Carbon radical geometry

To summarize the considerable available structural data with respect to fluorine substitution, one can conclude that non-conjugated carbon radicals bearing at least two fluorine substituents will be strongly pyramidal, a-radicals, while //-fluorine substituents appear to have little influence on the geometry of a radical. The strong a-character of CF3, CHF2, and perfluoro-n-alkyl radicals has a considerable influence on their reactivity. 

Bonding and Preferred Geometries in Carbon Radicals, Carbenium Ions and Carbanions... 

What are the geometries of carbon radicals, and how do they differ from those of carbenium ions or carbanions And what types of bonding are found at the carbon atoms of these three species First we will discuss geometry (Section 1.1.1). and then use molecular orbital (MO) theory to provide a description of the bonding (Section 1.1.2). 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

The structure and geometry of carbon radicals are similar to those of alkyl carboca-tions. They are planar or nearly so, with bond angles of approximately 120° about the carbon with the unpaired electron. The relative stabilities of alkyl radicals are similar to those of alkyl carbocations because they both possess electron-deficient carbons. 

Alkyl substituents apparently cause carbon radicals to adopt a pyramidal geometry. The butyl radical has been studied particularly extensively, and while experimental results have been interpreted both in terms of planar and slightly pyramidal structures, theoretical results favor the pyramidal structure. " Both theory and experiment agree that the barrier to inversion is very low, leading to rapid inversion. 

More sophisticated calculations of the semi-empirical SCF and ab initio types have been used to plot partial potential energy surfaces for methyl radicals with alkenes (55) (56) (57). They suggest that the lowest energy approach of the radical is from above the plane of the alkene directly above one or other of the carbons. The geometry of the transition state was found to be little changed from that of the reactants. So far this type of computation has not been attempted with fluoroalkyl radicals. 

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

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