The Formyl Radical

We return briefly to the formyl radical of Eq. (8-5), a by-product of the initiation reaction. The following sequence is believed to constitute a chain process that couples with the other sequence [Eqs. (8-5)—(8-8)] ... 

Both CO and C02 are reduced by eh. The immediate product of the first reaction is CO-, which reacts with water, giving OH and the formyl radical the latter has been identified by pulse radiolysis. The product of carbon dioxide reduction, C02-, is stable in the condensed phase with an absorption at 260 nm. It reacts with various organic radicals in addition reactions, giving carboxylates with rates that are competitive with ion-ion or radical-radical combination rates. 

At high pressures the presence of the H02 radical also contributes via HCO + H02 — H202 + CO, but H02 is the least effective of OH, O, and H, as the rate constants in Appendix C will confirm. The formyl radical reacts very rapidly with the OH, O, and H radicals. However, radical concentrations are much lower than those of stable reactants and intermediates, and thus formyl reactions with these radicals are considered insignificant relative to the other formyl reactions. As will be seen when the oxidation of large hydrocarbon molecules is discussed (Section H), R is most likely a methyl radical, and the highest-order aldehydes to arise in high-temperature combustion are acetaldehyde and propionaldehyde. The acetaldehyde is the dominant form. Essentially, then, the sequence above was developed with the consideration that R was a methyl group. 

As before, reaction (3.71) is slow. Reactions (3.72) and (3.73) are faster since they involve a radical and one of the initial reactants. The same is true for reactions (3.75M3.77). Reaction (3.75) represents the necessary chain branching step. Reactions (3.74) and (3.78) introduce the formyl radical known to exist in the low-temperature combustion scheme. Carbon monoxide is formed by reaction (3.76), and water by reaction (3.73) and the subsequent decay of the peroxides formed. A conversion step of CO to C02 is not considered because the rate of conversion by reaction (3.44) is too slow at the temperatures of concern here. 

The exothermicity of reaction (8.87) is sufficient to fragment the formyl radical and could be written as... 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

Divalent radicals are usually a radicals. The vinyl radical is bent, and the barrier for inversion through the linear form is 3 kcal/mol. Vinyl radicals with sigma substituents also are bent, but n substituents give linear vinyl radicals. The formyl radical and acyl radicals are bent. ... 

The first step promptly occurring in about 0.36 ps is the C—C cleavage. Some 50 ps later, a favorable configuration is found to enable the H atom transfer from the CHO group to the butyl radical, forming butane and a pentanoyl radical. A third pentanal at about 89.4 ps loses its H atom to the formyl radical resulting in formaldehyde and another pentanoyl radical. 

Once again, neither hexafluorodiacetyl nor hexafluoroacetone was detected, suggesting that the trifluoroacetyl radical, even when formed in thermal equilibrium with its environment, is too unstable to survive long enough to take part in combination reactions. The fate of the formyl radical is uncertain at low temperatures it may take part in wall reactions, while at higher temperatures it may decompose to yield hydrogen atoms capable of taking part in the chain reaction... 

Even though this is a chain-terminating step, the radical pool is rapidly replenished through the H + O2 reaction (Rl). Reactions between formaldehyde and O/H radicals lead to the formyl radical (HCO), which subsequently dissociates thermally (R30) or reacts with O2 to form CO (R31). 

The formyl radical is a major primary product of the photolysis of formaldehyde in the near ultraviolet. The formyl radicals produced in the atmosphere by sunlight may react with 02 to form CO and H02... 

The data given in Table 18 are therefore taken as characteristic of the formyl radical. Those of Adrian and co-workers (Adrian et al., 1962) are to be preferred since these workers obtained spectra which were somewhat better resolved than ours. This improved resolution... 

The formyl radical HCO has been the subject of very detailed experimental investigations in recent years, but surprisingly few theoretical studies have appeared. However, Thomson and Brotchie 506 have recently reported the results of a variety of SCF calculations. 

The main outline for the mechanism of the photooxidation has been discovered in the past decade, but the details are obscured—possibly due to the interference of surface reactions. The secondary reactions that follow the attack of radicals upon formaldehyde are best left to be discussed in the section on that subject. The evidence given above suggests that the formyl radical is chiefly oxidized to carbon monoxide with a little carbon dioxide. 

The stability of the formyl radical has recently been discussed by Calvert.88 84 There is evidence to support both a value of ca. 15 kcal. and of 27-30 kcal. for the energy of activation of reaction (79). 

In oxidation systems where formaldehyde, a primary product, is attacked by radicals, hydrogen atoms will only be produced at very low oxygen concentrations, i.e., where reaction (79) can compete with reaction (80). Even at temperatures as high as 500°C., formaldehyde is known to cause chain ending by the production of the formyl radical.8 It can therefore be safely assumed that formyl radicals normally react according to reaction (80). As with the oxidation of the acetyl radical, the acid (in this case formic acid) can be found in large quantities in the products or it can be entirely absent. By comparison, it is likely that this depends on the condition of the surface and the availability of hydrogen atoms for abstraction. 

Although the formyl radical, HCO, has been trapped and identified by e.s.r. (Adrian et al., 1962 Cochran et al., 1966 Brivati et al., 1962) the corresponding acetyl radical, CH3CO, which is an important intermediate in hydrocarbon oxidation had not been identified conclusively. In fact several different e.s.r. spectra have been attributed to this radical. 

The principal values of the gr-tensor are almost identical with those of the formyl radical (gr, = 1-9960 grj = 2-0034) which shows that the orbital occupied by the unpaired electron is very similar in both radicals,... 

The hyperfine coupling for the methyl protons (.<4i,o = 5T G) is markedly less than the proton hyperfine coupling observed for the formyl radical ( iBo= 130 G) and could arise from the normal hyperfine interactions observed in alkyl radicals in which the unpaired electron is located in a carbon 2p-orbital. However, the very large splitting in the formyl radical is attributed to a contribution of the excited state, H- C=0 (Adrian et al., 1962) to the electronic structure and it is feasible... 

The direct observation of the acetyl radical from the reaction of hydrogen atoms with acetaldehyde is particularly important because recent studies of the reaction in the gas phase have led to conflicting conclusions. McKnight et al. (1967) concluded that the acetyl radical is formed, whereas Lambert e< a . (1967) suggested that the initial reaction yielded the formyl radical and methane. Clearly the low-temperature result supports the former interpretation. 

The first step of the reaction is the generation ofthedifluoromethylcneiminyl radicals. Reaction with the perfluoroalkyl radicals leads to the perfluoro(A -methylenealkylamines) in 24 35% yield and the recombination with the formyl radicals gives the isocyanate in 20-36"/o yield. ... 

The photochemical production of H2CO proceeds with reaction H and CO formed in (1) and (2), leading to the formation of the formyl radical (HCO) ... 

The emission spectrum of the first-stage flame is truly that of excited formaldehyde [78, 79] whereas that of the second may be due to either the same emitter or to the formyl radical, depending primarily on the initial mixture ratio. 

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