Alkane acid-catalyzed transformations

Protonated alkanes are important highly reactive intermediates in the acid-catalyzed transformations of hydrocarbons. However, the simplest protonated alkane, the methonium ion, which exemplifies the entire family of nonclassical... 

Under the conditions applied the dynamic parameters for the transport of the weak base Ci-Cj alkanes are diffusion controlled and do not tell us anything about the dynamics of sorption, i. e., about the sorption sites. Stronger base olefins cannot be used as the sorptive for probing strong acidity since they are easily converted. Acetylene is quite inert to acid catalyzed transformation and its sorption is stronger than that of the alkanes. However, a full proton... 

One of the main difficulties in understanding the carbocationic nature of acid-catalyzed transformations of alkanes via the hydride abstraction mechanism was that no stoichiometric amount of hydrogen gas evolution was ever observed from the reaction mixture, although even in the early work of Nenitzescu et al. H2 gas was detected in measurable amounts. For this reason, an alternative mechanism was proposed The direct hydride abstraction by the Lewis acid [Eq, (6,10)],... 

Could acidic minerals catalyze the isomerization of methylhexanes to dimethylpentanes The answer is yes under certain conditions. As extensive thermal cracking generates mixtures of lighter n-alkanes and a-olefms, conversion of a-olefms into mixtures of methyl-substituted alkanes is typical for olefin transformation in the presence of acidic catalysts. The acid-catalyzed transformation of hydrocarbon such as isomerization, alkylation, and cracking play an important role in the industrial processes of petroleum industry (Brouwer and Hogeveen 1972 Poutsma 1976 Corma and Wojciechowski 1985 Boronat et al. 1996). 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

Side-Chain Isomerization. Arylalkanes undergo acid-catalyzed isomerization in the side chain in a way similar to the skeletal rearrangement of alkanes.70-72 There are, however, notable differences. Propylbenzene, for instance, yields only a small amount (a few percentages) of isopropylbenzene 73 Similarly, sec-butyl- and iso-butylbenzene are interconverted at 100°C with wet A1C13, but only a negligible amount of tert-butylbenzene is formed.74 In the transformation of labeled propylbenzene the recovered starting material was shown to have equal amounts of labeling in the a and p positions of the side chain, but none in the y position 73... 

The catalyst causes a classical carbenium ion to be formed by acid catalyzed activation reactions. The classical carbenium ion is transformed into the key intermediate which can be described as a protonated cyclopropane structure. After some rearrangements cracking occurs. The formation of branched paraffins is very fortunate since branched paraffins have high octane numbers and the isobutane produced can be used in alkylation. The preferred products are those of which the formation proceeds via tertiary carbenium ions. Carbenium ions can also be generated by intermolecular hydride transfer reactions between alkane and carbenium ions that are not able to form tertiary carbenium ions (see Chapter 4, Section 4.4). Under more severe conditions lower paraffins can also be cracked. 

Spirocyclic orthoesters, e.g. (397 equation 184), have been synthesized by Lewis acid catalyzed addition of epoxides to lactones. Lactones can be transformed to spirocyclic orthoesters, e.g. (398) and (399) (Scheme 73), by treatment with mixtures of either diols and orthoformates or l,2-bis(trimethyl-siloxy)alkanes and trimethylsilyl triflate. ... 

It must also be pointed out, however, that initiation of acid-catalyzed alkane transformations under oxidative conditions (chemical or electrochemical) can also involve radical cations or radical paths leading to the initial carbenium ions. In the context of our present discussion, we shall not elaborate on this interesting chemistry further and limit our treatment to purely protolytic reactions. ... 

The reaction network for isobutane selective oxidation catalyzed by POMs consists of parallel reactions for the formation of methacrolein, methacrylic acid, carbon monoxide, and carbon dioxide. Consecutive reactions occur on methacrolein, which is transformed to acetic acid, methacrylic acid, and carbon oxides. ° Methacrylic acid undergoes consecutive reactions of combustion to carbon oxides and acetic acid, but only under conditions of high isobutane conversion. Isobutene is believed to be an intermediate of isobutane transformation to methacrylic acid, but it can be isolated as a reaction product only for very low alkane conversion. ... 

Cracking of small saturated hydrocarbons, catalyzed by zeolites, can proceed via two mechanisms, both involving carbocations the bimolecular chain reaction, which involves carbenium ions that are further transformed by / -scission, and the unimolecular protolytic mechanism, involving alkanium ions that are formed by the direct protonation of the alkane by the Br0nsted acid OH groups of the catalyst. This latter mechanism, originally proposed by Haag and Dassau, is the predominant one at about 800 K in medium-pore zeolites, like HZSM-5, which favor monomolecular reactions. While rela-... 

A high-valent ruthenium complex is also reported to cleave the sp C-H bond. RuCl3 -3H20 catalyzes the transformation of cyclic alkanes to the corresponding ketones in the presence of peracetic acid, where oxoruthenium species is considered to act as the active species. Alcohol, as a primary product in this oxidation reaction, is obtained as an intermediate in the presence of trifluoroacetic acid (Scheme 14.11) [25]. 

These oxidative carbonylations also occur with alkanes, although with low conversions of alkane and relatively low turnover numbers. Fujiwara reported that cyclohexane reacts with CO and K S Oj in the presence of palladium acetate and copper acetate in trifluoroacetic acid to form cyclohexane carboxylic acid with about 20 turnovers and about 4% yield based on alkane (Equation 18.24). Fujiwara also reported the carboxylation of methane with KjSjOj as oxidant with V(0)(acac)j as catalyst (Equation 18.25). This reaction occurred in 93% yield based on methane and with 18 turnovers. Sen reported a palladium(II)-catalyzed oxidative carbonylation of methane with hydrogen peroxide as the oxidant. Subsequently, he showed that RhClj catalyzes the conversion of methane, CO, and oxygen to acetic acid at 100 °C in water (Equation 18.26). Periana has reported a somewhat related transformation of methane to acetic acid, although the reaction is conducted in the absence of CO, and both carbon atoms of the acetic acid arise from methane (Equation 18.27). In this case, the CO appears to arise from oxidation of the methaiie, as shown in Scheme 18.5. 

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