Radical weak acids

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

Unfortunately, it is not easy to measure acid strengths of very weak acids like the conjugate acids of simple unsubstituted carbanions. There is little doubt that these carbanions are very unstable in solution, and in contrast to the situation with carbocations, efforts to prepare solutions in which carbanions such as ethyl or isopropyl exist in a relatively free state have not yet been successful. Nor has it been possible to form these carbanions in the gas phase. Indeed, there is evidence that simple carbanions such as ethyl and isopropyl are unstable toward loss of an electron, which converts them to radicals. Nevertheless, there have been several approaches to the problem. Applequist and O Brien studied the position of equilibrium for the reaction... 

Syntheses are limited to mercuric salts of weak acids (2,110). Generally, increasing the length of the straight alkyl chain decreases the extent of decarboxylation (e.g., Ref. 133). Electron-withdrawing substituents suppress decarboxylation. For example, mercurials are not formed with Me02C, Cl, and Me(CH2)nO substituents on the a carbon (137,148,149), but some decarboxylation occurs with these on the j8 carbon (135-137). Chain decarboxylation predominated in reactions in benzene, butyric acid [R = Me(CH2)2] (150), or acetic acid (R = Me) (124). The chain reaction was also observed for R = Me(CH2)2 in the absence of solvent and in ethylacetate or heptane solution, but in these media the radical displacement reaction was dominant (2,150). When benzene was used as solvent... 

An extensive study132 has been presented on the polarizability effects of alkyl groups in RX moieties (R = Me, Et, i-Pr and i-Bu X = CH2, S, SO2, O and N) in families of weak acids and on the stabilities of adjacent anions and radicals in DMSO solution. Some of the results related to the 9-(dialkylamino)fluorenes are given in Table 12. The increases in acidity are believed to be caused by the progressive increases in anion stabilizing... 

For those systems where Ri = R2 = H or Ri = H, R2 = CH3, i.e. where the number of alkyl groups at C, is <1, and R3 = H to NO3, the alkoxynitroxyl radicals formed according to Eq. 7 under steady-state-ESR or pulse radiolysis conditions do not give rise to nitrobenzene radical anions. This means that the rate constants for heterolysis of the nitroxyls are < 10 s . This is not only true in weakly acidic (pH 4) or neutral but also in strongly alkaline solution (pH 13-14). The latter observation means that the nitroxyls are not susceptible to base catalyzed heterolysis. From this the rate constant for OH catalyzed decomposition can be estimated to be < 10 M s [19]. This low number for... 

Chlorpyrifos is stable in neutral and weakly acidic solutions (Hartley and Kidd, 1987). Chlorpyrifos reacts with OH radicals in water at a rate of 1.2 x lO /M-h at 25 °C (Armbrust, 2000). 

Differences in pK Values between Aromatic Weak Acids (RH) and Their Anion-Radicals (RH )... 

Although this mechanism is an oversimplification, it does give the basic idea. Chain termination is more complicated than in free radical polymerization. Coupling and disproportionation are not possible since two negative ions cannot easily come together. Termination may result from a proton transfer from a solvent or weak acid, such as water, sometimes present in just trace amounts. 

The guanine radical cation is a weak acid = 3.9) [22]. Therefore deprotonation will depend on the environment. Bachler and Hildenbrand [38] have studied the guanine oxidation product in aqueous solution of 5 -dGMP. The best fit to their EPR spectra seems to be from the radical cation (Gua " ). 

Table 8.2 shows some of the important acid-base equilibria in aqueous phases in the atmosphere. In addition to the S(IV) and C02 equilibria, the dissociation of H202 and the H02 free radical (generated in reactions described later) generate H+ and the reactive radical anions HO and O, respectively. Ammonia is the only gas-phase base found in the atmosphere. Uptake of strong acids such as HNO-, and HC1 also acidifies droplets, as do weak acids such as HONO and the organic acids. 

Radical stabilities may be measured experimentally by the determination of homo-lytic bond dissociation energies (BDEs) in the gas phase [140] or in solution by relating them empirically to the and the oxidation potentials, E ox(A ) of weak acids,... 

This could be explained in terms of disproportionation of the radical-anion to dianion with subsequent protonation. However, a much more complete explanation followed the realisation that, in most cases, the radical-anion acts not only as a base but also as a single electron-transfer agent (the so-called DISP mechanisms). In particular a comparison of observed cyclic voltammetric behaviour of substituted azobenzenes in the presence of weak acids with that predicted using digital simulation based on various mechanistic possibilities has established the DISPl route given in Eq. (3) (reactions 1-4). 

Similar processes also occur with 2,2 -bipyridine and 1,10-phenanthroline complexes of metals like Co, Cr, Ni and Ru. It is also known from the ESR study that, in the second step of Eq. (4.8), the electron is accepted not by the central metal ion but by the ligand, giving a radical anion of the ligand (see Section 8.2.2). The low-valency complexes are stabilized in aprotic solvents because aprotic solvents are of such weak acidity that they cannot liberate the coordinating ligand and its radical anion from the central metal ion. Aprotic solvents are suitable for studying the chemistry of low-valency metal complexes. 

The biological functions of vitamin C appear to be related principally to its well-established reducing properties and easy one-electron oxidation to a free radical or two-electron reduction to dehydroascorbic acid. The latter is in equilibrium with the hydrated hemiacetal shown at the beginning of this box as well as with other chemical species.1 Vitamin C is a weak acid which also has metal complexing properties. 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

It is path (c) of Scheme 1-29 that describes the acidity of anion radicals, pKa(RH ). Table 1-4 compares the pKa values of nitro-substituted aromatic weak acids (RH) and their anion radicals (RH-) in DMSO. 

---