Aromatic anion radicals protonation rate constants

Table III. Protonation Rate Constants for Some Aromatic Radical Anions in Isopropyl Alcohol at 25°C.

The protonic character of the alcohols is not the only important parameter which determines the magnitude of k2. There is a variation of at least two orders of magnitude in k2, for a given alcohol, for the radical anions investigated. Table III shows the values of the protonation rate constants in isopropyl alcohol for seven different aromatic radical anions. 

The reduction of organic halides in the presence of aromatic hydrocarbons, the subject of detailed kinetic studies, provide rate constants for the homogeneous ET [147-150] and the follow-up reaction [151]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers during the years 1978-80 [152-157]. Homogeneous ET also plays an important role in the protonation of anion radicals [158]. 

Protonation becomes a rapid reaction in protic solvents and in the presence of acids, as demonstrated for, e.g., -butyl acrylate in aqueous solution [207], methyl acrylate in EtOH [208], cinnamates in the presence of phenol in DMF [209], and benzaldehyde in ethanolic buffer solution [210]. Rate constants for protonation of aromatic radical anions (anthracene [211], naphthalene, 2-methoxynaphthalene, 2,3-dimethoxynaphthalene) by a number of proton donors including phenols, acetic acid, and benzoic acids in aprotic DMF were found to vary from 5.0 X 10 M- s-> (for anthracene, in the presence of p-chlorophenol) to 6.2 x lO s (for anthracene, in the presence of pentachlorophenol) [212]. For dimedone, PhOH, or PhC02H the rate of protonation depends on the hydrogen-bond basicity of the solvent and increases in the order DMSO < DMF MeCN [213],... 

Kinetic studies have revealed that aliphatic ketyl radical anions are very shortlived compared with aromatic (half life of acetone " in aqueous 2-propanol is 72 ps, whereas that for acetophenone " is 1.5 ms) [253]. The reductive dimerization of simple aromatic aldehydes has been studied in aprotic solvents, with the second order rate constant being larger in acetonitrile than in DMF, because of ion-pair effects [254]. Electron-withdrawing substituents reduce the speed of dimerization (benzaldehyde " k = 2.4x 10 m" s", p-cyanobenzaldehyde k = 5 M s" ) [255], whereas protic solvents lead to protonation before dimerization [256]. 

Reactive states of aromatic molecules in solution may be observed directly by the pulse radiolysis method. Extensive investigations of both aromatic molecule ions (particularly the radical anions) and electronically excited states have provided new information about not only the radiation chemical processes but also the general kinetic behavior of these reactive intermediates. Absolute rate constants have been determined for many elementary processes such as energy transfer and electron and proton transfer reactions. 

The recommended value of the reduction potential for SO," indicates that it is a moderate one-electron oxidant. This is supported by the rate constants measured for its reactions with one-electron reductants in aqueous solution, a few of which are listed in Table 3. The reactivity of SO," towards these compounds increases with decreasing reduction potential in some cases, for example for the aromatic amines, this leads to a reversal in reaction direction. Therefore, while N,N,A N -tetramethyl-/ -phenylenediamine and p-phenylenediamine are oxidized by SO,", the radical cations of A, A -dimethylaniline and aniline oxidize to SO," [65]. The oxidation of phenol and hydroxyphenols depends strongly upon the degree of deprotonation, reflecting both the lower reduction potentials of the anions and the requirement that the electron transfer from the neutral phenol also be accompanied by either deprotonation or proton transfer [66]. 

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