Alkenes reactions with peroxy radicals

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

The H atoms formed in reaction 15a can react with 02 (reaction 11) to form H02. The stabilized Criegee intermediate (CH200) can participate in further reactions, some of which will result in the formation of peroxy radicals. Larger alkenes react with ozone to produce organic peroxy radicals. 

The addition of the peroxyl radical to the double bond is governed by the electron density in the alkene bond and by electrophility of the radical. The rate constants of addition reactions increase with an increase of electron density on the double bond and with the increase of the electrophilic character of a radical (Table 6). The considerably larger electrophility of acyl peroxy radical (CH3CO3, C6H5CC>3) may explain by 5 orders faster addition of acyl peroxyl radicals [69] to a-methyl styrene at 20 °C. Electrophility of radicals leads to the marked reduction of activation energy of addition to alkenes methyl peroxyl radical has 47 kJ/mol, while acetyl peroxyl radical has 19 kJ/mol [70]. 

In the atmosphere the nitrooxy alkyl peroxy radical, > C(0N02) - COO( ) <, behaves like other alkyl peroxy radicals and will react with NO2, HO2, and other peroxy radicals. Reaction of nitrooxy alkyl peroxy radical with NO is unlikely because the conditions necessary for the formation of NO3 radicals (high O3) are incompatible with the presence of significant amounts of NO. For unsymmetrical alkenes the addition of NO3 radicals leads to the formation of two different peroxy radicals, e.g., for propene ... 

As discussed in section 4, reaction of the peroxy radicals with N02 gives thermally unstable peroxy nitrates. Reaction with H02 gives hydroperoxides and possibly carbonyl compounds. Reaction with other peroxy radicals (R 02) gives alkoxy radicals, carbonyls, and alcohols. The alkoxy radicals will then either isomerize, react with 02, or decompose (see Sect. 3). Thus, the NO3 radical-initiated atmospheric degradation of alkenes leads to oxiranes (generally in small yield), nitrooxy hydroperoxides, nitrooxy carbonyls, and nitrooxyalcohols. For a detailed listing of products from individual alkenes the reader should consult Calvert et al. [55]. 

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

The differentiation between PP and HDPE in the processing behaviour was linked to steric effects that limited direct reaction of a macro-alkyl (or alkoxy or peroxy) radical with the alkene CH2=C(CH3)- formed in PP, in contrast to the unhindered reaction of the radical with the -C=C- group in HDPE. Thus crosslinking was favoured in HDPE over PP. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

For longer chain alkanes ( 04) the reaction mechanism becomes more complex due to the IsomerIzatIons of the alkoxy radicals (47, 48), and to the fact that addition of NO to the alkyl-peroxy radicals (49) becomes more Important than NO to NO2 oxidation. For the alkenes and aromatic hydrocarbons the oxidation mechanisms In the atmosphere are more complex, and discussions of these systems, along with a more detailed treatment of the alkanes, are given later. 

Careful attempts to detect hydroperoxides and peroxynitrates, expected products of the reactions of the hydroxycyclohexadienylperoxy radicals with HO2 and NO2, respectively, were unsuccessful. This has been taken, in conjunction with work on the effect of NOx on the kinetics and products of the aromatic compound oxidation, as an indication that such peroxy radicals are very short-lived. It has been suggested, therefore, that the peroxy radicals are not able to oxidise NO as is the case for alkane and alkene atmospheric oxidation. 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

The reaction of OH radicals with alkanes results in the abstraction of a hydrogen atom followed by addition of oxygen whereby an alkylperoxy radical is formed In the reaction with alkenes OH is added to the double bond followed by addition of oxygen, generally at the neighbouring carbon atom. The peroxy radicals then enter into a reaction sequence of the type... 

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