Oxygen with alkanes

Radical cations are probably involved in the photooxidation of olefins [1-7, 24, 50-54, 56, 61, 66] (Eqs. 2 and 5). Many oxidizable arenes and dienes are, similarly, oxidatively cleaved or rearranged by routes consistent with initial formation of a radical cation. As we saw above, the observed oxidative cleavage product probably derives from the combination of a surface-bound radical cation with superoxide or adsorbed oxygen. With alkanes or simply substituted alkenes, however, the capture of a photogenerated hole is often thermodynamically forbidden. Thus, instead of radical cations, radicals formed by hydrogen atom abstraction by an activated oxygen species dominate the observed chemistry. With alkanes, oxygenation at sites... 

The reactions of atomic oxygen with alkanes, aldehydes, alcohols, and alkyl halides have been reported to proceed mostly as follows... 

The paramagnetic oxygen ions 0 , 01> and 0J have been formed on magnesium oxide and studied by EPR spectroscopy. The reactivity of these ions with hydrocarbons follows the sequence 0 >>03> >02. Both with alkanes and alkenes the initial reaction is thought to be hydrogen atom abstraction. 

Oxygenation of alkanes with hydrogen peroxide in homogeneous reactions can be obtained also with Keggin-type iron-substituted polyoxometalates. Evidence against radical reactions has been offered. 

The 02 ion on MgO does not react with CO or alkanes at 77 K but the EPR signal disappears slowly at room temperature (361). Similarly, on ZnO (390) it reacts only slowly with propylene at room temperature and not with CO, H2, or ethylene. A slow reaction with propylene is also observed for 02 on V2Os/MgO at room temperature (391). Yoshida et al. (392) have studied the reactivity of adsorbed oxygen with olefins on the V20j/Si02 system. Adsorption of propylene destroyed the signal from 02 slowly at room temperature and the reaction products, aldehydes with some acrolein, were desorbed as the temperature was raised to 150°C. More quantitative... 

Ozone has been shown to be protonated in the superacid media to ozonium ion H03+ 51 [Eq. (4.28)], which reacts with alkanes as a powerful electrophilic oxygenating agent.125 Similarly, ozone reacts with carbocations, giving alkylated ozonium ion that undergoes further cleavage reactions. These reactions are well-covered in Chapter 5. 

MePOR species and other complexes in cationic clays can be located at the edges of packed platelets, in the interlamellar space or in the mesopores present (Scheme 10.9). A review of the early data in this area is available.[86] The flat metallo macrocycles under clay synthesis conditions help to induce layer silicate formation, the complexes being intercalated between the layers. Whereas with monooxygen atom donors, alkanes can be oxygenated with significantly enhanced activities compared with the homogeneous case, in every case the expected products (ol/on) were obtained. Competitive oxygenation of adamantane and pentane shows lower... 

Zeolite encapsulated complexes catalyse the oxygenation of alkanes with peroxides according to oxo chemistry, following a mechanism very similar to the oxygen-rebound mechanism encountered in monooxygenase enzymes. 

Hill and co-workers have reported that TMSP is effective for the epoxidation of alkenes and the oxygenation of alkanes by use of f-BuOOH and PhIO as oxidants [67]. The characteristics of TMSP in alkene epoxidation, compared with those of metalloporphy-rins, Schiff base complexes, and triflate salts, are as follows [67] ... 

Reacts to form explosive products with alkanes + oxygen (forms peroxides), cyano-guanidine, perchloric acid (forms fluorine perchlorate gas), potassium chlorate (forms fluorine perchlorate gas), potassium hydroxide (forms potassium trioxide). 

Hydrocarbon polymers are particularly susceptible to attack by atomic oxygen in LEO. The reactions of atomic oxygen with hydrocarbon molecules in the gas phase serve as models for the relatively unstudied reactions of atomic oxygen with a hydrocarbon surface. A wealth of knowledge of gas-phase reactions is available, largely because these reactions are important in combustion and in atmospheric chemistry. Studies of both reaction kinetics and dynamics have revealed many of the mechanisms by which atomic oxygen reacts with gaseous alkanes and alkenes. A summary of probable reaction pathways is presented in Fig. 4. 

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