Quinones, half-wave potentials

Redox potentials for i-2 were determined in butyronitrile containing O.IM tetra-n-butylammonium perchlorate using a Pt disc electrode at 21. These potentials were measured relative to a saturated calomel electrode using ac voltammetry.(lQ) Both the one electron oxidations and reductions of i-2 exhibited good reversibility. The half-wave potentials for the one-electron oxidation and reduction of i-2, ZnTPP, and two model quinones are given in Table I. 

Benzoquinones are conveniently prepared in solution by the anodic oxidation of catechols. 1,2-Quinones are unstable in solution but they have a sufficient lifetime for the redox process to be reversible at a rotating disc electrode. Reaction involves two electrons and two protons and the half-wave potential varies with pH at 25 °C according to Equation 6.1. Some redox potentials for catechols and hy-droquinones are given m Table 6.6. 

The half-wave potential for the electrochemical oxidation of NADH to NAD is ca. -bO.6 V vj. SCE at pH 7. The formal potential for the NADH/NAD couple, however, is only —0.56 V. The overpotential therefore is about 1.2 V. As NAD acts as coenzyme in many enzyme-catalyzed oxidations of practical importance, it would be of interest to regenerate NAD electrochemically. For this purpose it is necessary to find a mediator system which is able to lower the overpotential. Mediator systems accepting two electrons or a hydride atom are most effective. Therefore, dopaquinone electro-generated from dopamine 2" and quinone diimines derived from diaminobenzenes applied successfully. 

Fig. 8.15 The relation between i cr of the spectra of charge-transfer complexes and the half-wave potentials of electron acceptors [53]. Electron acceptor derivatives of phthalic anhydride, quinone and nitrobenzene, and tetra-cyanobenzene and tetracyanoethylene.

It is certain that, in the first reduction step in aprotic solvents, an electron is accepted by the LUMO of the organic compound. However, it was fortunate that this conclusion was deduced from studies that either ignored the influence of solvation energies or used the results in different solvents. Recently, Shalev and Evans [55] estimated the values of AG V(Q/Q ) for 22 substituted nitrobenzenes and nine quinones from the half-wave potentials measured by cyclic voltammetry. For quinones and some substituted nitrobenzenes, the values of AG V(Q/Q ) in a given solvent were almost independent of the EA values. Similar results had been observed for other aromatic hydrocarbons in AN (Section 8.3.2) [56]. If AG V(Q/ Q ) does not vary with EA, there should be a linear relation of unit slope between El/2 and EA. Shalev and Evans [55], moreover, obtained a near-linear relation between AG V(Q/Q ) and EA for some other substituted nitrobenzenes. Here again, the Ey2-EA relation should be linear, although the slope deviates from unity.8)... 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

Fig. 8.17 Relationship between the half-wave potential and the standard rate constant for the first reduction wave of various organic compounds in AN. The influence of R4N+ of the supporting electrolyte [61 a). Compounds 1, hexafluorobiacetyl 2, p-benzoquinone 3, 1,4-naphthoquinone 5, oxygen 6, 9,10-anthra-quinone 8, p-nitrotoluene 9, 4,4 -methoxyben-zyl 10, 2,3-butanedione 11, nitromesitylene ...

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

The complex formation of PQ and PTQ with M" results in an increase in the electron acceptor ability of PQ and PTQ. The positive shifts of one-electron reduction potentials (Fred) of o-quinones caused by the complex formation with various metal ions were verihed by the electrochemical measurements (94). The CVs of PQ in the absence and presence of various metal ions exhibit a reversible redox wave, as shown in Fig. 30 (94). The Fred value of PQ without metal ions is determined from the half-wave potential as — 0.65 V (vs SCE), as shown in Fig. 30(a) (94). The addition of 2.0 x lO M Ba +, Ca +, Mg +, Y +, and Sc + to a deaerated MeCN solution of PQ results in large positive shifts of the Fred value of PQ, as shown in Figs. 30(b-f) (94). The largest positive shift of the... 

