Oxygen atoms, reaction

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

Leger, L. Oxygen Atom Reaction with Space Shuttle Materials at Orbital Altitudes, NASA Technical Memorandum 58246, May 1982... 

In this study, the metal centred radicals were formed indirectly by the photolysis of (Me3CO)2 yielding Me3CO radicals which then abstracted a hydrogen atom from the (n-Bu)3MH (M = Ge or Sn). The resulting metal based radical reacts with the carbonyl compound by adding to the oxygen atom (reaction 3). 

Because oxygen-atom reaction rate constants can be orders of magnitude greater than those for ozone, an experiment done on material subject to a reduced-pressure discharge is likely to signify damage done by oxygen atoms, rather than ozone. 

The absence of hydroxyl absorption in the reaction of oxygen atoms with hydrocarbons is evidence that oxygen atom reactions did not interfere with studying free-hydroxyl reactions, even if oxygen atoms were pumped out from the discharge zone in water vapor. 

In all experiments on oxygen atom reactions much attention was paid to the question whether reactions of 0 atoms with molecules may proceed on the surface. No such reactions were observed. Thus, all reactions discussed in this paper may be considered as proceeding in the gas phase. 

First experiments on oxygen atom reactions with hydrocarbons, with the zone of discharge in water vapor, as well as in O2, used as a source of 0 atoms, have shown that the reaction products are formaldehyde, acetaldehyde, acids, alcohols, peroxides, i.e., products of lower degrees of conversion than that yielding H20, CO, and C02. 

Thus, CH20 and CO are obtained in equal amounts. Other reactions of a lower probability are the formation of alcohol by reaction 0 + C2H6 - C2H6OH, possibly depending on pressure, and the formation of acetaldehyde 0 + C2H6 - C2H 0 + H2. It will be noted that the formation of alcohol was observed by Murad and Noyes63 in the course of oxygen atom reactions with ethane. 

It follows that oxygen atom reactions with saturated and unsaturated hydrocarbons proceed with scission of the C—C and C=C bonds, respectively. 

Cvetanovic67 was concerned with oxygen atom reactions with unsaturated hydrocarbons. The oxygen atoms were obtained in his experiments by mercury-photosensitized decomposition of N20. Cvetanovi6 came to the conclusion that the reaction of oxygen atoms with ethylene proceeded essentially with scission of the hydrocarbon bond, while with higher olefins this was not observed. Corresponding oxides (epoxides) and carbonyl compounds were formed in the course of the reaction. 

Great attention was paid to oxygen atom reactions with acetaldehyde. It will be mentioned that the study of this reaction encountered greater difficulties. 

The reaction of O atoms with acetaldehyde shows that the three possible types of oxygen atom reactions are scission of the C-C bond, the reaction yielding H20, and incorporation of the 0 atom at the C-H bond. 

In investigating the oxygen atom reaction with carbon monoxide 1 62 it was found that this reaction at pressures of 2.5 mm. Hg and higher obeyed essentially a bimolecular law, although a slow trimolecular reaction might have occurred as well. 

Besides the oxygen atom reactions discussed earlier, we studied those involving I.2-C2H4CI2,66 NH3,66 and acetylene, cyclohexane, and benzene. At first attempts were made to find an O atom reaction that would be similar and at the same time essential for all substances. This is the ease, for example, for hydrogen atoms and hydroxyl. Hydrogen reacts with saturated hydrocarbons by abstraction of the H atom, and with unsaturated hydrocarbons by addition as well. Hydroxyl is believed to react with hydrocarbons by abstraction of the H atom and formation of water. 

The basic conclusion on the mechanism of oxygen atom reactions is that the main primary steps of these reactions at temperatures up to 300 °C. are C—C, or C=C bond scissions. The formation of H20 and incorporation of the O atom at the C—H bond occurs less readily. The H atom abstraction with OH formation are least probable. 

In each of these equations the constant sought is found from the slope of the straight line and the intercept on the ordinate axis, though the intercept b and the slope values will be different for every case. A conclusion may be drawn that the final concentration of primary or secondary products, or the sum of these, as functions of the initial substance concentration should be known for determination of rate constants for oxygen atom reactions. But it is not indispensable to determine whether the products are primary, secondary, or the sum of these. 

Cvetanovi6,0>71 determined the relative rate constants of oxygen atom reactions with various molecules. 

It may be seen from Table I that the activation energy varies from 0 to 11 kcal., and the pre-exponential factor from 0.2 X 10 10 to 5 X 10-10. It may be stated, though tentatively (as information is scarce) that starting from a certain low value the pre-exponential factor drops with the activation energy. Table IV shows that activation energies for oxygen atom reactions vary from 0 to 8000 cal., while the values of pre-exponential factors range within 1.8 X 10 u to 3.4 X 10-11. 

It may be seen from Figure 5 that pre-exponential factors for oxygen atom reactions are lower than those for OH reactions by a power of ten. 

Rate Constants for Oxygen-Atom Reactions with Various Molecules... 

To explain these results it can be postulated that with increasing temperature the energy content of the intermediate biradicals (la and lb) at the moment when they are formed becomes greater and as a result the extent of their decomposition is increased. As will be seen later, the amount of this excess energy content may also depend on the kinetic energy of oxygen atoms. Reaction (13) should then be written as... 

An additional field where the NO2 technique offers particular advantages is in the study of oxygen atom reactions with olefins in the presence of molecular oxygen (90). This topic is outside the scope of the present article since it deals with secondary interactions. It is a most challenging field which is of great importance for the understanding of photochemical air pollution phenomena (67). 

In conclusion, it is evident that considerable progress has been made in the understanding of some of the fundamental features of oxygen atom reactions with olefins. A number of these features will no doubt prove to... 

In dilute aqueous solutions, the water vapor is decomposed into 01I plus H [reaction (16)] or molecular hydrogen and an oxygen atom [reaction (17)]. 

This mechanism omits the formation of OH bonds in order to accommodate the fact that neither H2O nor B(OH)3 were products. Reaction (7) is endothermic by about 35 kcal.mole" and presumably accounts for the observed activation energy. However, there is still no evidence for oxygen atoms reactions (8) and (9) are unverified. Reaction (11) is too complex to be an elementary step. Reaction (4) must be discarded since three-body reactions cannot be important below the lower explosion limit, and also for theoretical reasons. 

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