Hydroxyl radical alkene reactions

In contrast to the alkene theory the predominant mode of oxidation of the alkyl radicals is by oxygen addition and the alkylperoxy radical so formed then undergoes homogeneous intramolecular rearrangement (reaction (14)). Decomposition of the rearranged radical (reaction (16)) usually leads to a hydroxyl radical and stable products which include O-heterocycles, carbonyl compounds and alcohols with rearranged carbon skeletons relative to the fuel and alkenes. The chain-cycle is then completed by unselective attack on the fuel by the hydroxyl radical (reaction (12)). 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

Lloyd, A.C., Darnall, K.R., Winer, A.M., Pitts, Jr., J.N. (1976) Relative rate constants for reaction of the hydroxyl radical with a series of alkanes, alkenes, and aromatic hydrocarbons. J. Phys. Chem. 80, 189-794. 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

In addition to being oxidized by the hydroxyl radical, alkenes may react with the N03 radical as has been described by several investigators (52, 56, 66). Listed in Table I are some of the organic nitrates that have been predicted to be produced via reaction of OH and N03 with isoprene and pro-pene. Analogous compounds would be expected from other simple alkenes and from terpenes such as a- and (3-pinene. Other possible organic nitrates may be produced via the oxidation of aromatic compounds (53, 54) and the oxidation of carbonaceous aerosols (67). Quantitative determination of these species has not been made in the ambient atmosphere. 

Reaction pathway for substituted alkenes reacting with hydroxyl radical. 

The rate constants of the reaction of 2,6-dimethyloct-7-en-2-ol separately with ozone and hydroxyl radical, in the gas phase, have been determined. The OH radical can either abstract hydrogen or add to the double bond. Ozone adds to the double bond. The formation of acetone, 2-methylpropanal, 2-methylbutanal, ethanedial, and 2-oxopropanal was discussed.191 The rate laws and activation parameters for the ozone oxidation of alcohols in aqueous solution have been determined and explained on the basis of formation of an ozone-alcohol complex.192 The reactivity of alkenes towards ozone, in aqueous solution, correlates well with Taft s equation.193... 

In contrast to the water phase the HO radicals can have a much longer lifetime in gaseous media, i.e. up to 1 s for the OH and 60 s for the HO radical, respectively (Fabian, 1989). Despite the low concentration of OH radicals of about 10 molecules per cm in the sunlit troposphere (Ehhalt, 1999) they play an important role in controlling the removal of many organic natural and manmade compounds from the atmosphere (Eisele et al., 1997, Eisele and Bradshaw, 1993). Even in indoor environments, the formation of hydroxyl radicals is possible by ozone/alkene reactions (Atkinson et al., 1995). Steady-state indoor hydroxyl radical concentrations of about 6.7x10 ppb equivalent to 1.7x10 molecules cm were calculated at an ozone concentration of 20 ppb (Weschler and Shields, 1996). 

Salooja [21] carried out extensive studies of this phenomenon and considered that it was due to the inhibiting effect of alkenes produced as initial products and that the decrease with temperature in the concentration of hydroxyl radicals was accompanied by a corresponding increase in the concentration of the less reactive hydroperoxy radicals. Norrish s mechanism concurs with this reasoning. Enikolopyan [22] put forward two reaction schemes in an attempt to explain the negative temperature coefficient. In the first he considered the reactions... 

The oxidative properties of iodosylbenzene in the presence of Mn(TPP)Cl towards hydroxylation of alkanesand epoxidation of alkenes" were simultaneously reported by Hill and Groves. These oxidations have the characteristics of a stepwise radical reaction. 

Isomerization and the formation of addition products with alkenes are the most noted reactions of the benzene ring (16). Alkylbenzenes such as toluene form fairly stable epoxides upon the addition of hydroxyl radical, which are potential toxic and mutagenic compounds (17). The subsequent reactions of the epoxide could lead to the formation of epoxy carbonyls, which can react further with OH radicals or ozone until smaller molecules are formed. 

Aldehydes may be formed as intermediates in the oxidation of alkanes and alkenes, initiated by a hydroxyl radical, and by direct attack of ozone upon alkenes. Attack by OH is the dominant sink as shown in reaction (23)... 

Hydroxyl radicals have been observed to be formed from alkene- 03 reactions, sometimes with close to a unit yield (1 molecule of OH per 1 molecule of alkene reacted). Atkinson et al. (1995a) reported OH radical yields from a series of alkene-03 reactions ... 

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