Fig. 1. Variations of the half-wave potentials of isatogens (136a-e) and quinones with pH. , 1,4-benzoquinone (1st wave) x, 136b (1st wave) v 1,4-naphthoquinone (1st wave) 0, 136b (2nd wave) , 136a A, 136c 0, 136d H, 136e. Reproduced with permission from Bunney and Hooper. ...

The carbonyl group, including aldehydes, ketones, and quinones, produce voltammetric waves. In general, aldehydes are reduced at lower potentials than ketones conjugation of the carbonyl double bond also results in lower half-wave potentials. 

Electroactive monolayers of thiol-derivatized hydroquinones on the surface of platinum electrodes were prepared and studied by Hubbard and coworkers even before Nuzzo and Allara reported formation of the first gold-thiol monolayers . The redox potentials of quinone- or hydroquinone-containing monolayers vary with pH with a slope of 60 mV pH indicating a 2e -I- 2H+ reaction . This observation was used in an elegant study by Hickman and coworkers. They codeposited hydroquinone and ferrocene-containing thiols on the surface of a gold electrode. The half-wave potential of the latter does not depend on the pH value of the solution, and therefore can be used as a reference. The whole assembly acts thus as a pH-sensitive sensor which does not require any separate reference electrodes . 

Preceding cyclovoltammetric studies proved that on reduction in aprotic DMF solution (ch+ < 0.1 ppm), its second and reversible half-wave potential is lowered by 0.6 V( ) on addition of the soluble salt Li [B"(C6H5)4] and simultaneously becomes irreversible [13] (Fig. 14 CV). The presumed microscopic reduction pathway in the presence of excess lithium cation, which, owing to its small ionic radius (ru+ = 60 pm), possesses a high effective ionic charge, is supported by independent ESR/ENDOR measurements [13] in THF (Fig. 14 ESR) the solvated radical anion M ", with two equivalent quinone hydrogen atoms, can be detected by its (1 2 1) triplet. In the subsequently formed contact ion-pair radical [M Li ] , the Li countercation docks at the quinone radical anion. Due to reduced symmetry > Cs, a doublet of doublets ESR signal pattern results. 

Ferrocene is also called "dicyclopentadienyl iron" which accurately describes its sandwich structure. What may not be apparent is that the molecule is quite nonpolar it is soluble in hexane. The two cyclopentadienyl rings are formally aromatic, six-electron systems and as such are extremely electron rich. The ferrocene system can be functionalized by electrophilic aromatic substitution chemistry. The property of ferrocene most relevant to our efforts, however, is that an electron may be lost reversibly from iron [Fe(II) = e + Fe(ni)] so that the entire molecule becomes positively charged. The half wave potential for ferrocene is observed at about 400 mV (vs. calomel) in aqueous solution and the ferricinium species is stable in water for hours. This contrasts, for example with nitrobenzene or most quinones, which are generally less to much less stable than this upon reduction. These properties make ferrocene particularly attractive for applications in redox-altered chemisuy. 

A real example from the hterature is shown in Fig. 2.25 which utilises a cat-echin-immobihsed poly(3,4-ethylenedioxythiophene)-modified electrode towards the electrocatalysis of NADH in the presence of ascorbic acid and uric acid [11]. Interestingly, catechin has a quinone moiety in its oxidised state and the effect of pH on the redox properties of the modified electrode is shown in Fig. 2.25 over the pH range of 2-10 where the redox couple of the catechin molecules are shifted to less positive values with the increase in pH. The insert in Fig. 2.25 shows a plot of the half-wave potential of the catechin molecule as a function of pH. Note it... 

The half-wave reduction potentials for a series of annelated 1,4 naphthoquinones (102-106) increase upon alkylation, and decrease as ring size decreases (Table 13). The more cathodic reduction potentials of 2,3-dimethylnaphtho-l,4-qui-none (106, 0.846 V) and l,2,3,4-tetrahydro-9,10-anthroquinone (105,0.854 V) as compared to 1,4-naphthoquinone (0.685 V) are expected from inductive electron donation of alkyl groups. A decrease in reduction potential from 105 to 2,3-cyclobutanaphtho-l,4-quinone (103) (0.695 V) as ring size decreases is observed such that the reduction potential of 103 is only slightly higher than the parent 1,4-naphthoquinone. 

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