Radical species

The role of ketyl radicals derived from the p-0-4 linkage in the light-induced yellowing of mechanical pulps has been established using transient methods 

FIGURE 3.20 C-phenoxy cleavage of a-(aryloxy)acetophenones. The canonical form of the phenacyl radical with the nnpaired electron on oxygen absorbs at about 540 nm. 

FIGURE 3.21 There is no ground-state interaction between the benzoyl and phenol chromophores of these molecules, even though transient studies show that the linking chains are sufficiently flexible to allow interaction of the triplet carbonyl and the phenol. 

Neumann et al. [154] observed transient spectra of milled-wood lignins in deaerated solutions, using a xenon flash lamp for excitation. They interpreted the observed spectra as a combination of ketyl and phenoxy radicals, consistent with the conclusions of Fabbri et al. [128]. 

In highly scattering materials, absorbance is not directly observable, and changes in diffnse reflectance after the laser flash are monitored instead. Wilkinson and Kelly have described details of the techniqne [157]. Hurrell et al. [151] used diffuse-reflectance laser-flash photolysis to stndy the behavior of several 

The PH2 phosphinyl radical is formed as an intermediate in the photolysis of phosphine below, but the parent phosphinyl and phosphoranyl radicals are generally not easily obtained. They can, however, be produced by y-irradiation of phosphine trapped in a krypton matrix at 4.2 K (13.156), (2.1) and (2.2). 

The parent phosphonyl radical, H2P(0) (Table 13.12) is not known but derivatives R2P(0) are well established and can be regarded as phosphinyl radicals in which the unshared electron pair is involved in bonding to oxygen. 

Phosphite (phosphonate) and hypophosphite (phosphinate) radicals are produced from phosphites and hypophosphites by y-ray excitation of their crystalline salts, or by oxidation of their alkaline solutions. 

Most phosphorus radicals, like the well-established reactive methyl radical, CH3, are based on a system of seven outer electrons in contrast to the eight outer electrons found in normal phosphorus molecules or ions. The phosphoranyl radical, RjP and the phosphonium radical anion RsP, are exceptional since they have nine outer electrons and no nitrogen analogues. Phosphinidene, PH (Uke nitrene NH) has only six outer electrons. 

Phosphinyl radicals are generally fairly stable and can be obtained by homolytic fission of a diphosphine using heat or ultra-violet radiation (13.157) or by a displacement reaction (13.158). Irradiation of halophosphites (RO)2PX yields (RO)2P radicals. 

Silyl radicals have been studied theoretically less extensively than the corresponding cationic species336. Naturally, the most studied species is the parent H3Si. Calculations at all levels of theory find H3Si to be pyramidal (C3v symmetry) with a HSiH angle ranging from 107.2o265g to 115.1°68 281-307 33s exceuent summary of the calcu- 

FIGURE 60. Optimized geometries (6-31G ) of the radicals H3SiCH2 and CH3SiH2 and the transition structure for their interconversion. Bond lengths are in A, bond angles in deg343. 

The barrier to rotation around the C-C bond in H3SiCH2CH2 is 2.0 kcal mol-1 at 3-21G//3-21G, the orthogonal conformation 159, R = SiH3 being the more stable346. The 

Elimination of a hydrogen atom from H2Si=S produces two distinct HSiS equilibrium structures with the same 2 A symmetry. The electronic configurations of the two radicals are described by 161 and 162, which is consistent with the fact that the Si-S bond length is shorter in 161 than in 162220. 

Unpaired electrons can be present in charged species as well as in the neutral systems that have been considered up to thispoint. There havebeen many studiesofsuchradical cations and radical anions, and we will consider some representative examples in this section. 

In the presence of a proton source, the radical anion is protonated and further reduction occurs (the Birch reduction Part B, Section 5.5.1). In general, when no proton source is present, it is relatively difficult to add a second electron. Solutions of the radical anions of aromatic hydrocarbons can be maintained for relatively long periods in the absence of oxygen or protons. 

