Chapter 6: Radical Structure

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Chapter 6 is in a portable document file (pdf) and can be viewed by clicking the blue Chapter 6 button. The drawing below the button pictures the conformations of pyranos-1-yl radicals and shows the effect of C-2 substituents on Frontier-Orbital interactions in these radicals. Beside and below this drawing is a summary of Chapter 6.

This is a drawing showing Frontier-Orbital interactions in pyranos-1-yl radicals.
Summary of Chapter 6

Determining the structure of a radical is essential to understanding its reactivity. The process begins by establishing the structural formula for the radical, that is, by identifying the constituent atoms, their connectivity, and elements of stereochemistry. Remaining unknown at this point typically are radical-center configuration and radical confor-mation.

The structural formula of a radical often can be determined reliably from knowledge of the structure of the radical precursor, the method of radical formation, and the reaction products. In instances where this information is insufficient, direct observation of the radical by ESR spectroscopy sometimes is possible and can provide the additional information needed to establish a structural formula.

The configuration at a radical center defines the location in space of the atoms directly attached to the radical-bearing atom. Nearly every carbon-centered radical has a pyramidal configuration, but these radicals vary widely in how close their configurations are to being planar. If a radical is nearly planar, it is described as being π-type. If, on the other hand, a radical is much more pyramidal, it is considered to be a σ-type radical. Information about radical structure is obtained from molecular-orbital calculations and from observation of α-13C hyperfine coupling constants (determined from ESR spectra of the 13C-enriched radicals).

A conformation of a radical is one of the arrangements of atoms that can be formed by rota­tion about one or more single bonds. Pyranos-1-yl radicals have been extensively studied and some have been found to favor unexpected conformations. Perhaps most striking among these is the 2,3,4,6-tetra-O-acetyl-D-glucopyranos-1-yl radical, which exists in a distorted B2,5 boat conformation.

Information about radical conformations is derived from both experimental and theoretical studies. Study of pyranos-1-yl radicals has led to the identification of the quasi-anomeric effect as a general, controlling influence in determining conformations in many radicals. Understanding of radical conformation comes both from simple and complex applications of molecular-orbital theory. Frontier-Orbital interactions offer a simple, theoretical means for rationalizing radical conformation. The far more sophisticated ab initio molecular-orbital calculations also provide understanding of the reasons for a radical adopting a particular conformation.