Oxygen-centered radicals polarity

Not all radical aromatic substitutions are as immune to polar effects as is attack by phenyl. Some radicals reveal marked electrophilic or nucleophilic character. Oxygen-centered radicals, for example, are electrophilic, as would be expected if there is substantial polar contribution to the transition state. Table 9.13 lists partial rate factors for substitution by benzoyl radicals note that the orientation and activation trends found in typical electrophilic substitutions have begun to appear, but are still modest compared with the dramatic effects shown in Table 9.12 for a true heterolytic substitution.179... 

Eq. 4.54 shows the reaction of n-heptanol (151) with Pb(OAc)4 under high-pressured carbon monoxide with an autoclave to generate the corresponding 8-lactone (152). This reaction proceeds through the formation of an oxygen-centered radical by the reaction of alcohol (151) with Pb(OAc)4,1,5-H shift, reaction with carbon monoxide to form an acyl radical, oxidation of the acyl radical with Pb(OAc)4, and finally, polar cyclization to provide 8-lactone [142-146]. This reaction can be used for primary and secondary alcohols, while (3-cleavage reaction of the formed alkoxyl radicals derived from tertiary alcohols occurs. 

The C—H bond is normally not very polar. As a result, the <7Ch and ch orbitals are widely separated and more or less symmetrically disposed relative to a. A sluggish reaction is expected with carbon free radicals, but a rapid reaction may be anticipated with both electrophilic and nucleophilic free radicals. Examples of both kinds of reactions are ubiquitous in organic chemistry. An ab initio investigation of the former, involving oxygen-centered free radicals, has been carried out [237], The reactivity spectrum may be modified by substitution on the carbon bearing the hydrogen atom. As we have seen in Chapter 7, all three kinds of substituents stabilize the carbon-centered free-radical intermediate. 

One of the most important implications of these experiments is the conclusion that, due to the high polarity of the C=0 bond, organoelement a-ketones R3MCOR (M = Ge, Sn) are extraordinary effective radical traps. The electronegative oxygen atom of their carbonyl group could be attacked not only by element-centered radicals R3M (M = Ge, Sn), but also by thiyl radicals SR " and phosphorus-centered radicals. Indeed, the experimental estimates of the absolute reaction rate constants of the element-centered radicals with... 

As a last open shell system, we have chosen the H2C0 radical, which has been well characterized both at the theoretical and experimental level [77-80]. In this radical the symmetry of the singly occupied molecular orbital (a rt-in plane orbital located mainly on the oxygen center) [79] determines that only spin polarization effects contribute to isotropic hcc s of C and O, whereas spin densities at hydrogens have also a direct delocalization contribution. The results of table 5 show that, as expected, isotropic hcc s at the H atoms are well reproduced by aU the functionals, whereas the results for heavy atoms are much more scattered. In particular, the hyperfine constant of the carbon atom is -35 G at the PBEO level, in better agreement with the experimental value (-39 G)[80] than the B3LYP prediction (-34 G). It is noteworthy that the PBEO functional provides values for the H and C atoms which are sufficiently accurate. Unfortunately, no experimental value is available for the oxygen atom. 

A different picture is observed when a polar radical reacts with a C—H bond of a polar molecule. For example, the reaction of an oxygen atom with the methane C—H bond is characterized by the activation energy of thermoneutral reaction /ic0 54.6 kJ mol-1 and parameter bre= 13.11 (kJ mol-1)172 while the reaction with the methanol C—H bond is characterized by Ed) 50 kJ mol-1 and parameter brc 12.55 (kJ mol-1)172 [30]. For these values of bre, the difference between the activation energies is 4.6 kJ mol-1. The decrease in the activation energy can be explained by the fact that the polar O—H group in the O H C—OH transition state interacts with the O H C polar reaction center. 

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