Aromatic hydrocarbons electron transfer reactions

Electron transfer reactions involving alkali metals are heterogeneous, and for many purposes it is desirable to deal with a homogeneous electron transfer system. It was noticed by Scott39 that sodium and other alkali metals react rapidly with aromatic hydrocarbons like diphenyl, naphthalene, anthracene, etc., giving intensely colored complexes of a 1 to 1 ratio of sodium to hydro-... 

Recently, Weissman and his colleagues52 showed that the product is paramagnetic indicating that it results from an electron transfer process giving one unpaired electron to the hydrocarbon ion. Furthermore, they demonstrated30 that electron transfer reactions easily proceed in systems containing aromatic" ions and neutral aromatic hydrocarbon molecules, e.g., naphthalene" + phenathrene - naphthalene -j- phenanthrene". 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

The cation-radicals ArH+ were detected, but they originated from the fast reaction of a one-electron transfer, which does not affect kinetic constants of the oxidation. The rate constant depends linearly on Brown s a constants of substituents (Dessau et al. 1970). All these data are in agreement with the formation of the strong polar dication of an aromatic hydrocarbon as an intermediate. Because PF salts (in particular the diacetate) are not reductants, the two-electron transfer reaction proceeds irreversibly. 

Reactions involving electron transfer. Reaction of free ion radicals, oxidation of anion radicals of aromatic and heteroaromatic hydrocarbons. Usually an energy acceptor is required to be present... 

Photoinduced electron-transfer reaction of aromatic compounds with amines is one of the most fundamental reactions in the electron-donor-acceptor systems, which was recently reviewed by Lewis [35], Because of the low oxidation potentials of the amines, the photoinduced one-electron transfer from the amines to the excited singlet states of aromatic hydrocarbons ( Aril ) readily occurs to give the radical cations of amines and the radical anions of aromatic compounds even in the less polar solvents. 

The most common triplet state electron acceptors are ketones and quinones, whereas aromatic hydrocarbons, often bearing one or more cyano groups, are the most frequently used singlet state electron acceptors. For the generation of radical cations from a given donor it is important that the exothermidty of the electron transfer reaction can be adjusted to fall within an appropriate range, typically... 

Tertiary amines have also been employed in electron transfer reactions with a variety of different acceptors, including enones, aromatic hydrocarbons, cyanoaro-matics, and stilbene derivatives. These reactions also provide convincing evidence for the intermediacy of aminoalkyl radicals. For example, the photoinduced electron transfer reactions of aromatic hydrocarbons, viz. naphthalene, with tertiary amines result in the reduction of the hydrocarbon as well as reductive coupling [183, 184]. Vinyl-dialkylamines can be envisaged as the complementary dehydrogenation products their formation was confirmed by CIDNP experiments [185]. 

If these shift data really do represent the onset of an intermolecular electron transfer reaction in DABCO, ABCO, HMT clustered with amine, either, and aromatic solvents, one ought to be able to observe the reaction kinetics or dynamics. Consider the specific instance of DABCO. The singlet Rydberg state lifetime for DABCO (and all the other Rydberg molecules studied for this determination (Shang et al. 1993c, 1994a) is ca. 2 ps for the isolated molecule and ca. 1.2 ps for the nonpolar rare gas, hydrocarbon, and fluorocarbon solvents. This... 

Photoinitiated electron transfer reactions are among the earliest photochemical reactions documented in the chemical literature and (ground state) electron donor-acceptor interactions have been known for over one hundred years. Some aspects of plant photosynthesis were already known to Priestly in the eighteenth century. The photooxidation of oxalic acid by metal ions in aqueous solution was discovered by Seekamp (UVI) in 1803 and by Dobereiner (Fe,n) in 1830. The electron donor-acceptor interactions between aromatic hydrocarbons and picric acid were noticed by Fritzsche in the 1850s the quinhydrones are even older,... 

The formation of molecular radical ions by electron transfer reactions between alkali metals and a wide variety of aromatic and other organic compounds in polar solvents is well established. A very large number of radical anions have been prepared by this method and extensive studies of their e.s.r. and optical spectra have been made (Bowers, 1965 Gerson, 1967 Kaiser and Kevan, 1968). In solution the electron transfer reaction will be facilitated by the subsequent solvation of the two ions (or ion pair) by the polar solvent molecules. However, we have observed that similar electron transfer reactions occur readily when alkali metal atoms are deposited on a variety of relatively non polar substances at 77°K in the rotating cryostat. In most cases the parent compound acts as the matrix, though for some radical ions an inert matrix of a non-polar hydrocarbon has been used successfully. It is perhaps surprising that the reactions occur so readily as the energy of solvation of the ions must be quite small in most of these systems as compared with that in the polar liquids. 

Aromatic hydrocarbons also affect the efficiency of photoinduced electron transfer reactions. In such cases, aromatic hydrocarbons often act as redox photosensitizers or co-sensitizers. The role of these sensitizers is shown in Scheme 4. The primary photoinduced electron transfer occurs from ArH to A (or ArH to A ) to give ArH" and A". The succeeding secondary electron transfer from D to ArHproduces D and ArH in which D" is a real reactive... 

