Formation and Simple Reactions of Radicals

We saw radicals in Chapters 1 and 2 when we considered the formation of molecules such as hydrogen, methane, and ethane from their constituent parts the hydrogen atom, which is the smallest radical, and the methyl radical (see Sections 1.5, 2.4 and 2.5 Fig. 11.1). 

FIGURE 11.1 The formation of hydrogen, methane, and ethane through reactions of the radicals, H and CH3. 

Can we reverse the bond-forming process to produce radicals from these molecules Energy will surely have to be added in the form of heat, because bond forming is highly exothermic and bond breaking is highly endothermic. Rgure 11.2a 

FIGURE 11.2 (a) A schematic picture of the formation of a covalent bond through overlap of two half-filled orbitals, 

Recall that the energy required to break the sigma bond homolytically is called the bond dissociation energy (BDE, p. 337). Note that the bond breaks to give two neutral species, two radicals, and not to give two polar species, a cation and an anion (Fig. 11.3). 

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We begin with a section on the formation and simple reactions of radicals and then move on to structure before considering the more complicated chain reactions. 

The final contribution turns back to the more simple aspects of PET in homogeneous media. As was shown for ion pairs in one of the golden ages of physical organic chemistry, the controlled formation of various types of radical ion pairs by photochemical methods can be utilized to control the course of chemical reactions. At this point, the medium effects which govern the formation and the fate of radical ion pairs resemble the supramolecular effects of the arranged systems discussed in the first articles. 

Although computer treatment is ultimately the most convenient way to interpret the results accurately, a preliminary analytical treatment is useful in defining both the reactions involved and the approximate velocity constant ratios to use in the computer treatment. The analytical treatment also emphasizes the essential simplicity of the method—i.e.9 despite the apparent complexity of the H2 + 02 mechanism, the predominant reactions of the radicals H, O, OH, and H02 are Reactions 4, 3, 1, and 10, respectively. The relative rates of additive consumption and water formation are determined effectively by the competition between these reactions and the reactions of the corresponding radical with the additive, the remaining reactions of the H2 + 02 system merely affecting slightly the relative radical concentrations and the rate of water formation. Thus, with suitable approximations, relatively simple expressions for —d[RH]/d[H20] can be obtained for attack of H, O, and OH on the hydrocarbon, and the expression for H02 attack is more complex only because the competition between Reactions 10 and 24 depends on the H02 concentration. 

These experiments show that with increasing plasma energy the chemical equilibrium of the reaction (1) is shifted to the right in the C/N2 -system (transport direction 2 - Ei) whereas it is shifted to the left in the C/H2- and C/02-systems (transport direction -+E2). The mechanism of the transport was previously discussed for the C/N2-system28,35,43 and C/H2- and C/02-systems28,39,44. It was shown that the transport involves formation and subsequent decomposition of CN, simple hydrocarbon radicals and CO in the systems C/N2, C/H2 and C/02 respectively. 

It is also likely that radical cation formation occurs in reactions of very reactive arenes with Pd(II) (see Section II.B.3.b), which would also lead to biaryl formation. That the reactions of arenes with Pd(II) compounds are far from simple is illustrated by the work of Arzoumanidis and Rauch.573,574 In the reaction of Pd(02CCF3)2 with benzene or naphthalene in TFA, a variety of polynuclear complexes, containing both Pd(I) and Pd(II) and arenes, were isolated574 in addition to the usual biaryls. 

Study of nonrepetitive biological transient reactions in solution has led to the development of continuous-flow systems first pioneered by Piette (149), Chance (150), and their coworkers. This was subsequently extended by Stone and Waters (151) and Dixon and Norman (152) as a simple and convenient method for the formation of organic radicals and the study of radical reactivity. Some of the current activities of the flow technique in ESR studies have been reviewed (153,154). 

It was realized that another reaction, the unimolecular formation of the olefine with elimination of hydrogen iodide would also take place, but it was thought that measurement of the iodine would be a good measure of equation 4.2,4.1. This simple view depends upon the assumption that iodine atoms recombine to give iodine more rapidly than they take part in the reverse of the reaction of equation 4.2,4.1. This seems rather unlikely, because the recombination of iodine atoms requires three body collisions and the recombination of radicals with iodine atoms may well be a bimolecular process. Under these circumstances, it seems more likely that the prevention of complete reformation of the alkyl iodide is due to further reactions of the alkyl radical, such as... 

Simple alkyl azides are quite labile even at room temperature and have a tendency to detonate on rapid heating for these reasons, the majority of kinetic studies have been confined to the solution phase. As with azocompounds, the common nitrogen elimination reaction is the consequence of the relative stability of the resulting, divalent RN radical, called a nitrene, and the high heat of formation of the N2 molecule. In some cases, particularly in the thermolysis of aryl azides, Nj elimination follows a concerted path nevertheless, nitrene formation is of more common occurrence in both the photolytic and thermal decompositions. Decomposition and addition reactions of organic azides have been recently reviewed . 

The mechanism by which BHT, a simple phenolic, functions is shown in Figure 14.3. A hindered phenol, when used alone, will react with two radicals. It then becomes spent and it wiU no longer protect the polymer. These reactions are only simplihed overviews. More complex and undesirable side reactions can occur. For example, quinone formation by the reaction of an alkoxy radical at the para position of the phenoxy radical leads to color development because of the conjugated diene structure [18]. 

Under CO pressure in alcohol, the reaction of alkenes and CCI4 proceeds to give branched esters. No carbonylation of CCI4 itself to give triichloroacetate under similar conditions is observed. The ester formation is e.xplained by a free radical mechanism. The carbonylation of l-octene and CCI4 in ethanol affords ethyl 2-(2,2,2-trichloroethyl)decanoate (924) as a main product and the simple addition product 925(774]. ... 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Historically, the steady state approximation has played an important role in unraveling mechanisms of apparently simple reactions such as H2 + CI2 = 2HC1, which involve radicals and chain mechanisms. We discuss here the formation of NO from N2 and O2, responsible for NO formation in the engines of cars. In Chapter 10 we will describe how NO is removed catalytically from automotive exhausts. 

Zard and coworkers [32] reported a simple approach to create another group of natural products, namely the lycopodium alkaloids [15]. These authors first investigated the reaction of O-benzoyl-N-allylhydroxylamide 3-60 with tributyltin hydride and ACCN in refluxing toluene, which led (after formation of the N-radical 3-61 in a 5-exo-trig/5-exo-trig cyclization) to the undesired pyrrolidine 3-62 in 48% yield. Nevertheless, a small structural modification, namely the placement of a chlorine atom at the allyl moiety as in 3-63, induced a 5-exo-/G-endo- instead of the 5-exo-/5-... 

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