Alkyl radicals ionization potentials

The alkyl radical ionization potentials needed were calculated by the pseudo-ir-orbital method.20 It was found21 that these radical ionization potentials converge to a constant value at the pentyl radicals. For various structures the values were as follows n-pentyl, 8.60 e.v. sec-pentyl, 7.81 e.v. iso-pentyl, 8.48 e.v. teri-pentyl, 7.19 e.v. (see ref. 21 for details of these calculations). The molecular ionization potentials also converge to a constant value at about ten carbons. Whenever these constant values are reached the heats of formation of the ions in Table II vary only with the heat of formation of the associated alkane, and therefore decrease by 5 kcal. mole-1 for each successive carbon atom. 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

Until now, applications of semiempirical all-valence-electron methods have been rare, although the experimental data for a series of alkyl radicals are available (108,109). In Figure 9, we present the theoretical values of ionization potentials calculated (68) for formyl radical by the CNDO version of Del Bene and Jaffe (110), which is superior to the standard CNDO/2 method in estimation of ionization potentials of closed-shell systems (111). The first ionization potential is seen, in Figure 9, to agree fairly well with the experimental value. Similarly, good results were also obtained (113) with some other radicals (Table VII). 

Figure 3. Direct relationship between the ionization potentials of alkyl radicals (R ) with ID of the alkylmetals RHgMe (O) and R2SnMe2 (%),...

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

Figure 4. Correlation of the ionization potentials of alkylmetal donors with the electron-transfer rate constant (log kFe) for Fe(phen)s3+ (%), Fe(bpy)s3+ (O), and Fe(Cl-phen)s3+ ((D), (left). The figure on the right is the same as the left figure for Fe(phen)s3+ except for the inclusion of electron-transfer rates for some alkyl radicals as identified, (Note the expanded scale,)...

Tossing, F.P. Semeluk, G.P. Free Radicals by Mass Spectrometry. XLII. Ionization Potentials and Ionic Heats of Formation for C1-C4 Alkyl Radicals. Can. J. Chem. 1970, 48, 955-965. 

The rate of the reaction of methyl radicals with 03 has been studied from 243 to 384 K by monitoring the decay of methyl in the presence of excess 03 [99], With the temperature dependence of the rate constant it was estimated that less than 1% of the methyl radicals in the stratosphere react with ozone. The reactions of a series of alkyl radicals (CH3, C2HS, n-C3H7, i -C3H7, and t-C4H9) with ozone were investigated at 298 K [100]. The rate coefficients were found to correlate with the difference between the ionization potential of each radical, and the electron affinity of 03. 

Table 3. Experimental ionization potentials, electron affinities and absolute electronegativities of alkyl and fluorinated alkyl radicals [50-55]...

Oxygen atom reacts with alkyl radicals by two orders of magnitude faster than molecular oxygen, the rate constant being the highest for alkyl radicals with the lowest ionization potentials [54] (Table 3). 

The high-pressure limiting rate constants also correlate with the ionization potential of the alkyl radical the lower the ionization potential of the alkyl radicals, the higher is their rate constant with molecular oxygen [55, 56]. 

The yield of alcohol ROH depends on the nucleophility of the radical as well as on the degree of the delocalization of an unpaired dectron. If the unpaired electron is not ddocalized, the reactivity of the n-alkyl radical increases with its nucleophility which may be correlated with the ionization potential. On the other hand, if the unpaired spin is delocalized, the orbital overlap of the radical and the peroxidic oxygen becomes an important reactivity factor. A radical seemingly... 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

The Benson mechanism can in principle be extended without major modifications to insertion reactions of other divalent species, such as oxygen and sulfur atoms. With oxygen, Yamaaaki and Cvetanovid have shown recently that 0( Z>) atoms preferentially insert, while 0( P) atoms abstract. This despite the fact that the singlet state hes some 48 kcal./ mole above the ground triplet state. On the other hand, sulfur atom reactions seem compatible with the proposed mechanism. Thus, if one attempts to apply it for the insertion of S( D) atoms, one finds that its predictions are consistent with the experimentally observed trend in the reaction rates. Here the transition state, due to the ionization potential and electron affinity of S( Z)) atoms being higher than those of alkyl radicals (9.2 and 2.2 e.v., respectively) could be formulated as... 

In this section we shall briefly review the experimental data on the equilibrium (47, -47) and then move on to discuss the strong correlation between the rate coefficients for reaction (47) and the ionization potential of the alkyl radical. The direct determination of the enthalpies for reactions (47, -47) during the mid and late 1980 s produced values significantly larger than those estimated by group additivity methods [95]. The reasons for these discrepancies are discussed in the final part of this section. 

Figure 2.31 illustrates the strong correlation between the reaction cross-sections for oxygen addition and the ionization potentials of linear alkyl radicals and the physical reasons for this are discussed in the following section. It can be seen that the data of Cobos et al. [97] (CH3) and Kaiser et al. [101] (C2H5) lie someway off the best fit line to the remaining data points which have been determined by a number of different experimental techniques, indicating that these two determinations may be subject to some systematic errors. 

It has long been realized that excellent correlations exist between 1/2 or and molecular orbital parameters [172,178-181]. Correlations between voltammetric data and experimental observables such as ionization potentials (IP) [172,178,182,183], charge-transfer transition energies [172], and positions of p-bands in ultraviolet (UV) absorption spectra of the hydrocarbons [4,172] have been reported as well. The basis and limitations of such correlations have been examined critically [147,175,184]. For alkyl aromatic hydrocarbons (AAHs), the slope, a, of the correlation line [Eq. (56)], where E° is the standard potential for the reversible one-electron oxidation, is close to unity and has been used to suggest that in MeCN the solvation energies of the hydrocarbon radical cations are constant throughout the series [175]. 

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