Internal isotope scrambling

Internal hydrogen scrambling and isotope redistribution between propane and solid catalysts may proceed by a rather complex mechanism involving several pathways with different rates and activation energies. The competitive pathways cannot be distinguished if the temperature for activating propane on zeolites is... 

However, in other instances, postelimination reactions can change the nature of the products. The main difficulty arises from the readdition of the alkene initially produced to the hydrido complex. This can result in isotopic scrambling or isomerization of the alkene upon reelimination. The formation of internal alkene products from the [Pd(PPh3)4]-catalyzed decarbonylation of acyl cyanides (equation 2) may also arise from postelimination reactions between the alkene and [PdH(CN)(CO)(PPh3) ] initially formed. The importance of this reaction in isomerizing the anti-Bredt alkenes formed by the catalytic decarbonylation of 3-chlorocarbonyltricyclo[5.3.1.0 ]iindecane is shown in Scheme 3. 

That the rates of 0( D2) deactivation by N2O and CO2 are similar is fortuitous, the high efficiency of quenching by CO2 arising from the intermediacy of a long-lived CO3 complex, the existence of which can be inferred from isotope scrambling studies (Baulch and Breckenridge, 1966). Excitation of the internal modes of CO2 following... 

The results show that considerable isotopic scrambling takes place over all five catalysts. At low conversion, the unconverted -butane retained its isotopic identity, n-butane molecule contains two and two C atoms. However, the fragmentation pattern in the mass spectrum showed that considerable internal rearrangement of CH3- CH2- CH2- CHa to takes place. In this respect solid and liquid acid catalysis... 

Each of these units will still contain two C atoms no isotopic scrambling takes place. This statement is rigorously correct only if no internal rearrangement in the C4 monomers occurs prior to the formation of the Cg dimer. In reality, internal rearrangement is well documented. Its rate has, however, been found to be lower than the rate of isomerization for all catalysts. Randomization of the C label, as observed over sulfated zirconia would require an extremely fast internal atom rearrangement prior to the formation of the Cs intermediate, if only simple methyl shifts inside the dimer took place before p-fission. This model can thus be discarded. It follows that substantial carbon scrambling in the Cs carbocation is required to achieve randomization of the carbon atoms in the ultimate C4 entities. 

Howe, I. McLafferty, F.W. Unimolecular Decomposition of Toluene and Cyclohep-tatriene Molecular Ions. Variation of the Degree of Scrambling and Isotope Effect with Internal Energy. J. Am. Chem. Soc. 1971,93,99-105. 

A detailed and elegant study of the SnI solvolysis reactions of several substituted 1-phenylethyl tosylates in 50% aqueous TEE has enabled the rates of (1) separation of the carbocation-ion pair to the free carbocation, (2) internal return with the scrambling of oxygen isotopes in the leaving group, (3) racemization of the chiral substrate that formed the carbocation-ion pair, and (4) attack by solvent to be determined.122... 

McLafferty, F. W., and Howe, I. (1971). Metastable ion characteristics. XVII. Unimolecular decomposition of toluene and cycloheptatriene molecular ions. Variation of the degree of scrambling and isotope effect with internal energy. J. Am. Chem. Soc. 93, 99-105. 

Internal Ion Pair Return—collapse of a contact ion pair to covalent material, sometimes referred to as hidden return if the covalent material is formed without scrambling of isotopic label or without partial racemization ... 

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