Reactions of Phenyl Radicals

We observed reactive scattering signal at w/z=102 (CgHg + ) (acetylene), 

Product translation energy Ej (kJ/mol) Center of mass angle 6 (degree) 

To gain these important data and to gather crucial mechanistic information, it is important to inspect the derived center-of-mass functions and flux contour maps (Figs 11.7 and 11.8, respectively). 

FIGURE 11.8 Center-of-mass velocity contour flux map for the reaction of phenyl radicals (left 0°) with acetylene (a), ethylene (h), and D6-henzene (c) to form phenylacetylene (a), styrene (h), and D5-diphenyl (c) (right 180°). The colors connect data points with an identical flux and range from red (highest flux) to yellow (lowest flux). The units of axis are given in ms (see legend). 

It should be indicated that the methylacetylene and propylene are more complex reactants than the nonsubstituted counterparts and depict nonequivalent hydrogen atoms at the acetylenic and methyl group (methylacetylene) and at the vinyl and methyl group (propylene). Therefore, even the detection of the atomic hydrogen loss makes it difficult to elucidate if the hydrogen atoms are lost from the methyl group, the acetylenic/vinyl units, or from both positions. In these cases, it is very useful to conduct experiments with partially deuterated reactants J3- 

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Grossmann attributes these results to simple reactions of phenyl radicals, in competition with two effects of oxygen ... 

This supposition was validated by experimental studies that demonstrated the prevalence of different ROS at different temperatures. Using CRDS, Yu and Lin studied the reaction of phenyl radical and oxygen, noting that phenylperoxy radical was the only adduct formed at temperatures ranging up to 473 Venkat et al. completed flow reactor studies of benzene combustion at 1200K and identified phenoxy radical as a key intermediate. 

An example of the first type of reaction is provided by the preparation of the cyclohexyl radical by the reaction of phenyl radicals with cyclohexane. The preparation of the cyclohexyl radical without the use of a... 

However, caution is needed in applying this method since unexpected reactions may occur. For instance, the expected radical from the reaction of phenyl radicals with 2-methylpentane would be the 2-methylpent-2-yl radical formed by scission of the tertiary CH-bond. In fact the e.s.r. spectrum of the deposit showed that the radical (CH3)2CH.CH2.CH2.CH2 was formed exclusively. The reason for this unlikely selectivity is obscure but it is possible that the molecule adopts a conformation in the solid state such that the terminal methyl group more accessible to attack by radicals than the rest of the molecule. 

FIGURE 11.9 Schematic potential energy surfaces for the reactions of phenyl radicals with acetylene (a), ethylene (b), and D6-benzene (c) to form phenylacetylene (a), styrene (b), and diphenyl (c). 

Logan, C. F Chen, P. Ab initio calculation of hydrogen abstraction reactions of phenyl radical and p-benzyne, J. Am, Chem. Soc. 1996,118, 2113-2114. 

In Scheme VI the benzene is seen to be formed by a transfer reaction of phenyl radicals, and two types of phenyl radicals can be distinguished, namely those arising by decarboxylation of benzoyloxy radicals and those escaping the primary pairs benzoyloxy/phenyl and phenyl/phenyl. Now, scavenging of the ben-... 

The actual structural definition must await isolation of sufficient quantities of the components for spectroscopic studies under various conditions. The possibility of a hydrogenated p-terphenyl was carefully considered, but the only structure even remotely consistent with the infrared and NMR data would require that the center ring of p-terphenyl be nonaromatic with mono-substituted phenyl groups attached as in IX. Such a material could be formed by preferential hydrogenation of the center ring of p-terphenyl (statistically unlikely) or by the reaction of phenyl radicals and 1,3-cyclohexadiene as follows ... 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Frank et al. [168] studied the reactions of phenyl radicals with molecular and atomic oxygen in shock tube experiments in the temperature range of 1000 K to 1800 K and pressures from 1.3 to 2.5 bar. For the initiation reaction CeHs + O2, two products channels were reported CeHs + O2 CeHsO + O with the apparent activation energy for the formation of CeHsO of 6 kcal mol and a fast channel producing H-atoms, which they assigned as CeHs + 02 C6H4O2 + H. 

Figures 6.3 - 6.8 describe the pathways and energetics relevant for the reaction of phenyl radicals with molecular oxygen. The phenyl-peroxy (PhOO ) is formed with nearly 50 kcal mol of excess of energy and there are several forward reaction channels that require less energy for this chemically activated adduct to react to. The names and structures of the adduct/transition state/product have previously been described in Table 6.3.

Reaction of Phenyl Radical with O2 Thermodynamic Properties, Important Reaction Paths and Kinetics ... 

Reaction of Phenyl Radical with O2 Thermod5mamic Properties, Reaction Pathways and Kinetics Nadia Sebbar and Henning Bockhom... 

The fact that the intermediate phenyl radicals react before they are reduced seems to support the second explanation for the retention of configuration observed in the previous example. Since, however, phenyl radicals are much more reactive than alkyl radicals, it is just possible that the reduction of the intermediate radicals by electrons may be slow compared with the reactions of phenyl radicals but still fast compared with the rate of inversion of cyclopropyl. In this case the phenyl radicals would react before they could be reduced, while the cyclopropyl radicals would be reduced before they could invert. 

Trotman-Dickenson reported an activation energy of 6.7 kcal/mol for the reaction of phenyl radicals with isobutane in the gas pha.se. ( ) On the basis of this, we were confident that conformational interconversion (ki,k i) would be rapid with respect to other reactions and that the product composition would be determined by the ratio kj [SH]k-i/kj ki. Recent experimental work by J. P. Lorand,( ) however, shows that the rate of reaction of phenyl radicals with the tertiary hydrogens of alkanes is about 10 times faster than that reported by Trotman-Dickenson. A competition between kR[SH] and ki is, therefore, plausible. 

The intermediacy of phenyl radicals in the thermal decomposition of aryldiazo alkyl ethers (9)" and in the products from nitrosation of 1,3-diphenyltriazene (10)" has been demonstrated. Reactions of phenyl radicals with substituted 9-methylanthracenes and 9-halogeno-anthracenes invariably give 10-phenylated products."... 

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