Orbital hybridization free radicals

Structures. The methyl radical is planar and has D symmetry. Probably all other carbon-centerd free radicals with alkyl or heteroatom substituents are best described as shallow pyramids, driven by the necessity to stabilize the SOMO by hybridization or to align the SOMO for more efficient pi-type overlap with adjacent bonds. The planarity of the methyl radical has been attributed to steric repulsion between the H atoms [138]. The C center may be treated as planar for the purpose of constructing orbital interaction diagrams. 

ESR spectroscopy is perhaps the best method for the unequivocal detection and observation of free radicals, and ESR 13C hyperfine splitting (hfs) constants are considered to be a very useful indicator of a radical s geometry because non-planarity introduces s character into the orbital that contains the unpaired electron. The methyl radical s 13Ca value of 38 G is consistent with a planar structure. Fluoromethyl radicals exhibit increased 13Ca values, as shown in Table 1, thus indicating increasing non-planarity, with trifluoromethyl radical s value of 272 G lying close to that expected for its sp3 hybridization [4]. 

Like carbocations, free radicals are sp2 hybridized and planar (or nearly planar). Unlike carbocations, however, the p orbital perpendicular to the plane of the C—H bonds of the radical is not empty it contains the odd electron. Figure 4-15 shows the structure of the methyl radical. 

Many radical cations derived from cyclopropane (or cyclobutane) systems undergo bond formation with nucleophiles, typically neutralizing the positive charge and generating addition products via free-radical intermediates [140, 147). In one sense, these reactions are akin to the well known nucleophilic capture of carbocations, which is the second step of nucleophilic substitution via an Sn 1 mechanism. The capture of cyclopropane radical cations has the special feature that an sp -hybridized carbon center serves as an (intramolecular) leaving group, which changes the reaction, in essence, to a second-order substitution. Whereas the SnI reaction involves two electrons and an empty p-orbital and the Sn2 reaction occurs with redistribution of four electrons, the related radical cation reaction involves three electrons. 

We saw earlier (Sec. 2.21) that the methyl radical may not be quite flat that hybridization of carbon may be intermediate between sp- and sp For the allyl radical, on the other hand— and for many other free radicals—flatness is clearly required to permit the overlap of p orbitals that leads to stabilization of the radical. 

The C—H distance in acetylene is 1.08 A, even shorter than in ethylene (1.103 A) because of their greater s character, sp orbitals arc smaller than sp orbitals,-and 5p-hybridized carbon forms shorter bonds than 5/ -hybridized carbon. The C—H bond dis.sociation energy in acetylene is not known, but we would expect it to be even greater than in ethylene. Oddly enough, the same sp hybridization that almost certainly makes cleavage of the C—H bond to form free radicals homolysis) more difficult, makes cleavage to form ions heterolysis) easier, as we shall see (Sec. 8.10). 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

A free radical is a short-lived intermediate. It is a species possessing an unpaired electron due to deficiency of one electron and usually results from homolytic cleavage of a covalent bond or addition of radical to a multiple bond. A typical carbon radical is sp hybridized with the unpaired electron in the perpendicular unhybridized p-orbital. 

Molecules and ions with an unpaired electron are known as free radicals their existence can be explained by the molecular orbital (MO) theory of bonding (Chapter 14). Both nitrogen monoxide and nitrogen dioxide exist as resonance hybrids (see pages 134-135) of two Lewis structures. 

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