Hydrocarbon radicals relative stability

Figure 13.1 The relative stability of the allyl radical compared to 1°, 2°, 3°, and vinyl radicals. (The stabilities of the radicals are relative to the hydrocarbon from which was formed, and the overall order of stability is allyl > 3° > 2° > 1° > vinyl).

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

Thus, on the basis of relative C-H BDEs, (CH3)3C-H (96.4), (CH3)2CH-H (98.6), CH3CH2-H (101.1) and CH3-H (104.8 kcal/mol) [23,26], one reaches the conclusion that the stability of hydrocarbon radicals decreases 3° > 2° > 1 ° > CH3. It is, of course, recognized that the variable degree of steric strain of the molecules within this series limits the quantitative impact of these numbers vis-a-vis radical stability. 

In summary, perfluoroalkyl radicals exhibit extraordinary reactivity in both their alkene addition reactions and their hydrogen-abstraction processes, relative to their hydrocarbon counterparts. This reactivity can be attributed partially to the increased exothermicity of such reactions when compared to the analogous reactions of hydrocarbon radicals, and partially also to the fact that perfluoro-n-alkyl radicals are a-radicals. However the major source of the reactivity of 1°, 2°, 3° perfluoroalkyl radicals must be their high electronegativity, which gives rise to stabilizing polarization of the transition states of these radicals addition and hydrogen-abstraction processes. 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

We can now expand the sequence of radical stabilities (Sec. 6.22). Relative to the hydrocarbon from which each is formed, the relative stability of free radicals is ... 

As noted above, the correlation of the relative stabilities of hydrocarbon radicals with the selectivities in C-H substitution reactions is poor as hydrogen atom abstractions are mostly entropy controlled [130, 135]. Hence, the analogous radical... 

All these arguments for the stability of the M—Rf complexes are speculative and they neglect considerations which may be significant, such as the relative free energies of the fluorocarbon and hydrocarbon radicals. It should be noted that generally very little is known about the importance of factors which affect the stability of transition metal complexes and that a very small change in bond energy may have a marked effect on thermal stability. 

The simplest of the ethers would be ether that has the simplest hydrocarbon backbones attached those backbones are the radicals of the simplest hydrocarbon, methane. Therefore, the simplest of the ethers is dimethyl ether, whose formula is CH3OCH3. Dimethyl is used because there are two methyl radicals, and "di-" is the prefix for two. This compound could also be called methyl methyl ether, or just plain methyl ether, but it is better known as dimethyl ether. It is an easily liquified gas that is extremely flammable, has a relatively low ignition temperature of 66°F, and is used as a solvent, a refrigerant, a propellant for sprays, and a polymerization stabilizer. 

Some reactions occur thermally at temperatures too low for homolysis of any of the covalent bonds present to provide enough radicals to start the reaction. For example, mixtures of fluorine and methane explode at room temperature, and many hydrocarbons are oxidized slowly by molecular oxygen (see below). It has been postulated that the bimolecular reactions (6.10) and (6.11) are responsible. In (6.10), the formation of the very strong H-F bond makes this reaction much less endothermic than the simple homolysis of the fluorine molecule. In (6.11), a strong O-H bond is formed in the hydroperoxyl radical 18, whereas two relatively weak bonds are broken, the 0=0 n bond and the C-H bond in 16 which is weakened by the stabilization of the product benzylic radical 17. The occurrence of these molecule-induced homolysis reactions is difficult to prove because the compounds formed tend to be swamped by those from the subsequent radical reactions. 

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