Cyclooctatetraene provides a significant contrast to the preference of aromatic hydrocarbons for one-electron reduction. It is converted to a diamagnetic dianion by addition of two electrons. It is easy to understand the ease with which the cyclooctatetraene radical accepts a second electron because of the aromaticity of the 10-7t-electron aromatic system which results (Section 9.3). 

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

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. 

Various aromatic and polyolefinic hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are good examples. The spectrum of the benzene radical anion is shown in Fig. 12.2b (p. 506). Such reactions must be carried out in aprotic media, and ethers are the most commonly used solvents. The ease of formation of such radical anions increases as the number of fused rings 

In addition to chemical oxidations and reductions, electrochemical processes are an important means of generation of charged-radical species. One-electron oxidation at the anode of an electrolysis cell generates radical cations, while one-electron reduction at the cathode generates radical anions. 

Two classes of charged radicals derived from ketones have been well studied. The ketyk are radical anions formed by one-electron reduction of a carbonyl compound. The formation of the benzophenone radical anion by reduction with sodium metal is an example  

One-electron reduction of a-dicarbonyl compounds gives radical anions known as semidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

For general reviews of structure and reactivity of vinyl radicals, see W. G. Bentrude, Annu. Rev. Phys. Chem. 18, 283 (1967) L. A. Singer, in Selective Organic Transformations, Vol. II, B. S. Thyagarajan (ed.), John Wiley, New York, 1972, p. 239 O. Simamura, Top. Stereochem. 4,1 (1969). 

---

Figure Bl.16.16 shows an example of RTPM in which the radical species is TEMPO (10), a stable nitroxide radical, while the triplet state is produced by photoexcitation of benzophenone (11) [45].

Table 4.3 Ionization Energy of Molecular and Radical Species...

Several portions of Section 4, Properties of Atoms, Radicals, and Bonds, have been significantly enlarged. For example, the entries under Ionization Energy of Molecular and Radical Species now number 740 and have an additional column with the enthalpy of formation of the ions. Likewise, the table on Electron Affinities of the Elements, Molecules, and Radicals now contains about 225 entries. The Table of Nuclides has material on additional radionuclides, their radiations, and the neutron capture cross sections. 

The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

Chemical Interaction. Halogens and some phosphoms flame retardants act by chemical interaction. The flame retardant dissociates into radical species that compete with chain propagating and branching steps in the combustion process. 

Reversibility of Equation 2. Notwithstanding the problems and conflicts, there is widespread agreement that the NTC phenomenon may well be related to the reversibiUty of equation 2 (13,60,63—67) R- + O2 ROO-. In the low temperature regime, the equiUbrium Hes to the right and alkylperoxy radicals are the dominant radical species. They form hydroperoxides, the chain-branching agent, by reaction 3. 

As the temperature is increased through the NTC zone, the contribution of alkylperoxy radicals falls. Littie alkyl hydroperoxide is made and hydrogen peroxide decomposition makes a greater contribution to radical generation. Eventually the rate goes through a minimum. At this point, reaction 2 is highly displaced to the left and alkyl radicals are the dominant radical species. 

The overall requirement is 1.0—2.0 s for low energy waste compared to typical design standards of 2.0 s for RCRA ha2ardous waste units. The most important, ie, rate limiting steps are droplet evaporation and chemical reaction. The calculated time requirements for these steps are only approximations and subject to error. For example, formation of a skin on the evaporating droplet may inhibit evaporation compared to the theory, whereas secondary atomization may accelerate it. Errors in estimates of the activation energy can significantly alter the chemical reaction rate constant, and the pre-exponential factor from equation 36 is only approximate. Also, interactions with free-radical species may accelerate the rate of chemical reaction over that estimated solely as a result of thermal excitation therefore, measurements of the time requirements are desirable. 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

The second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

In mbber production, the thiol acts as a chain transfer agent, in which it functions as a hydrogen atom donor to one mbber chain, effectively finishing chain growth for that polymer chain. The sulfur-based radical then either terminates with another radical species or initiates another chain. The thiol is used up in this process. The length of the mbber polymer chain is a function of the thiol concentration. The higher the concentration, the shorter the mbber chain and the softer the mbber. An array of thiols have subsequendy been utilized in the production of many different polymers. Some of these apphcations are as foUow ... 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

---