Ethyl radicals, abstraction reactions

It is also necessary to explain why there are parentheses around the collision partner M in reactions (3.94), (3.95), and (3.99). When RH in reactions (3.94) and (3.95) is ethane and R in reaction (3.99) is the ethyl radical, the reaction order depends on the temperature and pressure range. Reactions (3.94), (3.95), and (3.99) for the ethane system are in the fall-off regime for most typical combustion conditions. Reactions (3.94) and (3.95) for propane may lie in the fall-off regime for some combustion conditions however, around 1 atm, butane and larger molecules pyrolyze near their high-pressure limits [34] and essentially follow first-order kinetics. Furthermore, for the formation of the olefin, an ethyl radical in reaction (3.99) must compete with the abstraction reaction. 

Treatment of a-iodo ketone and aldehyde with an equimolar amount of Et3B yielded the Reformatsky type adduct in the absence of PhaSnH (Scheme 21), unlike ot-bromo ketone as shown in Scheme 15 [22], Ethyl radical abstracts iodine to pro-duee carbonylmethyl radical, which would be trapped by EtsB to give the corresponding boron enolate and regenerate an ethyl radical. The boron enolate reacts with aldehyde to afford the adduct. The three-component coupling reaction of tert-butyl iodide, methyl vinyl ketone and benzaldehyde proceeded to give the corresponding adduct 38, with contamination by the ethyl radical addition product 39. The order of stability of carbon-centered radical is carbonylmethyl radical > Bu > Pr > Ef > Me . 

The results listed in Table 19 have been determined by methods analogous to those used to study methyl and ethyl H-abstraction reactions. The yields of the abstraction product, propane or cyclopropane, must be corrected for the disproportionation reaction. The reference reaction is the radical combination reaction. For n-propyl radical reactions TABLE 20... 

In a chain reaction, the step that determines what the product will be is most often an abstraction step. What is abstracted by a free radical is almost never a tetra- or tervalent atom (except in strained systems, see p. 989) and seldom a divalent one. Nearly always it is univalent, and so, for organic compounds, it is hydrogen or halogen. For example, a reaction between a chlorine atom and ethane gives an ethyl radical, not a hydrogen atom ... 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodide, and thus is a good initiator for iodine atom transfer reactions [13,33,34]. Indeed, the ethyl radical, issued from the reaction of triethylborane with molecular oxygen, can abstract an iodine atom from the radical precursor to produce a radical R that enters into the chain process (Scheme 13). The iodine exchange is fast and efficient when R is more stable than the ethyl radical. 

The explanation for the slow decrease in methanol is the same as for formaldehyde. The dramatic increase in the yields of ethyl and propyl alcohols at the pic darret clearly indicate the importance of abstraction reactions of the type above and of the appearance of alkoxy radicals at this point in the reaction. 

A similar three-component transformation can be achieved using triethylborane-induced radical reactions (Scheme 6.34) [53]. On exposure to air, triethylborane generates the ethyl radical, which abstracts iodine from alkyl iodides to generate the t-butyl radical. Addition of the resulting t-butyl radical to methyl vinyl ketone produces a radical a to the carbonyl group, which is trapped by triethylborane to form a boron enolate with the liberation of ethyl radical, thus creating a chain. 

Addition reactions with Mode B are not popular, but are occasionally useful. Eq. 4.7 indicates the reaction of ethyl bromoacetate and sugar vinyl ether with Bu3SnH initiated by AIBN. The ethyl acetate radical is electrophilic and it reacts with electron-rich sugar vinyl ether through SOMO-HOMO orbital interaction to form a ribosyl anomeric radical, as shown below. Then, the formed ribosyl anomeric radical abstracts... 

This reaction comprises firstly of SH2 reaction on the iodine atom of ethyl iodoacetate by an ethyl radical, formed from triethylborane and molecular oxygen, to form a more stable Chester radical and ethyl iodide. Electrophilic addition of the a-ester radical to electron-rich aromatics (36) forms an adduct radical, and finally abstraction of a hydrogen atom from the adduct by the ethyl radical or oxidation by molecular oxygen generates ethyl arylacetate (37), as shown in eq. 5.20. Here, a nucleophilic ethyl radical does not react with electron-rich aromatics (36), while only an electrophilic a-ester radical reacts with electron-rich aromatics via SOMO-HOMO interaction. 

Our success in synthesizing silyl ketals containing an aryl halide with (+)-ethyl lactate led us to explore the intramolecular radical translocation reaction (Scheme 29). The term radical translocation is described by Robertson et al. as the intramolecular abstraction of an atom (usually hydrogen) or group by a radical center this results in a repositioning of the site of the unpaired electron which can lead to functionalization at positions normally unreactive towards external reagents or whose selective modification is difficult In the most common cases the abstraction occurs at a site that is five atoms away from the radical 1,6 atom abstraction are less common, and l,n-abstractions where n > 6 are rare. This is because the shortest chain length that can accommodate the trajectory for atom abstraction contains six atoms, as in the case of the 1,5 atom abstraction. Entropic factors usually result in the failure of the process in the cases where n > 6 atoms. 

There is little rate data available for addition and abstraction reactions involving higher alpha-olefins. Where such data are available, they have usually been obtained from lower temperature studies (12,13,14), However, the available information indicates that addition should be competitive with abstraction in the temperature range of this study. For example, Steacie s data for ethyl radical reactions with 1-hexene and 1-heptene indicate that at 525°C, the addition rate would be about seven-tenths of the abstraction rate. For dodecene, one would expect this ratio to decrease because of the increase in abstractable hydrogen, but addition should still be a significant pathway. For methyl radicals and H atoms, available data (13,14) indicate that addition is somewhat faster relative to abstraction. 

The reason for the increase in the importance of the H abstraction path with increasing pressure is more subtle. While the reaction order for the radical addition path is 1.33 (the same as the overall reaction), the abstraction path is 1.40 order in dodecene (from the data in Table V). This may be attributable to a decrease in [H ] with increasing pressure because more ethyl radicals are stabilized to ethane and fewer are decomposed to ethylene and H- as the pressure is increased (see Figure 7). If H atoms are the most efficient addition species, a decrease in their relative concentration would result in a relative decrease in the contribution of the addition path and a corresponding increase in the role of the H abstraction route. 

The products of reaction (6) are the stable ethane molecule and the propionyl radical. This radical is unable rapidly to abstract hydrogen from any other species in the system. It readily decomposes, however, with a high frequency factor and a low activation energy, 20 5 kcal./ mole, to yield carbon monoxide and the ethyl radical. Thus this system is capable of yielding a chain reaction... 

The abstraction of a hydrogen atom from propionaldehyde by the ethyl radical has an activation energy of about 7.5 kcal./mole, and the rate constant of reaction (6) may be taken to be... 

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