Butanes butyl cations

When usiag HF TaF ia a flow system for alkylation of excess ethane with ethylene (ia a 9 1 molar ratio), only / -butane was obtained isobutane was not detectable even by gas chromatography (72). Only direct O -alkylation can account for these results. If the ethyl cation alkylated ethylene, the reaction would proceed through butyl cations, inevitably lea ding also to the formation of isobutane (through /-butyl cation). 

The difference in behaviour between pentyl and butyl cation systems (Figs. 3 and 4) has also been encountered in trapping experiments with carbonium ions, primarily formed from alkanes and SbFs, by CO (Hogeveen and Roobeek, 1972). In the case of n-butane the secondary butyloxocarbonium ion is the main product, whereas in the case of n-pentane only the tertiary pentyloxocarbonium ion is found. 

Figure 7.1. X-Ray structure of the tert-butyl cation and (CHBuMejClg) obtained by the reaction of n-butane and isobutane with CH3(CHBnMe5Cl5). (Adapted from reference 19.)...

The behaviour of the butyl system provides important information on the nature of the intermediate formed during the rearrangement of the isobutyl to the 2-butyl cation. Thus, from the observation that isobutyl chloride yields n-butane which has exchanged one proton with the acid, while the solvolysis of 2-butyl chloride in the same acid (2% HjO), yields unexchanged n-butane one might deduce that an intermediate was formed during the former s solvolysis which exchanged one proton with the acid before it converted to a secondary butyl ion. A reasonable mechanism is shown in Scheme 1. 

In the ethane-ethylene reaction in a flow system with short contact time, exclusive formation of n-butane takes place (longer exposure to the acid could result in isomerization). This indicates that a mechanism involving a trivalent butyl cation depicted in Eqs. (5.1)—(5.5) for conventional acid-catalyzed alkylations cannot be operative here. If a trivalent butyl cation were involved, the product would have included, if not exclusively, isobutane, since the 1- and 2-butyl cations would preferentially isomerize to the rm-butyl cation and thus yield isobutane [Eq. (5.9)]. It also follows that the mechanism cannot involve addition of ethyl cation to ethylene. Ethylene gives the ethyl cation on protonation, but because it is depleted in the excess superacid, no excess ethylene is available and the ethyl cation will consequently attack ethane via a pentacoordinated (three-center, two-electron) carbocation [Eq. (5.10)] ... 

When -butane 1 or isobutane 2 was reacted with HSO3F—SbF5 (Magic Acid), tert-butyl cation 4 was formed exclusively [Eq. (5.2)] as evidenced by a sharp singlet at 4.5 ppm (from TMS) in the 1H NMR spectrum. In excess Magic Acid, the stability of the ion is remarkable and the NMR spectrum of the solution remains unchanged even after having been heated to 110°C. 

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). 

Tetramethylbutane 36 was not formed when n-butane and. sec-butyl cation were reacted. The isomer distribution of the octane isomers for typical butyl cation-butane alkylations is shown in Table 5.7. 

Table 5.7. Isomeric Octane Composition Obtained in Typical Alkylations of Butanes with Butyl Cations...

In other conversions, better yields (Scheme 2) and product selectivity (Scheme 3) are obtained at lower temperatures.62 In the case of the Koch-Haaf butane carbonylation, it was noted the importance of temperature on the equilibria between the. sec-butyl and tert -butyl cations and their derived acyl cations. Besides providing better yields and product... 

The t-butyl cation was found by Olah and Lukas (1967) to be perfectly stable in FSOgH-SbFg solutions, even at 150°C, while all other carbonium ions from the protonation of butanes, pentanes and hexanes were observed to be converted into the t-butyl cation, following various fragmentation and isomerization pathways. 

Olah (17a) has also reported the alkylation reactions (at -10 with 1 1 HS03F-SbF5) of n-butane with ethylene to yield 38 weight percent of hexanes and of n utane with propylene to yield 29 weight percent of heptanes. The former reaction has also been reported by Parker (31) at 60 , but the product in this case more nearly resembles polyethylene degradation products. In our work with 10 1 HF-TaF5 at 40 , in a flow system, ethylene (14.1 wt.%) reacted with rv-butane to form 3-methyl-pentane as the initial product of 94% selectivity (Scheme 6, path a). The alternative, i.e., the direct reaction of ethylene with a secondary-butyl cation (path b), can be ruled out since butane does not ionize under these conditions (vide supra). 

Brouwer (8) studied the isomerization of isotopically labeled butane CH.3 — CH.2 — CH.2 — CH3. He proposed the conversion of s-butyl cation to give a substituted cyclopropyl carbenium ion intermediate ... 

Such an intermediate explains the - C scrambling in the butane carbon chain without the formation of isobutane ring opening at the C3 — C2 carbon bond results in the formation of x-butyl cation, whereby the labeled C is no longer a terminal carbon ... 

Lewis superacid-catalyzed direct alkylation of alkanes is also possible with alkyl cations prepared from alkyl halides and SbFs in sulfuryl chloride fluoride solution. " Typical alkylation reactions are those of propane and butanes by 2-butyl and ZerZ-butyl cations. The ClfU-Sbfs and C2H5F-SbF5 complexes acting as incipient methyl and ethyl cations besides alkylation preferentially cause hydride transfer. Since intermolecular hydride transfer between different carbocations and alkanes are faster than alkylation, a complex mixture of alkylated products is usually formed. A significant amount of 2,3-dimethylbutane was, however, detected when propane was propylated with the 2-propyl cation at low temperature [Eq. (6.36)]. No 2,2-dimethylbutane, the main product of conventional acid-catalyzed alkylation, was detected, which is a clear indication of predominantly nonisomerizing reaction conditions. 

This interpretation was challenged by van Hooff [18], who proposed that low i/n ratios, and especially the presence of doubly labelled butanes in the Chang-Chu experiment, could be explained by hydride transfer between propane and butyl cations. If hydride transfer were slower to t-butyl cation than to the secondary cation, low i/n butane ratios would result. Hydride transfer... 

Computed values at the MP2/6-311G(d,p) level for the hydride exchange between tert-butyl and bicyclo[2.2.2]octyl cations and anchored to the experimental data for tert-butyl cation and Ao-butane. Entropy corrections at the HE/6-31G(d) level. 

When the contact time between iso-butane and deuterated zeolites (or D2SO4) was prolonged to obtain extensive deuteration of the alkane (>90%), deuterium also appeared gradually in the tertiary position.The appearance of deuterium in this position cannot be explained by a simple hydride transfer from iso-butane to the tertio-butyl cation. Alkenyl and polyenyl ions that were previously identified in sulfuric acid as a product... 

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