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)] ... 

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 9. Determination of the first electron affinity, and the first and higher ionization potentials of formyl radical from the SCF orbital energies and electronic repulsion integrals, and K,j (cf. eqs. (90), (92), and (93)). The experimental value (112), 9.88 eV, for the first ionization potential corresponds to the theoretical value I . All entries are given in eV. With A and I a lower index stands for MO the upper one indicates the state multiplicity after ionization.

When formic acid was codeposited at 14 K with a beam of excited argon atoms, formyl radical, HOCO, was produced (12) in sufficient yield for the IR detection of most of its vibrational fundamentals (Jacox, 1988). Detailed analysis of the matrix spectra of isotopically (D, C and 0) labelled formyl radical showed absorptions at 3603, 1844 and 1065cm , which correspond to the stretching vibrations of O—H, C=0 and C—O bonds. 

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. 

The aldehydes contain the characteristic formyl radical group... 

The low-temperature hydrocarbon oxidation mechanism discussed in the previous section is incomplete because the reactions leading to CO were not included. Water formation is primarily by reaction (3.56). The CO forms by the conversion of aldehydes and their acetyl (and formyl) radicals, RCO. The same type of conversion takes place at high temperatures thus, it is appropriate, prior to considering high-temperature hydrocarbon oxidation schemes, to develop an understanding of the aldehyde conversion process. 

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 primary products are methyl and formyl radicals [36, 37] because potential energy surface crossing leads to a H shift at combustion temperatures [35], It is rather interesting that the decomposition of cyclic ethylene oxide proceeds through a route in which it isomerizes to acetaldehyde and readily dissociates into CH3 and HCO. Thus two primary addition reactions that can be written are... 

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

Hydroxymethylation of 2- or 4-nitroalkyl-benzenes, (44), takes place by passing a catalytic amount of charge through a solution of (44) and (CH20) Scheme 40 [140]. (44) is reduced at the electrode and electron transfer from (44) to CH2O is envisaged [140]. The resulting formyl radical anion then acts as an EGB toward... 

The most stable O2 addition adduct below T = 1250 K is 6-peroxyoxepinone (route A in Fig. 12), which can cyclize to form a 1,4-peroxy intermediate which subsequently releases CO2 to form a 5-oxopentanalyl radical. This species can cyclize and fragment, yielding formyl radical, furan, and carbon dioxide. Above T = 1250 K, the... 

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. 

Figure 15 Top Photographic latent image formation in undoped (left) and formate-doped and gold-sulfide sensitized AgBr crystals with the hole-scavenging step (center). Secondary reduction step by formyl radical (right). Bottom Sensitometry curves for gold-sulfide sensitized emulsions, undoped or formate-doped, and developed after 5 or 20 min (texp = 10 sec, development with aminophenol and ascorbic acid). The same absorbance is observed for a number of photons absorbed 5 or 10 times less, respectively, than in the undoped emulsion. (From Ref. 200.)...

Sources of H02 include the reactions of 02 with hydrogen atoms and formyl radicals, both of which are produced, for example, by the photodissociation of gaseous formaldehyde following absorption of solar actinic UV radiation. 

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 HCo(CO)4 complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO)4 to free CO, affording a formyl radical, HtO subsequent reaction with further HCo(CO)4 would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). 

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. 

H transfer from Ca to formyl radical, creating formaldehyde... 

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). 

Peroxy radicals are also formed in the troposphere through the photolysis of aldehydes (10, 11) and through nitrate radical (N03) reactions (12-14). The hydrogen atom and formyl radical that are formed then react with molecular oxygen (02) (reactions 11 and 12) under tropospheric conditions. 

As with the simple alkylcoumarins, phenylcoumarins readily eliminate carbon monoxide from the molecular ion. Mass spectral fragmentation of 4-phenylcoumarin follows the route given in Scheme 10. The molecular ion (122a) appears as the base peak. Formyl radical elimination from (122b) produces the fluorenyl cation (122c), m/e 165 (63TL891). 

Mass spectra of hydroxy- and alkoxy-coumarins have been very intensively studied. The decomposition sequence of 3-hydroxycoumarin is initiated by carbon monoxide loss from the molecular ion giving a 2-hydroxybenzofuran ion. Subsequent fragmentation occurs by two major pathways, involving a further loss of CO and expulsion of a formyl radical. The former leads to the base peak, and thence by another loss of CO to give the abundant benzene radical cation at m/e 78. The other main pathway gives a benzoyl cation which leads to the phenonium ion at m/e 77 (77IJC(B)816). 

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... 

In a potentially significant development in CO reduction chemistry, Rathke and Feder (88h) have found that HCo(CO)4 serves as a catalyst for conversion of CO/H2 mixtures to MeOH and HCOOMe under forcing conditions (300 atm, 200°C), albeit at very slow rates. An activation energy of 40.7 kcal/mol was obtained, and a scheme based on formyl radical chemistry was proposed. While the latter seems premature, the fact that reduction products were observed in the absence of cluster compounds brings into question the necessity of such a structural arrangement. 

Hart (1952, 1954) studied the oxidation of formic acid by the radiolysis method. In the presence of oxygen, hydroxyl radicals abstract hydrogen from HCOzH. Both the carboxyl radicals and formyl radicals are formed. These radicals undergo oxygen addition and subsequently dissociate ... 

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