Radical addition bond strength effects

The natural conclusion to be drawn from Table 12 is that steric effects play a very big part indeed in determining the rate and orientation of radical addition. We believe this conclusion to be correct, but, just as it is impossible to separate bond strength effects from polar effects, so it is impossible to separate steric effects in radical addition from polar effects. Any steric hindrance in the addition of a radical to an olefin will depend to a major extent on the shape of the radical. However, the shape of the radical is directly connected to the electron density at the trivalent carbon atom. 

Reactivities of a variety of alcohols, ethers, and amides toward hydroxy radicals derived from Fenton s reagent have been compared with those obtained from radiation chemistry in the absence of iron species.728 b Reactivities of different C—H bonds indicate that hydroxyl radical is a strongly electrophilic radical so that electron supply is more important than C—H bond strength in determining its reactivity. Fenton s reagent thus serves as a useful means for studying the one-electron oxidation and reduction of the resulting carbon-centered radicals with iron(II, III) species.73a,b Furthermore, in these systems the addition of copper(II) complexes that can intercept free radicals effectively often leads to enhanced yields of oxidation products.72 73... 

The regiochemical outcome of a radical addition to substituted alkenes generally depends on a complex blend of bond strength, polarity and steric effects . The more common result is the attack on the less substituted carbon. Recently Shaik and Canadell used a state correlation diagram (SCO) model to derive the regiochemical trends in radical addition to substituted alkenes. Regiochemistry was discussed in terms of the relative spin density in the state of the alkene (which directs the radical... 

Additions of alkyl radicals onto carbonyl groups are reversible and energetically unfavorable because of strong n bond strengths of carbonyl bonds. Fragmentation reactions of oxy radicals are faster than additions to carbonyl groups. Thus, it is anticipated that carbonyl derivatives cannot be used as elficient radical traps. Only several carbonyl derivatives are effective to some extent in radical cyclizations. The intermolecular addition of alkyl radicals to carboxylic acid derivatives represents a radical acylation reaction in which carboxylic acid derivatives are required to be... 

A second type of polar effect on activation energies is revealed in comparisons of the reactions of CF3 and CH3 radicals with non-polar molecules. In general, it is found that Eqhs Eqf3 2—3 kcal mole" for attack on non-polar alkanes or silanes. In terms of the Polanyi relations for CH3 and CF3, the difference in bond strengths D(CF3—H) — D(CH3-H) = 2 kcal mole" would account for about 1 kcal mole" difference in the activation energies. The additional 1—2 kcal mole" lowering in as compared with Fchs for tbe same substrate is... 

We can compare the stabilizing effects of substituents on radicals by comparing their effect on the strengths of, e.g., the CH bonds in methane. Addition of a vinyl or a carbonyl substituent to methane decreases the bond strength of the CH bond by approximately 13 kcal/mole. The effect is virtually independent of whether the substituent is of the or -t- type. 

The combined influences of polar and steric effects and of the strength of the newly formed bond was also recognized in the reaction of OE,0-unsaturated carbonyl compounds and similar electron deficient alkenes with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

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