Alkanes hydrogen exchange

Zheng X, Blowers P (2005) A computational study of alkane hydrogen-exchange reactions on zeolites. J Mol Catal A Chem 242(1-2) 18-25... 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

Until recently, halogen substituted alkanes were thought to be inert to xenon difluoride, but detailed examination has shown that common solvents such as dichloromethane (3) and chloroform (4) undergo fluorine-chlorine and fluorine-hydrogen exchange at room temperature due to hydrogen fluoride catalysis.15... 

The protolytic activation of the alkane is, however, only the apparent part of the reaction as long as the alkane or the acid is not isotopically labeled. When HF is replaced by DF and the isobutane-CO mixture is bubbled through the DF-SbF5 acid (6 1 molar ratio) at — 10°C, the apparent conversion based on ester or H2 formation is only 4% but the 1H/2H NMR analysis of the apparently unreacted isobutane (96%) shows extensive H-D exchange (18 atom% in the tertiary position and 9 atom% at each primary position).30 The most plausible rationalization of hydrogen exchange is via the formation of carbonium ions (here pentacoordinate transition states or intermediates) as described in Eq. (5.15). 

Several trails of evidence are left by transient alkane a complexes, including a common observation that hydrogen exchange takes place between the hydride and... 

G. A. Olah, R. H. Schlosberg, Chemistry in super acids. I. Hydrogen exchange and polycondensation of methane and alkanes in FSOjH-SbFs ( magic acid ) solution. Protonation of alkanes and the intermediacy of CHJ and related hydrocarbon ions. The high chemical reactivity of paraffins in ionic solution reactions, /. Am. Chem. Soc., 1968, 90, 2726-2727. 

G. A. Olah, G. Klopman, R. H. Schlosberg, Super acids. HI. Protonation of alkanes and intermediacy of aUcanonium ions, pentacoordinated carbon cations of CHJ type. Hydrogen exchange, protolytic cleavage, hydrogen abstraction, polycondensation of methane, ethane, 2,2-dimethylpropane and 2,2,3,3-tetramethylbutane in FSOsH-SbFs, J. Am. Chem. Soc., 1969, 91, 3261-3268. 

Ziegler has studied bond metathesis of CpjSc-R (R = H, Me, Et, Pr, vinyl, acetylide) with both Hj and alkanes, using DFT methods where Cp rather than Cl or H is used to model the experimental Cp system.A weak Hj adduct forms in the early stages of the hydrogen exchange reaction with Cp2ScD (using D to differentiate atoms) ... 

Another field of analytical applications is connected with polymers. Polypropylene can have different microstructures described as atactic, syndiotactic or isotactic and in one sample of the polymer combinations of these forms may be present. A detailed study of the NMR spectra can distinguish between these forms, although high-resolution spectra are usually needed to reveal all the necessary details A quite different analytical approach employing deuterium NMR of alkanes was demonstrated in a study of rhodium and platinum catalysts which catalyse deuterium-hydrogen exchange ... 

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