Rearrangement of alkanes

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

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... 

IV. Skeletal Rearrangement of Alkanes on Platinum and Other Noble Metals. 141... 

Acid-Catalyzed Reactions and Rearrangements of Alkanes, Cycloalkanes, and Related Compounds... 

In isomerization reactions only platinum, palladium and iridium are active metals for the skeletal rearrangement of alkanes. Hence, in the first part of this chapter we shall mainly focus on the catalytic behavior of these three metals. 

Skeletal rearrangement of alkanes, olefins, and functionalized compounds pinacolone rearrangement Wagner-Meerwein rearrangement epoxide rearrangement rearrangement of cyclic acetals... 

Isomerization (rearrangement) of hydrocarbons is of substantial practical importance. Straight-chain alkanes obtained from petroleum... 

The bases most commonly used to effect rearrangement are hydroxides, alkoxides, alcoholic sodium bicarbonate and, in some instances, amines. In the rearrangement of a series of l,l-dibromo-2-keto-alkanes, where a direct comparison has been made between triethylamine and sodium methoxide, the amine has given slightly better results ... 

Similar preference in replacement by fluorine of tertiary versus secondary and secondary versus primary hydrogens is observed in the fluorination of alkanes with chlorine trifluoride in 1,2-difluorotetrachloroethane at room temperature (Table 3). Skeletal rearrangements accompany the fluorination [31]... 

Thermal rearrangement of trans-l,2-dibromo compounds is known in the literature (refs. 6-10). In all case studies only one pair of bromine in each organic molecular was studied. Bellucci (ref. 10), for example, studied the kinetics of such trans-l,2-cyclo alkanes as cyclopentane, hexane, octane, etc. The intermediates suggested as an explanation for the experimental results are bromonium bromide I in polar solvents and four center transition state II in non-polar solvents. 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

Catalytic cracking When a mixture of alkanes from the gas oil fraction (C12 and higher) is heated at very high temperature (-500 °C) in the presence of a variety of catalysts, the molecules break apart and rearrange to smaller, more highly branched alkanes containing 5-10 carbon atoms. 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

This A cracking was shown to be 20 times faster than the rearrangement of type B involved in n-alkane hydroisomerization. 

Oppenlander, T. Zang, G. Adam, W. Rearrangements and Photochemical Reactions Involving Alkanes and Cycloalkanes. In The Chemistry of Alkanes and Cycloalkanes, Patai, S., Rappoport, Z., Eds. John Wiley and Sons, Chichester, 1992, 681 pp. 

Mention should be made of studies of slow, controlled combustion of alkanes, where formation of oxetanes can be detected. For example, oxetane is observed during combustion of propane, while 2-f-butyl-3-methyloxetane and 2-isopropyl-3,3-dimethyloxetane are observed from combustion of isooctane. While the yields are extremely low, the presence of these compounds, along with the other products found, have provided evidence for the mechanism of combustion. The oxetanes are believed to result from rearrangement of peroxy radicals in the radical chain process (equation 114) (70MI51300,73MI51301). 

The rearrangement of radical cations often leads to products not formed from the parent neutral compounds, and this fact is increasingly exploited in reactions initiated by single electron transfer. 1,8-Naphthoquinodimethane and its alkane-derivatives have been characterized spectroscopically as persistent species in... 

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