Alkanes neopentane isomerization

Anderson and his co-workers examined the reactions of small alkanes mainly on platinum and palladium 45-48). Isobutane was isomerized to n-butane on platinum and on palladium, neopentane isomerized to isopentane on platinum, whereas other metals (including palladium) caused hydrogenolysis predominantly or exclusively. It was proposed that the slow step in the isomerization was the formation of a bridged intermediate (C) from an aory-triadsorbed species (A, B) (Fig. 11).1 Hiickel MO calculations based on this proposal suggested,... 

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... 

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... 

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. 

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... 

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. 

Because transition metals other than Pt (as far as they have been examined) have not such a propensity to form a metallocyclobutane directly from a gem-dimethyl group the way to ready isomerization of neopentane is barred. However, an alkane such as isobutane may have an indirect route to the ay-diadsorbed species provided that 1,2-migration of hydrogen is relatively easy (60d), as follows. 

But nearly every alkane can have a number of isomeric structures, and there must be an unambiguous name for each of these isomers. The butanes and pentanes are distinguished by the use of prefixes w-butane and isobutan penta, isopentane and neopentane. But there are 5 hexane T lTieptanes, and 75 decanes it would be diflicnli to devise, and even more difficult to remember, a different prefix for each of these isomers. It is obvious that some systematic method of naming is needed. 

Fresh tungsten carbides catalyze hydrogenolysis of branched (neopentane, 3,3-dimethyl pentane, etc) and linear (hexane, heptane, etc) alkanes at high rates and with high selectivities to lower alkanes (47). The different C—C bonds are cleaved at about an equal rate, and there is negligible isomerization activity. The absence of isomerization products is not the result of rapid secondary hydrogenolysis of the products on the WC surface. Again, the rates and the product distribution are similar to those of Ru. 

Recall from Section 3.2 that two or more compounds with the same molecnlar formula but different properties are called isomers. Those with different arrangements of bonded atoms are constitutional (or structural) isomers alkanes with the same number of C atoms but different skeletons are examples. The smallest alkane to exhibit constitutional isomerism has fonr C atoms two different compounds have the formula C4H10 (Table 15.3). The unbranched one is butane (common name, n-butane n- stands for normal, or having a straight chain), and the other is 2-methylpropane (common name, wobntane). Similarly, three compounds have the formula C5H12. The unbranched isomer is pentane (common name, n-pentane) the one with a methyl group at C-2 of a four-C chain is 2-methylbntane (common name, isopentane). The third isomer has two methyl branches on C-2 of a three-C chain, so its name is 2,2-dimethylpropane (common name, neopentane). 

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