Frontier orbitals reacting with radicals

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene 7.36, to anthracene 7.37 and to thiophene 7.38, with the regioselectivity shown on the diagrams. In all three cases, the frontier orbitals are clearly in favour of this order of reactivity (because of the symmetry in these systems, both HOMO and LUMO have the same absolute values for the coefficients). 

Radicals should be very soft entities—most of them do not have a charge, and in most chemical reactions they react with uncharged molecules. Thus the Coulombic forces are usually small while the frontier orbital interactions remain large. In a sense this is borne out by such well-known reactions as the attack of radicals at the conjugate position of oc,/ -unsaturated carbonyl compounds,... 

The discussion on pp. 29-31 established the many reasons why an allyl anion and an allyl cation react with electrophiles and nucleophiles respectively at C-l (and C-3) of the allyl system. The force of these arguments is less when they are applied to the reaction of an allyl radical with a radical. Although the frontier orbital interaction (Fig. 5-10a) will still favour attack at C-l in the usual way, the interaction of the lowest filled 7E-orbital will not be negligible, especially with a radical having a low-energy SOMO (Fig. 5-10b). Since the lowest filled 7t-orbital has the larger coefficient on C-2, reaction at this site... 

It is worth nothing that the use of both I and A for both reactants gives the best correlation with the rate constants. The more common procedure in frontier orbital theory is to consider only I for the donor and A for the acceptor. This works for the alkyl olefins, but fails badly for the chloro olefins. If a nuleophilic radical, such as (CH3)3C, is used as the common reagent, then the order in Table 3.14 is completely inverted." The chloro olefins now react very rapidly, and the alkyl olefins are slow. 

Till now, we have seen that radicals can be considered to be either nucleophilic (reacting fastest with electron-poor alkenes) or electrophilic (reacting fastest with electron-rich alkenes). These tendencies can be nicely rationahzed in terms of frontier molecular orbital (FMO) theory. Recall the key ideas of this theory ... 

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