Abstraction Reactions of Radicals

The kinetic results are again qualitative in that the abstraction reactions can be divided into only two categories, those which occur and those which do not occur under cryogenic conditions. Clearly, molecules which have labile OH-bonds, such as the higher olefins discussed in the previous section, will undergo abstraction reactions with less reactive radicals more easily than molecules which contain strong CH bonds, such as cyclohexane or hexamethylethane. 

The reaction of toluene is of interest as toluene has been used extensively in the toluene-carrier gas technique as a means of removing radicals from a reacting system (Szwarc, 1950). The results from the rotating cryostat show that n-heptyl radicals react with toluene at 77°K by abstracting a methyl hydrogen to give the benzyl radical. As expected, addition to the aromatic ring does not occur. 

In principle it is also possible to use the technique to determine the site of attack by a radical on a molecule, provided that the e.s.r. spectra of the product radicals are sufficiently different. However, in practice this application has not been studied to any extent. 

Two of the abstraction reactions listed in Table 6 will be discussed further viz. (a) the reaction of n-heptyl radicals with tetramethyl-ethylene which was studied specifically to help elucidate the conditions under which the reactions take place and (b) the reaction of peroxy-radicals with an oxidation inhibitor 2,6-di-t-butyl-4-methylphenol (lonol) which is of particular interest in that two distinct mechanisms have been proposed to account for its antioxidant properties. 

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Some Representative Arrhenius Parameters for the H-Atom Abstraction Reactions of Radicals from Molecules R + R H - RH + R ... 

Once again it would be extremely difficult if not impossible to account for such large A factors in terms of the relatively tight 4-center transition state of reaction H for which if anything a very small A factor of the order of 106 to 10 liter/mole-sec. might be expected. Compared to the H-abstraction reactions of radicals (Tables I and II) for which A = 108-5 liter/mole-sec., the value A = 10 2 is high by about 103-6. 

Addition of the strong CH bond of cyclopropane (106 Kcal/mole) is favoured over insertion into the relatively weak C—C bond this result rules out hydrogen abstraction reaction of radicals which favour CH bond with low bond energies. In fact, cyclopropane is the cyclic hydrocarbon with the highest reactivity towards CH insertion (Figure 1). 

Many chemical reactions can be classified by either abstraction or addition-elimination mechanisms. Abstraction mechanisms are common in the reaction of radicals with closed-shell species, such as the reaction... 

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Scheme 3.1 Possible reaction steps in the hydrogen abstraction reaction of fluoranil with 2-propanol. Note FA= fluoranil, (CH3)2CHOH = 2-propanaol, FAH = fluoranil ketyl radical, FA = fluoranil radical anion.

Addition reaction of peroxide-generated macroalkyl radicals with the reactive unsaturation in MA is shown in reaction scheme 4. The functionalised maleic-polymer adduct (II, scheme 4) is the product of hydrogen abstraction reaction of the adduct radical (I, scheme 4) with another PP chain. Concomitantly, a new macroalkyl radical is regenerated which feeds back into the cycle. The frequency of this feedback determines the efficiency of the cyclical mechanism, hence the degree of binding. Cross-linking reaction of I occurs by route c ( scheme 4). 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

Along with free radical atom abstraction reactions, reactions of radical substitution are known where a free radical attacks the weak Y—Y bond and abstracts radical Y [56] ... 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

DFT-based descriptors can be applied for studying the most potential site for hydrogen abstraction reaction of a radical from a substrate. For H-abstraction... 

The presence of halogen additives substantially increases the tendency of all fuels to soot under diffusion flame conditions [69], The presence of H atoms increases the soot pyrolysis rate because the abstraction reaction of H + RH is much faster than R + RH, where R is a hydrocarbon radical. Halogenated compounds added to fuels generate halogen atoms (X) at modest temperatures. The important point is that X + RH abstraction is faster than H + RH, so that the halogen functions as a homogeneous catalyst through the system... 

The feasibility of hydrogen abstraction at the peptidyl a-carbon hydrogen bond by 1,4-aryl diradicals has been determined by examining a model reaction, i.e. abstraction of deuterium from dideuterioglycine by aryl radicals. The results have biological implications for the reactivity of the enediyne anti-tumour antibiotics with proteins. The non-Arrhenius behaviour of hydrogen-abstraction reactions by radicals has been investigated. For a number of reactions studied the enthalpy of activation was found either to increase or to decrease as a function of temperature. 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

The radical source must have some functional group X that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters and selenides are reactive. Allyl tris(trimethylsilyl)silane can also react similarly.232 Scheme 10.11 illustrates allylation by reaction of radical intermediates with allylstannanes. 

An alternate potential fate for the CH3SCH20 radical is the well-known abstraction reaction of alkoxy radicals with 02, forming methyl thioformate (Crutzen, 1983 Butkovskaya and Le Bras, 1994) ... 

The bulk of them also have activation energies in the range of 8 3 kcal./mole. In fact the atom abstraction reactions of most free radicals from molecules, when exothermic, have activation energies in this same range. Table II lists a representative series of such reactions. 

If we assiime that there is no activation energy for the disproportionation or recombination, then fcj 109-5 liter/mole-sec. (see Table III). This is about a factor of 10 higher than the values to be expected of H-abstraction reactions of alkyl radicals. It is furthermore anomalous in having a negligible activation energy compared to the expected 8 3 kcal. Note that if we assign 1 kcal. of activation energy to the disproportionation then Ad 101 U liter/mole-sec. 

In addition to acetonylacetone and acetone, disproportionation and abstraction reactions of the radicals can give rise to hydrogen chloride... 

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