Hydrogen transfer, radical

Hydrogen Transfer Radical reacts with a hydrocarbon H-transfers from hydrocarbon to radical forming a lower-molecular-weight compound and a new radical... 

Radicals or Hydrogen transfer Radical ions Substrate or solvent... 

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

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). 

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].

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

If hydrogen transfer is under thermodynamic control, then the vitamin will experience cleavage of the weakest CH (or OH) bond. Compare energies ofradicals derived from hydrogen abstraction at different positions from a model a-tocopherol (R = CH3). Which radical is most stable Are there alternative radicals of similar stabihty ... 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

Acrylamide readily undergoes polymerization by conventional free radical methods, ionizing radiation, ultrasonic waves, and ultraviolet radiation. The base-cata-lized hydrogen transfer polymerization of acrylamide yields poly-/3-alanine (Nylon 3) a water insoluble polymer that is soluble in certain hot organics. All current industrial production is believed to be by free radical polymerization. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

Free radicals, unlike carbocations, do not normally undergo isomerization by methyl or hydrogen migration. However, hydrogen transfer (chain transfer) occurs when a free radical reacts with other hydrocarbons. 

Analysis of the products from the thermal decomposition of the mixed azo compound 6 showed that in the cross-reaction of radicals 5 and 7 ld/A tt(90oC) is 0.61.179 This study also found that in disproportionation, hydrogen transfer from 5 to 7 is ca 2.2 times more frequent than transfer from 7 to 5. Both self-reactions involve predominantly combination (Scheme 7.14). The values of Ar1j/Aru.(80°C) are 0.16 and 0.05 for radicals 5 (Section 5.2.2.1.1) and 7 (Section 5.2.2.1.3) respectively. It is clear that values of kJkK for homotermination cannot be used as a guide to the value for kjkyt in cross-termination. 

The rare [1,4] hydrogen transfer has been observed in radical cyclizations. With respect to [1,7] hydrogen shifts, the rules predict the thermal reaction to be antarafacial. Unlike the case of [1,3] shifts, the transition state is not too greatly strained, and such rearrangements have been reported, for example,... 

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