Hydrogen transfer reaction

Reactor temperature. An increase in the reactor temperature will also reduce delta coke by favoring cracking reactions over hydrogen transfer reactions. Hydrogen transfer reactions produce more coke than cracking reactions. 

Certain catalyst properties appear to increase coke formation. Catalysts with high rare earth content tend to promote hydrogen transfer reactions. Hydrogen transfer reactions are bimolecular reactions that can produce multi-ring aromatics. 

A proposed mechanism for the Cp lr-catalyzed Tishchenko reaction is illustrated in Scheme 5.35. In this reaction, hydrogen transfer from the hemiacetal to aldehyde catalyzed by the Cp lr complex would be crucial. 

Rearrangement Reactions. Hydrogen transfer from a metal site to the /3-carbon of a /u.,i72-vinyl complex may also yield edge bridging al-kylidenes (Scheme 24). The only report of such an internal rearrangement... 

Moreover, they involve several fragmentation reactions, hydrogen transfer and protonation equilibria of potential biological significance vide infra). 

In kinetic NMR experiments, rate constants for the intermolecular hydrogen transfer from several carbohydrates to cysteinyl radicals were found to be of the order of k29 = (1-3) x lO s at 37 °G [77]. These values agree with previous, pulse radiolytically determined rate constants for thiyl radical-mediated hydrogen abstraction from various model alcohols and ethers [74, 78, 79]. In contrast, the reverse reaction, hydrogen transfer from thiols to carbohydrate radicals, proceeds with k 29 > 10 s [80, 81], indicating that equilibrium (3.29) is normally lo-... 

The corresponding reverse reactions are radical termination, H-transfer, and radical addition, respectively. Figure 3 shows free radical mechanism of n-hexane cracking at high pressure (20-200 MPa), summarized by Domine (1987) and Kressman et al. (1990). Initiation reactions are necessary to produce the first radicals from pure hexane, but most of further n-hexane cracking is due to bimolecular reactions (hydrogen transfer) that are much more rapid as soon as minute radical concentration are reached. 

Quasiclassical calculations are similar to classical trajectory calculations with the addition of terms to account for quantum effects. The inclusion of tunneling and quantized energy levels improves the accuracy of results for light atoms, such as hydrogen transfer, and lower-temperature reactions. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Propylene. Propylene alkylation produces a product that is rich in dimethylpentane and has a research octane typically in the range of 89—92. The HF catalyst tends to produce somewhat higher octane than does the H2SO4 catalyst because of the hydrogen-transfer reaction, which consumes additional isobutane and results in the production of trimethylpentane and propane. 

Miscellaneous Reactions. Ahyl alcohol can be isomerized to propionaldehyde [123-38-6] in the presence of sohd acid catalyst at 200—300°C. When copper or alumina is used as the catalyst, only propionaldehyde is obtained, because of intramolecular hydrogen transfer. On the other hand, acrolein and hydrogen are produced by a zinc oxide catalyst. In this case, it is considered that propionaldehyde is obtained mainly by intermolecular hydrogen transfer between ahyl alcohol and acrolein (31). 

A second process has two steps. The first step is oxidation of propylene [115-07-1] to acrolein and the second step is reduction of acrolein to ahyl alcohol by a hydrogen transfer reaction, using isopropyl alcohol (25). 

Stabilized by hydrogen transfer. The stabilized free radicals undergo secondary cracking reactions as they come in contact with the hot coke. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

This reasoning was set forth by Johnston and Rapp [1961] and developed by Ovchinnikova [1979], Miller [1975b], Truhlar and Kupperman [1971], Babamov and Marcus [1981], and Babamov et al. [1983] for reactions of hydrogen transfer in the gas phase. A similar model was put forth in order to explain the transfer of light impurities in metals [Flynn and Stoneham 1970 Kagan and Klinger 1974]. Simple analytical expressions were found for an illustrative model [Benderskii et al. 1980] in which the A-B and B-C bonds were assumed to be represented by parabolic terms. 

Let us now turn to the influence of vibrations on exchange chemical reactions, like transfer of hydrogen between two O atoms in fig. 2. The potential is symmetric and, depending on the coupling symmetry, there are two possible types of contour plot, schematically drawn in fig. 17a, b. The O atoms participate in different intra- and intermolecular vibrations. Those normal skeleton... 

During the high-temperature operations, intermolecular hydrogen transfer reactions occur, transforming some indene to indane. The high indane concentration in the resin feedstock causes low yield and poor quality in the polymerization process. The indene loss can be reduced by decreasing the temperature and the residence time during distillation. 

The hydrogen transfer (from carbon to the heteroatom). Reactions of the earbon radical. 

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