Redox photosensitization or co-sensitization by aromatic hydrocarbons has been utilized for enhancement of the efficiency of photoinduced electron transfer reactions. For example, the efficiency of the 9,10-dicyanoanthracene-sensitized photooxygenation of 1,2-diphenyloxirane in acetonitrile is enhanced appreciably by adding biphenyl as a co-sensitizer, giving 3,5-diphenyl-1,2,4-trioxolane in good yield [31-32]. This photoreaction does not take place in the absence of biphenyl. Schaap proposed that in this photoreaction the primary electron transfer reaction occurs from biphenyl (BP) to DCA to produce biphenyl radical cation BP and DCA . The secondary electron transfer from the oxirane to BP produces BP and the radical cation of the oxirane which is converted into the trioxolane (Scheme 5). 

Dyotropic Rearrangements and Related cr-o- Exchange Processes, 16, 33 Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Electron-Transfer Reactions of Mononuclear Organotransition Metal Complexes, 23, I Electron-Transfer Reactions of Polynuclear Organotransition Metal Complexes, 24, 87 Fast Exchange Reactions of Group 1, 11, and 111 Organometallic Compounds, 8, 167 Fischer-Tropsch Reaction, 17, 61 Fluorocarbon Derivatives of Metals, 1, 143... 

Photochemical electron transfer reactions of electron donor-acceptor pairs in polar solvents provide a convenient and effective method for the generation of radical cations which can be trapped by complex metal hydrides. One of the most effective systems is based on irradiation of a solution of substrate, sodium borohydride and 1,4- or 1,3-dicyanobenzene. A range of bi- and poly-cyclic aromatic hydrocarbons has been converted into the dihydro derivatives in this way. An especially important aspect of this route to dihydroaromatic compounds is that it may give access to products which are regioisomeric with the standard Birch reduction products. Thus, o-xylene is converted into the 1,4-dihydro product (229) rather than the normal 3,6-dihydro isomer (228). The m- and p-xylenes are similarly reduced to (230) and (231), respectively. ... 

The photoreduction of carbonyl compounds or aromatic hydrocarbons by amines was one of the early electron-transfer reactions to be studied. Observation of products from primary electron transfer depends on the facility of a deprotonation of the amine, which must be fast compared to back electron transfer. For amines without a hydrogens, quenching by back electron transfer is observed exclusively (Cohen et al., 1973). The solvent plays a quite important role since it determines the yield of radical ion pairs formed from the exciplex (Hirata and Mataga, 1984). 

While electron-transfer reactions of aromatic hydrocarbons are in general reactions of the state, electron transfer in carbonyl compounds can... 

It follows from the discussion of electron transfer reactions in Sec. II.B that a reversible process when studied by CV inevitably passes into the quasi-reversible regime at some value of V when v is allowed to increase. For k° = 3cms a value typical for many aromatic hydrocarbons, for example [71], it is seen from Eq. (27) that this happens at approximately v = 170 Vs Thus, studies of electron transfer rates in this region require voltage sweep rates in the range of 200-1000 Vs Ultramicroelectrodes are superb for this purpose, as demonstrated, for example, in a study of the oxidation of ferrocene [190]. Peak potential separations recorded at sweep rates between 500 and 3000 Vs at an electrode with a surface diameter of 10 pm over a range of substrate concentrations resulted in k° equal to 1.10, 1.13, and 1.13 cm s A more recent example is the oxidation of selenanthrene and related compounds [191]. 

Majima, T., Pac, C., Nakasone, A., Sakurai, H., Redox photosensitized Reactions. 7. Aromatic Hydrocarbon photosensitized Electron transfer Reactions of Furan, Methylated Furans,l,l Diphenylethylene, and Indene with p Dicyanobenzene J. Am. Chem. Soc. 1981, 103, 4499 4508. 

Exciplex formation can also be responsible for quenching in micellar solutions, for example, the fluorescence quenching in aromatic hydrocarbon-dicyanobenzene and -A, A -dimethylaniline systems.The photoinduced electron-transfer reaction of duroquinone (DQ) -A -ethylcarbazole (ECz) system (equation 4), has been investigated in micellar solutions and microemulsions. 

The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated in the reaction of bromine with various aromatic compounds. With benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxidative addition occurs (6) however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C8Br (II, 12). 

An alternative starting point in chemically reacting fossil fuels is to treat them as if they were graphite. As noted earlier, graphite and larger polynuclear aromatic hydrocarbons are far from inert with respect to electron-transfer reactions, and thus the use of chemistry known to work for graphite may be of possible use in the investigation of coal, petroleum, and their derivatives. In the next two sections, we will discuss aspects of reduction and oxidation of carbonaceous solids and thereby parallel the chapters in this book on the reduction and oxidation of polynuclear aromatic hydrocarbon molecules. 

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