Radical reactions elementary

Our treatment of chain reactions has been confined to relatively simple situations where the number of participating species and their possible reactions have been sharply bounded. Most free-radical reactions of industrial importance involve many more species. The set of possible reactions is unbounded in polymerizations, and it is perhaps bounded but very large in processes such as naptha cracking and combustion. Perhaps the elementary reactions can be postulated, but the rate constants are generally unknown. The quasi-steady hypothesis provides a functional form for the rate equations that can be used to fit experimental data. 

A sequence of elementary steps of radical reaction leading to the regeneration of the original radical is called the chain cycle, whereas the particular reaction steps are the events of chain propagation. 

Kaufman, F., Kinetics of Elementary Radical Reactions in the Gas Phase, J. Phys. Chem., 88, 4909-4917 (1984). 

Most radicals are highly reactive, and there are few examples where one would produce a stable radical product in a reaction. Reference to a radical reaction in synthesis or in Nature, almost always concerns a sequence of elementary reactions that give a composite reaction. Multistep radical sequences are discussed in general terms in this section so that the elementary radical reactions presented later can be viewed in the context of real conversions. The sequences can be either radical chain reactions or radical nonchain reactions. Most synthetic apphcations involve radical chain reactions, and these comprise the bulk of organic synthetic sequences and commercial applications. Nonchain reaction sequences are largely involved in radical reactions in biology. Some synthetic radical conversions are nonchain processes, and some recent advances in commercial polymerization reactions involve nonchain sequences. 

With chain and radical reactions (including photochemical ones) the intermediate steps are elementary reactions of atoms and radicals with molecules. The lifetimes of atoms and radicals are relatively short. 

Although Reactions 8 to 10 have been written as distinct unimolecu-lar processes, it is possible that they may occur via Reaction 11, followed by decomposition of the QOOH radical. Reactions 9 to 11 are to be regarded as reaction types rather than as single reactions—i.e., each represents several distinct elementary processes. 

Kinetics and Mechanism for HO + CO. Atmospheric interest in the HO + CO reaction stems from its well-recognized role as the sole process for converting CO to C02. Although this reaction is among the most extensively studied gas phase free radical reactions, a detailed understanding of its kinetics and mechanism is still lacking. Presumably, it is not a simple elementary reaction but most likely involves an addition complex which can yield HOO and C02 in the presence of 02, e.g.,... 

The term substitution in an unrestricted sense is rather too broad to be useful in classification of radical reactions, since most of them result in replacement of one group by another. We have already seen typical examples of bond homolysis, in which a molecule dissociates to yield two radicals which combine with each other or with another molecule. We are primarily concerned in this section with those elementary reaction steps in which a radical attacks directly an atom of another molecule (Equation 9.64), displacing from the site of attack another group, and with the overall reaction schemes in which these elementary reactions occur. 

The text assumes elementary knowledge of the common organic spectroscopic techniques. Nevertheless, we have included a description of the recently developed method of chemically induced dynamic nuclear polarization (CIDNP), which has already proved to be of great importance in the study of radical reactions and which has not yet found its way into books covering spectroscopy of organic compounds. 

QSAR Models for Elementary Hydroxyl Radical Reactions... 

Tang, W.Z. and Hendrix, T., Development of QSAR models to predict kinetic rate constants for elementary hydroxyl radical reactions, in Water and Wastewater Industry and Sustainable Development In Honor of Professor Xu Baojiu s 80th Anniversary, Tsinghua University Press, Beijing, China, 1998, pp. 44CM51. 

The Hammett correlations for substituted benzenes were similar for elementary hydroxyl radical reaction and UV/Ti02 because both correlations have negative slopes. The similarity of the correlations may be due to the same oxidation species, such as hydroxyl radical. For both reactions, the benzenes substituted with electron withdrawing groups were difficult to degrade. 

Table 5.2 summarizes elementary radical reactions with participation of the above-mentioned free radicals and their kinetic parameters, taken from the periodicals. 

Before discussing catalytic hydrogen mediated radical reactions, processes based on preformed organometallic reagents will be discussed, because in this manner the elementary steps can be highlighted in a more accessible manner. 

---