Neopentane, isomerization

Fig. 4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known ( ) and the speculative aspects ( ) of the 3C complex formation are indicated. 

In the early 1970s it was known that palladium films isomerize n-butane and isobutane (63, 70) but not neopentane (63). Similarly, only pure cracking of neopentane was observed on Pd/Si02 catalysts (71). The observed inability of Pd to catalyze neopentane isomerization was confirmed in the case of more complicated molecules having a quaternary carbon atom (64). On the other hand, platinum seemed to be the unique metal in isomerizing neopentane. Comparing the catalytic behavior of both metals, Gault came to the conclusion that palladium, due to its well-established propensity toward formation of 7r-olefinic and ir-allylic adspecies, can... 

Very recent work (60b) has confirmed that Ir films do not isomerize neopentane most of the transition metals as well as palladium (60c) rearrange isobutane to k-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the ay species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting... 

However, it was recently shown that fresh palladium films do isomerize neopentane initially, but their activity dies off very quickly (66, 67). 

Three isomeric alkanes have the molecular formula C5H12 The unbranched isomer is as we have seen n pentane The isomer with a single methyl branch is called isopen tane The third isomer has a three carbon chain with two methyl branches It is called neopentane... 

There are three isomeric pentanes, ie, saturated aHphatic hydrocarbons of molecular formula C H 2- They are commonly called / -pentane [109-66-0] isopentane [78-78-4] (2-methylbutane), and neopentane [463-82-1] (2,2-dimethylpropane). 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Other types of non-micro-channel, non-micro-flow micro reactors were used for catalyst development and testing [51, 52]. A computer-based micro-reactor system was described for investigating heterogeneously catalyzed gas-phase reactions [52]. The micro reactor is a Pyrex glass tube of 8 mm inner diameter and can be operated up to 500 °C and 1 bar. The reactor inner volume is 5-10 ml, the loop cycle is 0.9 ml, and the pump volume adds a further 9 ml. The reactor was used for isomerization of neopentane and n-pentane and the hydrogenolysis of isobutane, n-butane, propane, ethane, and methane at Pt with a catalyst. 

The isomerization of the butanes and of neopentane has been studied over various types of evaporated platinum films by Anderson and Baker (68) and Anderson and Avery (108,24). Table II gives some typical results. It is clear that the proportion of parent hydrocarbon reacting to isomeric rather than to hydrogenolytic product is considerably smaller for a hydrocarbon with an unbranched as opposed to a branched chain containing an isostructural unit indeed, neopentane was studied as the archetypal molecule of the latter class. 

The isomerization of neopentane has also been observed on a variety of dispersed platinum catalysts, including supported platinum as well as platinum powder, by Boudart and co-workers (121, 122). On a 1% plati-... 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

Anderson and Avery s bond shift mechanism has the consequence of predicting that a quaternary carbon atom cannot be generated in the hydrocarbon product. In fact, Anderson and Avery (24) showed that in the isomerization of isopentane over platinum films, only a very small amount (<1%) of neopentane was produced (although the equilibrium constant for isopentane <= neopentane is 0.16 at 278°C). Furthermore,... 

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. 

Neopentane does not undergo isomerization 185) on chromia/alumina (non-acidic) at 537°C, the only significant reaction been hydrogenolysis to methane and iso-C4. However, the reality of isomerization is made clear from, for instance, the formation of xylenes from 2,3,4-trimethylpentane. For o- and p-xylene, the reactions are (24) and (25) 182, 93). These processes are formally quite analogous to those we have described in previous... 

In considering ways to rationalize the differences in product distribution in Table IV, it is speculated that the ability of the surface atoms of certain metals to exhibit different valence states may be important (23). It has previously been suggested by Boudart and Ptak (27) that the ability of platinum and iridium to catalyze the isomerization of neopentane could be a consequence of the variable surface valency of these metals proposed by... 

In a later report, Schmidt and Allen (1970) extended their measurement to 38 pure liquids and mixtures at room temperature and to 5 liquids as a function of temperature. The free-ion yields are arranged by the alkanes and their isomeric and cyclic counterparts, which show considerable differences in the results. Thus, the free-ion yield in neopentane (NP) is about seven times that in n-pentane. Some of the results are shown in Table 9.1. In mixtures of NP with CC1, or CS, the observed decrease of Gf with the additive concentration has been interpreted by Mozumder and Tachiya (1975) as due to epithermal electron scavenging (vide infra). 

The proposed mechanism of the bond shift isomerization of neopentane is shown in Scheme I Cl-3). There are now good models for each step in the proposed sequence, but no simple transition metal complex can accomplish all steps since there cannot be sufficient co-ordination sites. The first steps involve a,y-dinstallation of the alkane, for which there are good precedents in both platinum and iridium chemistry (4, 5, 6). The... 

Exchange reactions of neopentane have already lead to the conclusion (755, 158, 162) that 3C complexes, bound to the surface by the ay carbon atoms, may be formed on some metals (e.g., Rh or Pt). However, it was evident from those experiments that 3Cay complexes are formed by metals much more reluctantly than the 2Ca(1 or the aa-bound complexes. It means that their formation can only be studied at (much) higher temperatures than those suited for the study of the HC/D2 exchange reactions. In this case one can advantageously use the skeletal reactions of neopentane themselves as evidence for the formation of 3C complexes. When neopentane is being isomerized or split into Q and C3 fragments, 3C complexes are certainly... 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

In order to interpret the remarkably high activity of platinum to promote isomerization of neopentane to isopentane, the direct formation and involvement of metalla-cyclobutane intermediate 18 was suggested.152,158 This a,y diadsorbed species bonded to a single platinum atom is in accordance with the existence of platinum complexes and the ability of platinum to catalyze a-y exchange.156,159... 

The carbides and nitrides of the early transition metals have attracted considerable attention. Much of this can be attributed to the catalytic properties of these materials. Levy and Boudart1 were the first to note the Pt-like catalytic behavior of WC, which for many reactions is more catalytically active than metallic tungsten.2 Others have noted that the early transition metal carbides and nitrides, in general, are excellent catalysts for reactions characteristic of the more expensive noble metals.3,4 For example, WC has been found to catalyze the isomerization of neopentane to isopentane,1 a reaction that previously had been known to be catalyzed only by iridium and platinum.5... 

Propane and cyclopentane give isopropyl chloride and cyclopentyl chloride, respectively, whereas isobutane is transformed to ferf-butyl chloride under the same reaction conditions (yields are 69%, 74%, and 76%, respectively). Neopentane undergoes isomerization to yield 2-chloro-2-butane (88%). When saturated, hydrocarbons were allowed to react with methylene bromide and SbF5 bromoalkanes were obtained in comparable yields (64-75%). Formation of the halogenated product can be best explained by the mechanistic pathway (I) depicted in Scheme 5.55. Since SbF5 always contains some HF, mechanism (II) may also contribute to product formation (Scheme 5.55). 

Fig. 2. Mechanisms of bond-shift isomerization on metal catalysts, (a) Anderson-Avery mechanism (63). (b) Muller-Gault mechanism for isomerization of neopentane on Pt (64). (c) McKervey-Rooney-Samman mechanism (69). (d) Muller-Gault mechanism of isomerization of isobutane on Pd (64). (e) Clarke-Rooney mechanism (46). (f) Garin-Gault mechanism (65).

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