Isobutane hydride abstraction from

The tot-butyl cation (1) reforms isobutane via hydride abstraction from isobutane according to Bartlett et al.,834 Nenitzescu et al.,835 and Schmerling836 involving the tertiary C—H bond only through intermediate structure 463, and thus not exchanging the methine hydrogen. 

The tert-butyl cation (50) reforms isobutene, according to Bartlett et al. Nenitzescu et al., ° and Schmerling, by hydride abstraction from isobutane involving the tertiary C-H bond only, through intermediate structure 52, and thus no methine hydrogens would exchange with the deuterosulfuric acid. 

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... 

Dimethyl hexanes are believed to result mainly from codimerization of butene-l and isobutene, followed by abstraction of a hydride ion from isobutane. This mechanism differs significantly from previously published theory. Thus, high initial concentrations of butene-l favor dimethylhex-ane formation. Some isomerization of dimethylhexyl carbonium ions occurs. 

Excessive olefin polymerization followed by hydride ion abstraction from isobutane probably accounts for the formation of saturated residue. 

Isobutane-to-Olefin Ratio with Propylene Feed. The isobutane-to-olefin ratio has long been recognized as an important process variable in the alkylation of isobutane with either butenes or pxopylene (Phillips Petroleum Company, 1946). By maintaining a sufficiently high concentration of isobutane in the reaction zone, the abstraction of hydride ions from isobutane is favored over abstraction from product isoparaffins. 

Of further interest is the fact thot ii-butyl chloride reacts in the presence of excess ethane, also at 40 C, to form butylenes %) and some isobutane (15%)(eq. 6a). These products lead to the conclusion that rearrangement of the "free" trivalent carbenium ion is more rapid than hydride abstraction from another n-butyl chloride molecule. The t-butyl carbenium ion thus formed, being too weak an acid to abstract a hydride, deprotonotes to form butylene products. No isohexane alkylation products are formed (eq. 6). 

Carbonium ions undergo reactions that provide electrons to complete the octet of the positively charged carbon atom. Hence, one reaction that CH3-CH2 can undergo is to abstract a hydride ion from isobutane,... 

A new carbonium ion is formed. Now, if neohexane had been produced, it would have resulted from the cation abstracting a hydride ion from isobutane, which would, in turn, produce the t-butyl cation to start the entire cycle over again ... 

Products do not contain 2,2,3-trimethylbutane or 2,2,3,3-tetramethylbutane, which would be expected as the primary alkylation products of direct alkylation of isobutane with propylene and isobutylene, respectively. In fact, the process iavolves alkylation of the alkenes by the carbocations produced from the isoalkanes via intermolecular hydride abstraction. 

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). 

Intermediates and causes them to abstract hydride Ions more rapidly from Isobutane or any other potential donor. Increased hydride transfer converts more of the carbonlum Ions at the add Interface to saturates faster, yielding product while minimizing polymerization and side reactions. It Is also likely that the surfactants physically block alkyl Ions from one another in the surface film and thus Impede Ion + olefin polymerization. In such a film the carbonlum Ion concentration must also be lower than In the absence of surfactant and mass law effects will therefore also lead to less polymerization and cracking. The fact that steady state hydride transfer rates In H2SO are subject to control through the use of acid modifiers which act In the bulk acid and at the acid-hydrocarbon Interface Is the key to the control of sulfuric acid alkylation. 

Thus, the most direct route to chain-carrying, tertiary butyl carbonium Ions is offered in isobutene-isobutane alkylation (Equation I). When initiating with either a linear butene or propylene, a second step is necessary to form the tertiary butyl carbonium ion, i.e., abstraction of a hydride Ion from an isobutane molecule while forming a molecule of normal alkane. (Equation 2, 2-A, 3, 3-A). Reaction sequences in these equations are often referred to as hydrogen- or hydride transfer reactions and will be discussed subsequently. 

Chain Propagation. In the chain propagation step, an olefin molecule reacts with a tertiary butyl carbonium ion as postulated by Whitmore (1934). This addition reaction produces a larger carbonium ion which then either undergoes isomerization or abstracts a hydride from an isobutane molecule. (Under some circumstances, the larger carbonium ion may add a second molecule of olefin this reaction will be discussed under "Polymerization".) Hydride abstraction regenerates a chain-carrying, tertiary butyl carbonium ion and also forms a molecule of isoparaffin. Reactions follow ... 

Mechanistically, as elucidated by Schmerling (19) and as illustrated for isobutane-ethylene (alkane-alkene) alkylation (Scheme 1), the reaction is initiated by protonation of the alkene (ethylene) to form a very acidic primary ethyl cation (step 1) which rapidly abstracts a hydride ion from an isobutane molecule to generate the chain carrying t-butyl cation (step 2). This can then alkylate another molecule of ethylene to form the secondary-2-methyl-t-butyl carbenium ion (step 3). This cation rapidly undergoes a... 

Aliphatic alkylation is widely used to produce high-octane gasolines and other hydrocarbon products. Conventional paraffin (alkane)-olefin (alkene) alkylation is an acid-catalyzed reaction it involves the addition of a tertiary alkyl cation, generated from an isoalkane (via hydride abstraction) to an olefin. An example of such a reaction is the isobutane-ethylene alkylation, yielding 2,3-dimethylbutane. 

Figure 3.9 compares four mass spectra of the molecule of allethrin the spectrum recorded in electron ionization and those recorded in positive chemical ionization with methane, isobutane, and ammonia. The molecular weight of allethrin is 302. The molecular ion is absent in the El mass spectrum. In the spectrum recorded in methane Cl, the pseudo-molecular ion MH at m/z 303 constitutes the base peak. It is accompanied by a m/z 301 ion (m/z = M -1), which is not very abundant. It results from a hydride abstraction reaction and a m/z 331 ion (m/z = M + 29), which corresponds to an adduct between the molecule of allethrin and C2H5+. 

Excess Polymerization. A small amount of high-boiling heavy "tail" or residue Is formed in Isobutane alkylation, even urxJer the most favorable reaction conditions. The polymer molecule is in reality on isoparaffin formed from two or more molecules of olefin plus one molecule of Isobutane. Polymer is formed because of the inherent tendency of larger carbonium ions, e.g., Cj or C0 ions, to complete with tertiary butyl carbonium ions for addition of olefin molecules before abstracting hydride ions and becoming isoparaffin molecules. Reactions follow ... 

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