Branched alkanes rearrangement

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. 

Very little skeletal rearrangement occurs via pyrolysis, a fact inherent in the failure of free radicals to readily isomerize by hydrogen atom or alkyl group migration. As a result, little branched alkanes are produced. Aromatization through the dehydrogenation of cyclohexanes and condensation to form polynuclear aromatics can take place. Additionally, olefin polymerization also can occur as a secondary process. 

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

Similar to linear and branched alkanes, cycloalkanes also give rise to radical cations in zeolites, spontaneously or upon y-radiolysis. This brief discussion of selected examples is intended only to give a flavor of the work being done. Thus, a 13-line radical cation spectrum (a = 0.17 mT, g = 2.003) obtained upon incorporation of 1-methylcyclohexane, 43, into zeolites [71] was identified as 1,2-dimethylcyclopentene radical cation, 44 + (two sets of protons with hyperfine couphng constants in the ratio of ca 2 1 a = 1.67 mT, 2 CH3 a = 3.42 mT, 4H) [72]. The formation of 44 + was rationalized by protonation of the 3° carbon of 43, followed by loss of H2. Loss of a proton from a rearranged carbocation may generate 44, which is oxidized to 44 + by a Lewis site. 

Branched alkanes with less than 6 C atoms in their main chain also produce benzene. They must, however, undergo skeletal rearrange-... 

Carbonium ion rearrangements. At low temperatures, strong acids (- Ho about 10) induce methyl shifts in branched alkanes (a hydride acceptor must be present to form the carbonium ion). 

Rearrangement or isomerization reactions proceed typically at carbocations or other electron-deficient positions of a molecule. In rearrangement reactions the substrate stabilizes itself by rearranging its structure without changing the number and type of its atoms. Thus, rearrangement reactions proceed without addition/release of molecules other than substrate and product. Rearrangement reactions of technical importance are the isomerization of linear alkanes to branched alkanes (important to increase the quality of fuels) and the rearrangement of cyclohexanone oxime to e-caprolactam (Scheme 2.2.4). 

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. 

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