Geometry of Carbon Radicals

Arrange these radicals in order of increasing stability  

If an attempt were made to apply the rules of valence shell electron pair repulsion theory to radicals, it would not be clear how to treat the single electron. Obviously, a single electron should not be as large as a pair of electrons, but it is expected to result in some repulsion. Therefore, it is difficult to predict whether a radical carbon should be sp2 hybridized with trigonal planar geometry (with the odd electron in a p orbital), sp3 hybridized with tetrahedral geometry (with the odd electron in an sp3 AO), or somewhere in between. Experimental evidence is also somewhat uncertain. Studies of the geometry of simple alkyl radicals indicate that either they are planar or, if they are pyramidal, inversion is very rapid. 

This model shows the electron density for the odd electron of the planar methyl radical. The radical electron is in a p orbital perpendicular to the plane of the atoms. 

A trigonal planar A rapidly inverting pyramidal radical 

The important consequence of this is that reactions that involve radicals, like reactions that involve carbocations, result in the loss of stereochemistry (racemization) at the radical carbon. 

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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). 

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. 

For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (a-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 

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. 

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. 

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. 

The synthesis of the trismethylenemethane iron tricarbonyl complex [(CH2)3C]-Fe(CO)3 was reported by Emerson et al. in 1966 (27). The geometry of this compound in the gas phase was investigated by Almenningen et al. (28) using electron diffraction methods. These authors pointed out some structural peculiarities which were not amenable to a simple explanation, in particular, why the hypothetical planar (CH2)3C radical is distorted when bound to the Fe(CO)3 conical fragment in such a way that the carbon atoms of the CH2 groups are displaced toward — the iron atom (Fig. 9). 

Free radical addition of HBr to buta-1,2-diene (lb) affords dibromides exo-6b, (E)-6b and (Z)-6b, which consistently originate from Br addition to the central allene carbon atom [37]. The fact that the internal olefins (E)-6b and (Z)-6b dominate among the reaction products points to a thermodynamic control of the termination step (see below). The geometry of the major product (Z)-(6b) has been correlated with that of the preferred structure of intermediate 7b. The latter, in turn, has been deduced from an investigation of the configurational stability of the (Z)-methylallyl radical (Z)-8, which isomerizes with a rate constant of kiso=102s 1 (-130 °C) to the less strained E-stereoisomer (fc)-8 (Scheme 11.4) [38]. 

The reactivity trend in terms of the ring-opening reaction for the series 43, 47, and 53 can be analyzed in terms of the geometries of imino and vinyl radicals. Most of the spin density in imino radicals is concentrated in the p-orbital on nitrogen, but vinyl radicals have a nonlinear, vp -like structure, where the spin density is in the i /j -orbital of the carbon. In the o-quinone diradicals 43,47, and 53, the / -orbitals of the nitrogens (accomodating the odd electrons) are better... 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

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