Acyl radical, atmosphere

Figure 1. Phoiochcmicat smog formation in the atmosphere (R alkyl radical, RO alkoxy radical. RCO acyl radical. RO> Pcroxyalkyl radical. RC(X) Acyipcroxyl radical)...

Once thought to be of importance only in polluted urban atmospheres, PANs are now recognized to be ubiquitous, having been detected in urban, rural, and global environments (Roberts, 1990). By virtue of their photochemical inertness, relative insolubility in water, and low OH rate constant, PANs can have an appreciable atmospheric lifetime. The principal loss mechanism is thermal decomposition by reaction 5.91 or 5.95 back to the peroxy-acyl radical and NO2. The thermal decomposition is highly temperature dependent at temperatures of the upper troposphere PANs are quite stable and can be transported long distances. 

Hydroxy radical initiated oxidation of alkynes is important from the point of view of both atmospheric and combustion chemistry. Hatakeyama and coworkers have measured rate constants for the reaction of HO with acetylene, propyne and 2-butyne under atmospheric conditions. It has been suggested, based on product studies, that the jS-hydroxyvinyl radicals further react with molecular oxygen to form the corresponding peroxyl radicals and their subsequent reactions give carboxylic acid, a-dicarbonyl compounds and acyl radicals. 

The larger oxygenates behave in a similar fashion and react with OH in the atmosphere to generate peroxy alkyl radicals (CH3O2, C2H5O2,. .. RO2) and the peroxy acyl radicals [CH3C(0)02, CH3CH2C(0)02,. .. RC(0)02] that are also important reactants that drive NO to NO2,... 

Reaction of butanal with all three reactive species leads primarily to the formation of the butanoyl radical by abstraction of the aldehydic H-atom. Papagni et al. (2000) noted an increase in the rate coefficient of OH with butanal, compared with that with propanal, which is compatible with the sensitization of the C—H bond to the carbonyl group proposed by Kwok and Atkinson (1995). Application of their structure-activity relations (SARs) gives 17% abstraction at this site with a channel yield of 78% for production of the acyl radical. Reaction of the acylperoxy radical with NO2 forms peroxybutanoyl nitrate, while reaction with NO forms CO2 and the -propyl radical, which then reacts with O2 to form the n-propylperoxy radical, whose main atmospheric fate is conversion to propanal. 

Saturated fatty acids are very stable but unsaturated acids are susceptible to oxidation the more double bonds the greater the susceptibility. Unsaturated fatty acids, therefore, have to be handled under an atmosphere of inert gas (e.g. nitrogen) and kept away from (photo) oxidants or substances giving rise to free radicals. Anti-oxidant compounds have frequently to be used in the biochemical laboratory just as organisms and cells have to utilize similar compounds to prevent potentially harmful attack of acyl chains in vivo (section 8.11). 

Peroxy acyl nitrates dissociate quite quickly at 298 K, to regenerate peroxyacyl radicals. For example, PAN has a lifetime of about 50 min. The lifetime increases rapidly at the lower temperatures experienced at higher altitudes and is several months at the temperatures ( 250 K) of the upper troposphere. This long lifetime provides a mechanism for the transport of NOjc from polluted areas to less polluted areas, by transfer of peroxyacyl nitrates from the boundary layer to the free troposphere subsequent subsidence can return them to the boundary layer where they dissociate at the higher temperatures encountered there. The atmospheric reactions of the nitrates are discussed in detail in chapters VIII and IX. 

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