Formyl radical, from decomposition

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). 

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 . 

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. 

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. 

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. ... 

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). 

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. 

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