Isobutane hydride transfers from

Proton and C-nmr, ESCA, and Raman studies provide a wealth of information which unfortunately is not subject to a unique interpretation. The main conclusion to be drawn therefore is that the structure of the solvent stabilized cation is still unproven. Gas phase estimates of the heat of formation of the norbomyl cation imply a rather marked stability of the stmcture relative to other secondary ions (Kaplan et al., 1970). When combined with other estimates of the heat of formation of the t-butyl cation, however, these data suggest that hydride transfer from isobutane to the norbomyl ion will be endothermic by 6 to 15 kcal mole . This is contrary to experience in the liquid phase behaviour of the ion, and the author s conclusion that their observation of enhanced stability is evidence of stabilization by bridging deserves further scmtiny. 

The strong competition between alkylation and hydride transfer appears in the alkylation reaction of propane by butyl cations, or butanes by the propyl cation. The amount of C7 alkylation products is rather low. This point is particularly emphasized in the reaction of propane by the terf-butyl cation, which yields only 10% of heptanes. In the interaction of isopropyl cation 31 with isobutane 2 the main reaction is hydride transfer from the isobutane to the isopropyl ion followed by alkylation of propane by the isopropyl ions (Scheme 5.20). 

There are several reports of activation of acyl cations by superacids, suggesting the involvement of gitonic superelectrophiles.61 As discussed in Chapter 2, hydride transfer from isobutane to the acetyl cation has been reported when the reaction is carried out in excess HF-BF3. At the same... 

Finally, these carbenium ions suffer rapid hydride transfer from isobutane, leading to the different octane isomers and regenerating the fert-butyl cation to perpetuate the chain sequence ... 

Transfer from Alkanes to Methyloxocarbonium Ion. Reel. Trav Chim. Pays-Bas 1973, 92, 689-697. (b) Brouwer, D. M. Kiffen, A. A. Hydride Transfer Reactions III. Rates of Hydride Transfer from Isobutane to Hydroxy-carbonium Ions. Reel Tray Chim. Pays-Bas 1973, 92, 809-813. 

A second question is what might be done to improve selectivity beyond the usual practice of refiners to maximize mixing, maximize the isobutane to olefin ratio, lower the temperature and reduce the olefin space velocity. One approach is to decide what s rate determining and then to develop a chemical solution. This paper will be concerned with developing evidence that hydride transfer from a tertiary paraffin is generally slow and may be considered to be the rate determining step. The fact that a cation abstracts from Isobutane relatively slowly compared to... 

The probability that hydride transfer from Isobutane to carbonlum Ion Intermediates Is the kinetically slow step affecting product quality was raised by an experiment In which a stream of 96 percent H2SO4 was rapidly mixed with a stream of Isobutane plus Isobutylene In a mixing tee, after which the products were Immediately quenched In a large vessel containing cold caustic (13). The Cg fraction of the product contained trlmethylpentenes but no trlmethylpentanes. 

Thermodynamic Information indicates that 2-butenes would be the predominant olefins released. The resulting 2-butenes presumably react in this process with a t-butyl cation to produce a trimethyl pentyl ion. Hydride transfer from isobutane or more likely an acid-soluble hydrocarbon would result In the production of a trimethylpentane. 

C.-Cg cations and olefins. The latter olefins also quickly pro-tonateo forming cations. Hydride transfer from isobutane or acid-soluble hydrocarbons resulted in the production of C to Cg isoparaffins. Heavy ends and conjunct polymers were, of course, produced to some extent during the second-step reactions, but most of these compounds were probably produced during the first-step reactions. 

Initiation (or olefin protonation) In this step, a f-butyl cation is formed from isobutene. A sec-butyl cation is formed from 1-C4= or 2-C4=. The sec-butyl cation can form a f-butyl cation by methyl shift, or it can undergo hydride transfer from isobutane, forming n-C and a t-butyl cation. 

