Free radical isomerization

The isomerization of a free radical takes place by the internal transfer of a hydrogen atom. The activated complex is a cyclic intermediate whose size is equal to 4 for a 1-3 transfer, to 5 for a 1-4 transfer, to 6 for a 1-5 transfer... 

The property AjS° is equal to the difference between the entropy of the activated complex and that of the free radical. It consists mainly of two terms. 

The translation term is equal to zero, as the mass of the activated complex and of the free radical is the same, the terms due to external rotations, vibrations and the electronic terms are negligible therefore, finally  

The property AjH° is calculated by adding the activation energy of the reaction of 

Calculate the rate constant for the isomerization reaction of the neopentylperoxy radical by a 1-5 internal transfer of an H atom at T = 1000 K. 

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The experimental data on free radical isomerization reactions with H-atom abstraction were analyzed by the IPM method [36]. The kinetic parameters of isomerization were compared with those of the intermolecular H-atom abstraction (see Chapter 2). The results... 

The free radical isomerization of allylic hydroperoxides (3) in dilute nonpolar solvents appears to give a reliable measure of their relative... 

J. Marco-Contelles, P. Ruiz-Femandez, and B. Sanchez, New annulated furanoses A new free-radical isomerization of an S-methyl hex-5-enylxanthate to an 5-(cyclopentylmethyl) S-methyl dithiocarbonate, J. Org. Chem. 58 2894 (1993). 

Paquette and coworkers [127], in a synthesis of the complex polyol antibiotic amfidinolu-3, made masterly use of the Julia-Kodenski olefination reaction. Two examples taken from that work are presented below. Reaction of sulfone 285 with aldehyde 284 (Scheme 92) carried out with KHMDS in THF (at - 78 °C to rt) gave the building block 286 with 90% yield but with relatively poor selectivity, EjZ 1>I. Free-radical isomerization of the mixture (benzene, AIBN, reflux) increased the isomer ratio, /2 6/1. 

The mechanism of the process is not yet clear. This rearrangement was considered at one time as intramolecular, but it was later concluded that the transformation is rather intermolecular and can occur through formation of a carbenium ion intermediate25,26. However, evidence was obtained more recently in favor of a free radical mechanism involving an /V-alkyl bond fission27-29. An intermolecular free radical isomerization was described also for a thermal decomposition of (V-benzyl-A-methylaniline hydroxides30,31. 

Suppose you made the following observation [Heiba, E-A. I. Dessau, R. M. Free-radical isomerization. I. Novel rearrangement of vinyl radicals /. Am. Chem. Soc. 1967, 89, 3772-3777]. How might you... 

Propagation. The free radicals experience reactions of hydrogen transfer, decomposition, and creation of new low molecular weight free radicals, isomerization, cyclization, condensation. 

FIGURE 4 20 The bond dis sociation energies of methy lene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals The secondary radical is more stable than the primary... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

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. 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

This sequence of reactions forms a product rich in C, and Cj, and a fair amount of alpha-olefins. Free radicals undergo little branching (isomerization). 

Isomerization reactions occur frequently in catalytic cracking, and infrequently in thermal cracking. In both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions they shift around and crack to produce branched molecules (Equation 4-10). (In thermal cracking, free radicals yield normal or straight chain compounds.)... 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

The investigated cw-stilbene derivatives, 4-methoxy, 4,4 -dimethyl, unsubstituted, and 4,4 -bis(trifluoromethyl)stilbenes, had k2 values spanning 6-7 powers of ten both in methanol and in acetic acid. Products 2, 4, 5 and 6 were formed. Table 8 reports the results of the cis-trans isomerization test in acetic acid (ref. 29). No acid catalyzed or free radical process was found to be responsible for these isomerizations. 

Detection of an Intermediate. In many cases, an intermediate cannot be isolated but can be detected by IR, NMR, or other spectra. The detection by Raman spectra of NOj was regarded as strong evidence that this is an intermediate in the nitration of benzene (see 11-2). Free radical and triplet intermediates can often be detected by ESR and by CIDNP (see Chapter 5). Free radicals [as well as radical ions and EDA complexes] can also be detected by a method that does not rely on spectra. In this method, a doublebond compound is added to the reaction mixture, and its fate traced. One possible result is cis-trans conversion. For example, cis-stilbene is isomerized to the trans isomer in the presence of RS- radicals, by this mechanism ... 

For many radicals, step 1 (C=C+Y- -C—C—Y) is reversible. In such cases, free radicals can cause cis — trans isomerization of a double bond by the pathway. ... 

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