reactant hydrocarbon, 10/1 total reactant pressure 100 Torr.

Remainder of reaction by isomerization Anderson and Shimoyama (30, 135). n-H = n-hexane 2-MP = 2-methylpentane MCP = methylcyclopentane. 

Microwave activation of alkane transformations was studied in detail by Roussy et al., who summarized their results in several papers [2, 28, 29, 79]. Isomerization of hexane, 2-methylpentane, 2-methyl-2-pentene, and hydrogenolysis of methylcydo-pentane have been investigated, and the diversity of possible effects has been specified [2]. The course of 2-methylpentane isomerization on a 0.3% Pt/Al203 catalyst depended on the mode of heating - the distribution of hexane products was different... 

Disubstituted Alkenes. Simple 1,2-disubstituted alkenes such as 2-octene or cyclohexene, which produce only secondary aliphatic carbocation reaction intermediates, do not undergo reduction upon treatment with a Brpnsted acid and an organosilicon hydride. Even when extreme conditions are employed, only traces of reduction products are detected.192 203 207-210,214 An exception is the report that 4-methyl-2-pentene forms 2-methylpentane in 70% yield when heated to 50° for 20 hours with a mixture of Et3SiH/TFA containing a catalytic amount of sulfuric acid. It is believed that 4-methyl-2-pentene is isomerized to 2-methyl-2-pentene prior to reduction.203... 

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

Gault et al. noticed in their early papers (757) that the product pattern of methylcyclopentane (MCP) hydrogenolysis is sometimes surprisingly similar to that of hexane or methylpentane(s) isomerizations. They suggested that isomerization proceeded via a cyclic, methylcyclopentane-like intermediate. Later it appeared that the similarity was not always found, but an important idea was already born and, more importantly, was brilliantly confirmed by later papers from the laboratory of Gaults. 

The idea of the evidence is rather simple and can be elucidated by means of the following experiment. Let us consider, for example, a molecule of 2-methylpentane labeled in a branched position by 13C 2-methyl- 13C(2)-pentane. If the consecutive reactions in the adsorbed state are with a given metal of low extent, and this is certainly true for Pt or Pd, then the appearance, among the product, of 3-methyl-l3C(3)-pentane is very strong evidence of the operation of the 5C (cyclic) intermediates. Only via a ring closure at one place and an opening at another place of the molecule can a label move simultaneously with the branch. On the other hand, when the branch and labeled atom become separated by isomerization, this is evidence of the operation of the 3Cay complexes (see Fig. 5). 

The intramolecular nature of most carbocationic isomerization was proved by means of labeling experiments. [l-13C]-Propane was isomerized in the presence of aluminum bromide promoted by hydrogen bromide to form [2-13C]-propane. None of the propane product contained more than one l3C atom per molecule.64 Similarly, very little label scrambling was observed in the isomerization of labeled hexanes over SbF5-intercalated graphite.65 Thus simple consecutive 1,2-methyl shifts can account for the isomerization of l3C-labeled methylpentanes (Scheme 4.3). 

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). 

A. Fish The suggestion that the yield of O-heterocycles is a maximum when the fuel has a C5 chain is supported by the fact that the yields from 2-methylpentane (2) (32%) and n-pentane (28%) are similar. If, as generally agreed, O-heterocycles are formed by the isomerization and decomposition of alkylperoxy radicals, their formation competes with the 0-scission of hydroperoxylalkyl radicals. 

Barusch and Payne (6), in 1951, were successful in stabilizing a cool flame in a straight tube, and used this device to investigate the relationship between the octane number of the fuel and its tendency to form cool flames. Using a similar device Ober-dorfer (30), working in the author s laboratory was able to study the cool flames of the isomeric hexanes. In this manner, n-hexane, 2-methyl pentane, 3-methylpentane, and 2,2-dimethylbutane were readily brought to cool-flame combustion. The fuel-air ratio... 

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. 

Isomerization processes clearly illustrate thermodynamic relationships. For example, for the isomeric hexanes the highest level of free energy (relative to standard states) is that of n-hexane (—1.04 kcal/mole) while isomeric 2-methylpentane is —1.95 kcal/mole. Further branching gives —2.82 kcal/mole for 2,2-dimethylbutane. Similar relationships hold for cyclic hydrocarbons, and the same energy drives account for disproportionation and hydrogenation ... 

As a final step in the fractionation sequence, the hexane fraction is separated into a dimeihyibutanes concentrate as a net overhead product and an -hexane-rich bottoms stream to be recycled for the further isomerization of ihe n-hexane and methylpentancs. Wilh economically practical fractionation, Hie methylpentanes split between the overhead and bottoms of the deisohexanizer column. For the Cs fraction, the boiling points of the two isomers are far enough apart to make a relatively clean split economically feasible. For the C<, fraction, the greater number of... 

The hexasila-Dewar benzene 13 is thermally stable at —150 °C, but it gradually reverted to the hexasilaprismane 1243. The half-life is 11/2 = 0.52 min at 0 °C in 3-methylpentane. The activation parameters for the isomerization of 13 to 12 are a = 13.7 kcalmol-1, A= 13.2 kcalmol-1 and A= — 17.8 cal K-1 mol-1. The small Ea value is consistent with the high reactivity of Si=Si double bonds. Most probably, the small HOMO-LUMO gap of 13 makes it possible that the Si=Si double bonds undergo a formally symmetry forbidden [2 + 2] thermal reaction. Hexasila-Dewar benzene is a key... 

The 13C-labelling experiments allowed one to determine the relative contributions of cyclic and of bond-shift mechanisms in the isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane over Pt TiC>2 catalysts prepared by different methods566. 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

The relative reactivity of hexane and 3-methylpentane (about 1000) in the isomerization mode was shown to be the same as found for isomerization in HF-SbF5.102 In the cracking mode, however, the ratio is about 10, resulting from the dramatic acceleration of the reaction of hexane compared to that of 3-methylpentane. Further characteristics of the cracking mode are a large excess of branched isomers in the C4—C5 fractions, the absence of unsaturated cracking products, and formation of... 

When 3-methyl[3-13C]pentane (17 ) is isomerized to 2-methylpentane, the label distribution shows that the isomerization cannot be explained by a simple 1,2-methide shift 30% of the 2-methylpentane has the 13C label in a position that can be best explained by the ethyl shift (Scheme 5.12). The recovered 3-methylpentane (96%) also shows a very large degree of internal shift [Eq. (5.45)]. 

This result can only be explained by the /3-scission of the trivalent 4-methylpent-2-yl ion 30 as the initial step in the cracking process. Based on this and on the product distribution versus time profile, a general scheme for the isomerization and cracking process of the methylpentanes has been proposed103,104 (Scheme 5.16). 

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