Formyl radicals, decomposition

Below, some of the results concerning the pressure dependence of the radical decomposition and recombination processes will be discussed. No deviation was observed from the second-order kinetics of the termination step in the experiments of Grahame and Rollefson , while Dodd S found it necessary to consider the pressure dependence of the methyl recombination at around 10 torr. Dorman and Buchanan came to the conclusion that the decomposition of the formyl radical is in its pressure-dependent region below a few atm, whilst that of the acetyl radical seems to be pressure-dependent below about 50 or 150 torr. The results of Style and Summers also indicate the formyl radical decomposition to be pressure-dependent under the conditions where the photolysis of acetaldehyde was usually studied. 

Mode-specific decomposition has been illustrated for the H—C—C — H -f C==C model Hamiltonian (Swamy et al., 1986). Lifetimes for different resonance states are given in table 8.2. The states are listed according to the quantum numbers for the HC and CC stretch modes. The CC stretch progression of resonance states with zero quantum in the HC stretch has the longest lifetimes, while the progression with two quanta in the HC stretch has the shortest lifetimes. Such a finding is characteristic of mode-specific decomposition. A similar mode-specific decomposition is observed for formyl radical decomposition, HCO —> H + CO (Wang and Bowman, 1994 Werner et... 

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... 

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. 

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 first-order initiation and second-order termination steps suggested seem to be in agreement with this observation and, at the same time, also satisfy the kinetics observed experimentally. The decomposition of the formyl radical is, however, probably in its fall-off region at the pressures studied. 

Recent investigations on ethane formation in the photolysis of acetaldehyde indicate that decomposition into methyl and formyl radicals occurs from the triplet state which is also removed by first-order internal conversion and, to some extent, by second-order deactivation. In the mercury-photosensitized reaction methyl radicals are formed by direct dissociation of the excited aldehyde molecules, as well as by collision of excited mercury atoms . 

One of the most disputable questions is the mechanism of hydrogen formation. Experiments with CH3CDO show that hydrogen comes mainly from the formyl group of the aldehyde molecule both at low and high temperatures. It is beyond doubt that hydrogen is formed by the decomposition or some other reactions of the formyl radical. 

Allyl alcohol decreases the quantum yields of product formation. The quantum yields level off at higher allyl alcohol concentrations. The limiting value is approximately the same at 27.6 and 73.0 °C. It may well be assumed that the limiting quantum yields are related to an intramolecular non-radical decomposition. However, according to Chen and Volman, the residual reaction is essentially a geminate one occurring between methyl and formyl radicals that have been formed from the same aldehyde molecule in a solvent cage, viz. 

The formyl radical can be produced in several other ways, one of which is the photolytic decomposition of formaldehyde. The formaldehyde system is interesting because the intensity of the HCO spectrum does not increase mono-tonically with ultraviolet irradiation time but reaches a maximum and then falls off. This indicates that the formyl radical is decomposed by the ultraviolet... 

There are fewer experimental examples of mode specificity for the unimolecular decomposition of covalently bound molecules. One example is the decomposition of the formyl radical HCO, namely... 

In contrast to the studies for van der Waals molecules, there have been substantially fewer theoretical studies of state-specific decomposition for molecules with covalent intramolecular potentials. A survey of the theoretical work is given in table 8.1. For the most part the studies are two-dimensional (i.e., one coordinate coupled to a reaction coordinate) and involve model potentials. An important counterexample are the studies for the formyl radical that is, H—C=0 H -f C O (Gazdy et al., 1991 ... 

Dissociation of formaldehyde, CHgO, at comparably low temperatures is obviously determined by a complex decomposition mechanism. Conclusions on the unimolecular dissociation can only be drawn from measurements at high temperatures under shock wave conditions. In this system the primary dissociation leading to formyl radicals is followed by decomposition of CHO and subsequent reactions of H atoms with CH2O and CHO. By analysing the chain mechanism the rate constant of the unimolecular reaction was derived. ... 

The fate of this radical (which is chemically similar to the formyl radical HCO) is not decomposition but O2 addition onto the carbon radical, giving the peroxo acyl... 

Finally, the CH2OH radicals react with O2 to give HCHO and HO2 (5.327). Thus, C2 is broken down into Ci species. Fig. 5.34 shows schematically the C2 gas phase chemistry. It is obvious that there is no ethanol formation and acetic acid decomposition, whereas acetaldehyde provides many pathways back to Ci chemistry. Glycolaldehyde is a highly water-soluble product from several C2 species (ethene, acetaldehyde and ethanol) other bicarbonyls, however, are likely to be produced preferably in solution. The aqueous phase produces other C2 speeies but also deeomposes them (Fig. 5.35). By contrast, in aqueous solution from Ci, C2 species can be given as shown by the formation of glyoxal from the formyl radicals (5.351) the latter is... 

Anticipated products from the reaction of sym-dichloromethyl ether with ozone or OH radicals in the atmosphere, excluding the decomposition products formaldehyde and HCl, are chloromethyl formate and formyl chloride (Cupitt, 1980). 

It should also be mentioned that Allen and Pitts (1) studied the CH3-radical-sensitized decomposition of trans-crotonaldehvde to show that the 2-butene formation observed in the earlier photolysis studies is due to methyl-radical displacement of the formyl group. 

The decompositions of the formyl and acetyl radicals are certain to be in the fall-off region at the pressures used in the investigations of the acetaldehyde pyrolysis. However, the complexity of the mechanism impedes any conclusion to be drawn from this system. 

Below about 200 °C, the photo-decomposition of acetaldehyde becomes involved as a result of the increased stability of the formyl and acetyl radicals. The occurrence of new elementary steps, in addition to those already mentioned, renders the kinetics of the reaction rather complex. 

The kinetics of the photolysis is much more complex at lower temperatures than at around 300 °C. The role of rate-determining step, i.e. the hydrogen atom transfer reaction (20) at high temperatures, is taken over by the decomposition of the acetyl radical as the temperature decreases. At the highest temperatures, the chains are terminated almost exclusively by the recombination of the methyl radicals, while at medium and low temperatures the disproportionation step (26) as well as self combination of the formyl and acetyl radicals are dominant. The first-order wall reaction of the radicals, such as reactions (22) and (31), may also play an important role, especially at low light intensities and pressures. On account of the aforesaid, it seems almost impossible to attempt a general discussion of the kinetics of the reaction. Instead, only selected questions will be dealt with in detail. 

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