Pentacoordinated carbonium ion

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

The reverse reaction of carbenium ions with molecular hydrogen, can be considered as alkylation of H2 through the same pentacoordinate carbonium ions that are involved in C—H bond protolysis. Indeed, this reaction is responsible for the long used (but not explained) role of H2 in suppressing hydrocracking in acid-catalyzed... 

It is well known that trivalent carbenium ions play an important role in electrophilic reactions of 7t- and -donors systems. Similarly, pentacoordinate carbonium ions are the key to electrophilic reactions of o-donor systems (single bonds). The ability of single bonds to act as o-donors lies in their ability to form carbonium ions via delocalized two-electron, three-center (2e-3c) bond formation. Consequently, there seems to be in principle no difference between the electrophilic reactions of n- and O-bonds except that the former react more easily even with weak electrophiles, whereas the latter necessitate more severe conditions. 

Results of protoly tic reactions of hydrocarbons in superacid media were interpreted by Olah as indication for the general electrophilic reactivity of covalent C H and C—C single bonds of alkanes and cycloalkanes. The reactivity is due to the tr-donor ability of a shared electron pair (of cr-bond) via two-electron, three-center bond formation. Consequently, the transition state of the reaction, is of three-center bound pentacoordinate carbonium ion nature [Eq. (5.5)]. 

Consequently, the reverse reaction of protolytic ionization of hydrocarbons to carbenium ions—that is, the reduction of carbenium ion by molecular hydrogen — can be considered as alkylation of H2 by the electrophilic carbenium ion through a pentacoordinate carbonium ion. Indeed, Hogeveen and Bickel have experimentally proved this point by reacting stable alkyl cations in superacids with molecular hydrogen [Eq. (5.7)]. 

Propane as a degradation product of polyethylene (a byproduct in the reaction) was ruled out because ethylene alone under the same conditions does not give any propane. Under similar conditions but under hydrogen pressure, polyethylene reacts quantitatively to form C3 to C6 alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. Siskin also found that when ethylene was allowed to react with ethane in a flow system, n-butane was obtained as the sole product, indicating that the ethyl cation is alkylating the primary C—H bond through a five-coordinate carbonium ion [Eq. (5.66)]. 

These results show a high selectivity in monolabeled propane 13CH3CH2CH3, which can only arise from direct electrophilic attack of the ethyl cation on methane via pentacoordinate carbonium ion [Eq. (5.67)]. 

More recently, it has been proposed (78, 79) that on zeolite catalysts the reaction can also start by protonation of a C-C bond by the acid site of the zeolite forming a pentacoordinated carbonium ion transition state. This can then eliminate H2 or a short alkane molecule leaving an adsorbed carbenium ion on the zeolite (protolytic cracking), as shown below ... 

Mechanistically, two pathways are logical (Scheme 3). The ethyl cation can directly alkylate methane via a pentacoordinated carbonium ion (Olah) (path a), or alternatively, although a less favorable pothway (b), the ethyl cation could abstract a hydride ion from methane. The methyl cation thus formed, which is less stable by 39 kcal/mole (26), could then react directly with ethylene. In the latter case, propylene and/or polymeric material would probably be formed since the hydrogen required for a catalytic reaction has been consumed by the formation of ethane. 

The nonexistence of alkane-alkene equilibrium in superacid medium has been elegantly demonstrated by the behavior of isobutane in deuterated superacid medium (DSOsF-SbFs or DF-SbFs). Isobutane at low temperature undergoes hydrogen-deuterium exchange only at the methine position through the involvement of a three-center bound pentacoordinate carbonium ion (Scheme 17). 

Olah and his group ° also investigated the direct ethylation of methane with ethylene using Relabeled methane over solid superacid catalysts, such as T s-AlFs, TaFs and SbFj-graphite. A high selectivity (up to 96% of label content of C3 fraction) for monolabeled propane ( RCC2H8) was observed. This clearly indicated a direct electrophilic attack of the ethyl cation on methane via a pentacoordinated carbonium ion, as in Scheme 19. 

Similarly, Siskin " found that when ethylene was allowed to react with ethane in a flow system, only n-butane was obtained. This was explained by the direct alkylation of ethane by ethyl cation through a pentacoordinated carbonium ion (equation 127). The absence of a reaction between ethyl cation and ethylene was explained by the fact that no rearranged alkylated product (isobutane) was observed. 

The electron donor character of C-H and C-C single bonds that leads to pentacoordinate carbonium ions explains the detailed mechanism of acid-catalyzed isomerization of n-butane (1) as shown in Scheme 6.6 to isobutane... 

The ethylation of methane with ethylene has also been investigated using C-labeled methane (99.9% C) over solid superacid catalysts such as TaFs-AIF3, fal s and Sbf. graphilc. The results show a high selectivity in mono-labeled propane, which can only arise from a direct electrophiUc attack of the ethyl cation on methane via a pentacoordinate carbonium ion [Eq. (6.35)]. 

El Tanany et al. (ref. 22) have investigated the n-heptane hydroconversion at 428 K and 1013 mbar on H-erionite. According to Haag et al. (ref. 26) and their results the formation of C3/C4 species should take place via a pentacoordinated carbonium ion by protolytic cracking. In a following chain process higher hydrocarbons may result. 

I+. For the definition of carbocations, including differentiation of tetra- and pentacoordinated carbonium ions frcm trivalent carbenium ions, see Olah, G.A., J. Amer. Chem. Soc. (1972)... 

The last few years brought the rapid development of acid catalyst preparations especially the synthesis of solids with high acid strength - the so-called superacids. At the same time the investigations of acid catalysed reactions showed that apart from classical carbocations, pentacoordinated carbonium ions, cation-radicals and even radicals may be considered as possible transition states. The mechanism of non-classical intermediate formation has not been fully explained yet and needs further investigations. 

Further evidence for the pentacoordinate carbonium ion mechanism of alkane protolysis was obtained in the H-D exchange reaction observed with isobutane. When isobutane is treated with deuterated superacids (DS03F SbF5 or DFiSbFs) at low temperature (—78°C) and atmospheric pressure, the initial hydrogen-deuterium exchange is observed only at the tertiary carbon. Ionization yields only deuterium-free t-butyl cation and Recovered isobutane from the reaction mixture shows at low... 

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