Single 2-methylpentane

NMR MEASUREMENTS. Initially, the reduction of 2-chloro-4-methylpentane was carried out in order to ascertain the ideal temperature which would lead to complete reduction in about six hours. The progress of this reduction was followed by lH NMR, recording a single scan every thirty minutes. It was found that at 500 C the reaction reaches 80% of completion after 5 hr. All subsequent reductions were carried out at this temperature. 

For each of the hexanes, more than one singly substituted I3C positional isomer is possible there are five for 2-methylpentane, four for 3-methyl-pentane, and three for n-hexane. Clearly, if only Re, Rs, and R are determined, so that only o5 and o4 are available, Eqs. (5) cannot give the proportions of all the positional isomers in 2- and 3-methylpentane. This could, in principle, be achieved by extending (5) to include a3 and 02. However, it turns out that a2 is very dependent on source conditions, and thus is unreliable, while, particularly with 3-methylpentane, 03 is very insensitive to positional isomer composition. The result, in practice, is that one is limited to determining... 

Referring first of all to the reactions over 0.2% platinum/alumina (Table V) the major features of the product distributions may be explained by a simple reaction via an adsorbed C5 cyclic intermediate. For instance, if reaction had proceeded entirely by this path, 2-methylpentane-2-13C would have yielded 3-methylpentane labeled 100% in the 3-position (instead of 73.4%) and would have yielded n-hexane labeled 100% in the 2-position (instead of 90.2%). Similarly, 3-methylpentane-2-I3C would have yielded a 2-methylpentane labeled 50% in the methyl substituent (instead of 42.6%), and would have yielded n-hexane labeled 50% in the 1- and 3-positions (instead of 43.8 and 49% respectively). The other expectations are very easily assessed in a similar manner. On the whole, the data of Table V lead to the conclusion that some 80% or so of the reacting hydrocarbon reacts via a simple one step process via an adsorbed C5 cyclic intermediate. The departures from the distribution expected for this simple process are accounted for by the occurrence of bond shift processes. It is necessary to propose that more than one process (adsorbed C6 cyclic intermediate or bond shift) may occur within a single overall residence period on the catalyst Gault s analysis leads to the need for a maximum of three. The number of possible combinations is large, but limitations are imposed by the nature of the observed product distributions. If we designate a bond shift process by B, and passage via an adsorbed Cs cyclic intermediate by C, the required reaction paths are... 

Unlike the behavior over 0.2% platinum/alumina, the main features of the labeled product distributions obtained over 10% platinum/alumina and over platinum film catalysts (Tables VI and VII respectively) cannot be explained in terms of a single dominant reaction pathway via an adsorbed C6 cyclic intermediate. Again, parallel, multiple-step reaction pathways are involved. The results from 2-methylpentane-2-13C have been qualitatively accounted for (84) by the pathways... 

The energy of a single photon is obviously insufficient to ionize an organic compound. As early as the nineteen forties (3, 4), however, it -was observed that Wurster blue cation radical is produced by photoirradiation of 3-methylpentane glass containing N,N-tetramethyl p-phenylenediamine (TMPD) at 77° K. The recent detailed study of this system by electric conductivity measurement (5, 6) and electronic spectroscopy (7) provided conclusive evidence that the ionization is brought about via excitation to the triplet state followed by successive photoabsorption at the triplet state. This mechanism is supported by the facts that the life-time of the photochemical intermediate is identical with that of phosphorescence and the formation of Wurster blue, and that phosphorescence is inhibited in the presence of triplet scavengers. 

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. 

One means of classification depends on the way in which carbon atoms are connected. Chain aliphatic hydrocarbons are compounds consisting of carbons linked either in a single chain or in a branched chain. Cyclic hydrocarbons are aliphatic compounds that have carbon atoms linked in a closed polygon (also referred to as a ring). For example, hexane (single) and 2-methylpentane (branched) are chain aliphatic molecules, while cyclohexane is a cyclic aliphatic compound. 

