Structural and Stereochemical Properties of Free Radicals

ESR studies and other physical methods have provided insight into the geometry of free radicals. Deductions about structure can also be drawn from the study of the stereochemistry of reactions involving radical intermediates. Several structural possibilities can be considered. If discussion is limited to alkyl radicals, the possibilities include a rigid pyramidal structure, rapidly inverting pyramidal structures, or a planar structure. 

Precise description of the pyramidal structures also requires that the bond angles be specified. The ESR spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a shallow pyramid with a very low barrier to inversion. The IR spectrum of methyl radical at very low temperature in frozen argon puts a maximum of about 5° on the deviation from planarity. A microwave study has also indicated the methyl radical is planar. Various MO calculations indicate a planar structure.  

The tendency for pyramidal geometry is reinforced by an interaction between the p orbital on carbon and the a antibonding orbitals associated with the C—F or C—O bonds. The interaction increases electron density on the more electronegative fluorine or oxygen atom. This stabilizing p- j interaction is increased by pyramidal geometry. 

Entry 1 is a chlorination at a stereogenic tertiary center and proceeds with complete racemization. In Entry 2, a tertiary radical is generated by loss of C=0, again with complete racemization. In Entry 3, an a-methylbenzyl radical is generated by a fragmentation and the product is again racemic. Entry 4 involves a benzylic bromination by NBS. The chirality of the reactant results from enantiospecific isotopic labeling of ethylbenzene. The product, which is formed via an a-methylbenzyl radical intermediate, is racemic. 

Cyclic molecules permit deductions about stereochemistry without the necessity of using resolved chiral compounds. The stereochemistry of a number of reactions of 

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