Otvos suggested that under the reaction conditions a small amount of t-butyl cation is formed in an oxidative step, which then deprotonates to isobutylene. The reversible protonaton (deuteration) of isobutylene was responsible for the H-D exchange on the methyl hydrogens, whereas tertiary hydrogen is involved in intermolecular hydride transfer from unlabeled isobutane (at the CH position). Under superacidic conditions, where no olefin formation occurs, the reversible isobutylene protonation cannot be involved in the exchange reaction. On the other hand, a kinetic study of hydrogen... 

Mechanism 3 involves two types of reactions. First, heavy cations, in perhaps Cio—Ci6 or maybe even Cio—C20 range, are obtained as more than one olefin reacts with a -C4C9+. Second, hydride transfer from CPs and isobutane, results in the formation of Ciq—Cie (or even Cio—C20) isoparaffins. Hydride transfer occurs before the heavy cations fragment, as occurs in mechanism 2. Alkylates produced via mechanism 3 are obtained as 2 or more moles of olefins react with 1 mole of isobutane. The isoparaffins produced by mechanism 3 often have fairly low octane numbers, often in the 80-85 RON range (20). 

Aluminum chloride (40) could be used for alkylation, but its selectivity is relatively low, producing a large number of heavy ends. To decrease heavy ends formation, a fast product desorption is required that involves faster hydride transfer from isobutane to trimethylpentyl carbenium ions. To do this, higher concentrations of isobutane in the acid phase are desired, this being achieved by using as catalysts mixtures of complexes of AICI3 with dimethyl, diethyl, and methyl-ethyl... 

Pig. 6. Simplified alkylation cycle including the three main reaction steps (a) alkene addition to a tert-butyl cation, (b) isomerization of the formed trimethylpentyl cation, and (c) hydride transfer from isobutane to the TMP cation. 

Figure 6.10.2 shows the simplified main reaction cycle in refinery alkylation. An olefin is added to the ferf-butyl cation to give the corresponding Cg carbocation. This Cg carbocation may isomerize via hydride and methyl shifis to form a more stable carbenium ion and subsequently undergoes, again, hydride transfer from isobutane. This latter step forms the saturated hydrocarbon and regenerates the tert-butyl cation to perpetuate the catalytic cyde. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

With both liquid acid catalysts, but presumably to a higher degree with sulfuric acid, hydrides are not transferred exclusively to the carbenium ions from isobutane, but also from the conjunct polymers 44,46,71). Sulfuric acid containing 4-6 wt% of conjunct polymers produces a much higher quality alkylate than acids without ASOs (45). Cyclic and unsaturated compounds, which are both present in conjunct polymers, are known to be hydride donors (72). As was mentioned in Section II.B, these species can abstract a hydride from isobutane to form the -butyl cation, and they can give a hydride to a carbenium ion, producing the corresponding alkane, for example the TMPs, as shown in reactions (7) and (8). 

Fast hydride transfer reduces the lifetime of the isooctyl cations. The molecules have less time to isomerize and, consequently, the observed product spectmm should be closer to the primary products and further from equilibrium. This has indeed been observed when adamantane, an efficient hydride donor, was mixed with zeolite H-BEA as the catalyst (78). When 2-butene/isobutane was used as the feed, the increased hydride transfer activity led to considerably higher 2,2,3-TMP and lower 2,2,4-TMP selectivities, as shown in Fig. 5. 

Typical alkylation reactions are those of propane, isobutane, and n-butane by the ferf-butyl or sw-butyl ion. These systems are somewhat interconvertible by competing hydride transfer and rearrangement of the carbenium ions. The reactions were carried out using alkyl carbenium ion hexafluoroantimonate salts prepared from the corresponding halides and antimony pentafluoride in sulfuryl chloride fluoride solution and treating them in the same solvent with alkanes. The reagents were mixed at —78°C warmed up to — 20°C and quenched with ice water before analysis. The intermolecular hydride transfer between tertiary and secondary carbenium ions and alkanes is generally much faster than the alkylation reaction. Consequently, the alkylation products are also those derived from the new alkanes and carbenium ions formed in the hydride transfer reaction. 

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