Alkenes Radical isomerisation

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

Irradiation of phenyldisulfide cleaves the weak S—S bond to give a pair of thiophenyl radicals. One of these then adds to the less-substituted olefinic carbon of 25 to generate the tertiary alkyl radical 26 (Scheme 14.5). Bond rotation then ensues to give the more stable rotamer 27 in which there is minimal steric repulsion between the C(7)-methyl and the tetrahydrofuran framework. Elimination of the thiophenyl radical from 27 finalises the isomerisation to alkene 19. 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

As with the pyridine series, competing alkene isomerisation proved to be a significant problem within the azastilbene series. It was especially prominent when aryl bromides were employed as radical precursors. For example, exposure of the bromide analogue of 193 to tributyltin hydride and AIBN returned the starting material as a mixture of cis- and trans-isomers. Alkene isomerisation was most pronounced when the alkene was conjoined at C4 of the quinoline. In this case, even carbon to iodide bond homolysis was slow by comparison (Scheme 54). 

By way of contrast, when attached at C3, no alkene isomerisation was observed with iodide 202 and it was only a minor side reaction with the corresponding aryl bromide. With two orr/zo-cyclisation pathways available to the radical intermediate, it is perhaps... 

Easic Principles Practical Photochemistry General Considerations Carbonyl Compounds a-Cleavage Carbonyl Compounds Hydrogren Abstraction Steroids Carbonyl Compounds Cycloaddition Enone and Dienone Rearrangements Alkenes Isomerisation and Rearrangement Alkenes Cycloaddition Alkenes Photo-Cxidation Terpenoids Aromatic Compounds Isomerisation and Cycloaddition Practical Photochemistry Scale-up Aromatic Compounds Substitution and Cydisation Alkaloids Photoinitiated Free-radical Chain Reactions. 

Double-bond migration of terminal alkenes forms more of the Z-isomer than corresponds to thermodynamic equilibrium. A study of the exchange and isomerisation of 1-pentene-1,2- 2 on various types of nickel catalyst has shown that this is due to crowding at the active centre, so that the pentyl radical on losing a... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

The present experiments were aimed at a quantification of NO/NO2 conversion factors in the first oxidation step of a series of alkanes, alkenes and aromatics. Moreover, the experimental approach, namely time-resolved measurement of OH and NO2 evolution together with numerical simulation permitted the extraction of individual rate coefficients for the decomposition/isomerisation of oxy radicals. It has been shown that these rate coefficients increase with the size of the molecule up to a limiting value of - 10 at 298 K. The NOCON factors derived may be used to extract condensed mechanistic equations for the primary oxidation steps of hydrocarbons. The values derived in this work are integrated quantities comprising weighted averages of all fractional yields of detailed mechanisms as initiated by the attack of the primary oxidant to the various positions of the parent compound. 

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