Aromatic hydrocarbons, electrochemical reduction

Paradoxically, although they are electron-rich, S-N compounds are good electron acceptors because the lowest unoccupied molecular orbitals (LUMOs) are low-lying relative to those in the analogous carbon systems. For example, the ten r-electron [SsNs] anion undergoes a two-electron electrochemical reduction to form the trianion [SsNs] whereas benzene, the aromatic hydrocarbon analogue of [SsNs], forms the monoanion radical [CeHg] upon reduction. ... 

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

Naphthalene and other aromatic hydrocarbons can be reduced by one electron to produce the anion radical. The reduction can be carried out with sodium in an ether solvent or electrochemically in a polar aprotic solvent. 

FIGURE 1.22. Solvent reorganization energies derived from the standard rate constants of the electrochemical reduction of aromatic hydrocarbons in DMF (with n-Bu4N+ as the cation of the supporting electrolyte) uncorrected from double-layer effects. Variation with the equivalent hard-sphere radii. Dotted line, Hush s prediction. Adapted from Figure 4 in reference 13, with permission from the American Chemical Society. 

Thus, electrochemical data involving both thermodynamic and kinetic parameters of hydrocarbons are available for only olefinic and aromatic jr-systems. The reduction of aromatics, in particular, had already attracted much interest in the late fifties and early sixties. The correlation between the reduction potentials and molecular-orbital (MO) energies of a series of aromatic hydrocarbons was one of the first successful applications of the Hiickel molecular orbital (HMO) theory, and allowed the development of a coherent picture of cathodic reduction [1], The early research on this subject has been reviewed several times [2-4],... 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

DMF has been widely used as an electrochemical solvent, especially for the reduction of aromatic hydrocarbons.88 The polarography of a number of metal ions in DMF also has been reviewed.89 In general, die voltage range attained in reductions is comparable to acetonitrile and dimethyl sulfoxide, but DMF is less suitable for the study of oxidations. It has been suggested that the cyclic amide, iV-methylpyrrolidone, may have most of the favorable properties of DMF, but with less tendency to hydrolyze.90,91 However, it is less available and more expensive. 

Certain aromatic hydrocarbons, such as 9,10-diphenylanthracene, give relatively stable radicals and cation radicals upon electrochemical reduction and oxidation, respectively. If one arranges to have the radical ions from both processes mixed, either by normal DC electrolysis in a suitably designed cell or by using an alternating current for the electrolysis, the phenomenon of electrochemiluminescence appears (Hercules, 1971 McCapra, 1973). 

The electrochemical behavior of the A-D compounds (selected structures are shown in Fig. 16) agrees well with that expected on the basis of the electrochemical properties of both the donor and the acceptor moieties [134-138]. That they can be reversibly reduced and oxidized to the corresponding radical cation and anion has been ascertained by cyclic voltammetry. The standard reduction potentials, are close to the values found for the parent aromatic hydrocarbons or acridine [124]. In a similar way, the standard oxidation potentials, °, are congruent with those found for the corresponding amines [148]. The electrochemical reaction of A-D compounds can be formulated as follows ... 

Rappaport, S.M. Jin, Z.L. Xu, X.B. High-performance liquid chromatography with reductive electrochemical detection of mutagenic nitro-substituted polynuclear aromatic hydrocarbons in diesel exhausts. J. Chromatogr. 1982, 240, 145-154. 

Arylsilanes serve as a typical example of this system. The reduction potentials of arylsilanes are slightly less negative than those of the parent aromatic hydrocarbons [199-203]. This seems to be attributed to the dj -pj interaction between the aromatic ring and the silicon atom. The electrochemical behavior of silyl-substituted cyclooctatetraene is interesting [204]. The second reduction potential becomes less negative by the silyl substitution. The stabilization of the dianion (aromatic 10 7r-system) by dj -p interaction seems to be responsible for this phenomenon. 

Those EGBs for which proton-transfer rates are easily measured are radical anions derived by one-electron electrochemical reduction from azobenzenes (Sec. III.A.l), aromatic (Sec. III.C.3), and heteroaromatic hydrocarbons (Sec. III.A.3), and dioxygen (Sec. III.B.l). In those cases the protonated EGB is removed in a fast disproportionation reaction (cf. Sec. II.B, Eq. 2-4), and the proton-transfer step therefore is made effectively irreversible. In CV experiments with addition of an acidic substrate, protonation of the already mentioned radical anions is observed as an increase in the cathodic peak current (change from a one-electron to a two-electron process) and a decrease in the anodic peak current. Where the proton transfer reaction is fast compared to the time scale of the CV experiment, the cathodic peak current is doubled and the anodic peak completely vanishes. If the CV at low scan rates is unchanged after addition of (an excess of) acidic substrate, the EGB is too weak a base to deprotonate the substrate at a reasonable rate. 

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