To illustrate this method of approach, results of three of the experiments by Corolleur et al. are shown in Tables XIII and XIV. In the examples selected here, a 13C-labeled 3-methylpentane and a labeled 2-methylpentane are reacted, in turn, and distribution of 13C is shown for the methylpentane products in Table XIII and for the n-hexane products in Table XIV. Distributions expected for a pure cyclic mechanism (C) and for a methyl shift (T) are indicated. Detailed discussion by the authors of abnormal products (i.e., those not predicted by the single-stage purely carbocyclic mechanism) led to the conclusion that these are formed on a second, less numerous, type of surface site by the action of which a succession of several rearrangements, according to a cyclic or a bond-shift mechanism, takes place. In Table XIV it can be seen that assumption of a simple skeletal rearrangement of the type... 

Because of the greater selectivity of the bromine atom, radical brominations can be useful in synthesis as long as the compound to be brominated has one hydrogen that is considerably more reactive than the others. The reaction of 2-methylpentane with bromine, shown previously, gives predominantly a single product because there is only one tertiary hydrogen. Because allylic and benzylic radicals are stabilized by resonance, bromination at these positions can also be successfully accomplished. An example is provided by the following equation ... 

Radioactive tracer studies of 2-methylpentane oxidation at 515 K enabled Cullis et al. [200] to show that the overall mechanism was more strongly affected by abstraction from the four secondary C—H sites than fi"om the single tertiary C—H site, despite its lower bond strength. The temperature at which this study was performed would suggest that abstraction by OH radicals might be rather more important than abstraction by HO2 or RO2 radicals. 

Fig. 14. Isobaric excitation profiles for 1-propanol (a) and 3-methylpentane (b) at different pressures. The y axis gives the potential enthalpy difference between the amorphous basin sampled preferentially at a given temperature and the single crystalline basin. Calculations performed according to Eq. (55), using the experimental heat capacity data of Takahara et al. (1994). (From Lewis, 2000.)...

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... 

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). 

Although thermodynamic calculations show appreciable quantities of di-methylbutanes at equilibrium (about 30-35% of the total hexane isomers at 500°C), such quantities are not observed in reforming. Equilibria are established readily between n-hexane and the methylpentanes, but not between these hydrocarbons and the dimethylbutanes. The reaction kinetics are not favorable for the rearrangement of singly branched to doubly branched isomers (11). This limitation apparently does not exist for the rearrangement of the normal structure to the singly branched structures. 

The compound on the left is numbered correctly since the methyl group is located at carbon 2 of the pentane chain in the compound on the right, the methyl group is located at carbon 4. Thus the name of the compound is 2-methylpentane, and not 4-methylpentane. Note that the branch name and the parent name are written as a single word, and a hyphen follows the number. 

The name is 3-methylpentane. We have now exhausted all possihilities for placing a single methyl group on pentane. 

Scheme IIL7. Single hydrogenolysis of 2-methylpentane leading via C2 mode to isopentane.

The loading dependence of 2-methylpentane is similar to earlier results showing a decrease of 2-methylpentane diffusivity with loading in single-... 

Figure 13 displays the self-diffusivities of n-hexane and 2-methylpentane in silicalite-1 and H-ZSM-5 as a function of the ratio of the hydrocarbons. The self-diffusivities of both hexanes linearly decrease with increasing gas-phase fraction of the branched hexane in the gas phase for the non-acidic and acidic zeolite. In H-ZSM-5, the mobility of alkanes is approximately two times slower than in silicalite-1. Obviously, the presence of acid sites strongly affects the molecular transport due to stronger interactions with the n-hexane molecules. A similar effect of Bronsted sites on the single component diffusion of aromatics was observed in MFI zeolites with different concentration of acid sites [63-65]. The frequency response (FR) technique provided similar results... 

The current work indicates the strong effect of acid sites on the interaction and diffusivity of hydrocarbons. To further study this effect, we determined the single-component diffusion coefficients and specifically the activation energy for diffusion. Activated diffusion is described by the Arrhenius-type Eq. 8. The pre-exponential factor Djnf is related to the jump frequency between adsorption sites in the zeolite lattice, while the exponential expresses the chance that the molecules are able to overcome the free energy barrier - act between these sites. The loadings of n-hexane and 2-methylpentane in H-ZSM-5 and silicalite-1 have been measured at temperatures between 373 and 533 K at intervals of 20 K. The hydrocarbon pressure was taken identical... 

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