Reduction potentials hydrocarbons, table

Table 12.1. Oxidation and Reduction Potentials for Some Aromatic Hydrocarbons"...

V. D. Parker [56] obtained in acetonitrile the oxidation and reduction potentials (EQx and ERea) of alternant aromatic hydrocarbons (AAH) by cyclic voltammetry and examined how those potentials are related to the ionization potential (IP) and the electron affinity (EA) of the compounds (Table 8.8). As expected, he found linear relations of unit slopes between E0x and IP and between ERed and EA. Moreover, he found that E0x and ERed of each AAH was symmetrical with respect to a common potential MAAH (-0.31 V vs SCE). The values of (E0x-MAAH) and (ERed Maa ) are correlated with the values of IP and EA, obtained in the vacuum, by E0x-Maah = IP- +AGsV+ and ERed-MAAII = liA-r/t-AG, respectively (Fig. 8.21). Here, is the work function of graphite and equal to 4.34 eV, and AGj v+ and AG v are the differences in solvation energies for the 0/+1 and 0/-1 couples of AAH. Experimentally, AG°V+ and AG°V were almost equal, not depending on the species of AAH, and were equal to -1.94 eV in AN. 

TABLE 3.7 Half-Wave Reduction Potentials for Hydrocarbons in 75% Dioxane-HjO and Energies of Lowest Vacant Molecular Orbitals... 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

Table 8. Reduction potentials of polycyclic hydrocarbon-radical anions relative to FeCp2 in DMF .

Table 4.3 lists AAG values, the ECD Ea, and the Ea from 1/2, of several aromatic hydrocarbons obtained in this manner. The electron affinity for pentacene was determined by TCT, while that of coronene is the value obtained from reduction potentials [6]. The Ea are verified using CURES-EC. The calculated values are given in Table 4.3. Also listed are the weighted average of the values that cluster about the current evaluated values from a 1983 compilation [27]. The consistency of the Ea values in this table support the gas phase experiment and the assignments of lower values to excited states.

This method was first applied to relative electron affinities of substituted nitro-benzenes. All but one of these has been measured by HPMS TCT studies. However, the Ea of s-butyl nitrobenzene has only been determined by collisional ionization and is still listed in the NIST tables as 2.17(20) eV. This value is referenced to a high value for nitrobenzene and should be about 1 eV lower [60]. The electron affinities of aromatic hydrocarbons have been reported using the collisional ionization method. The value for biphenylene is larger than that obtained from half-wave reduction potentials. The values for pyrene, anthracene, and c-CgHg are consistent with other reported values, but the values for benzanthracene, coronene, and benzo[ghi]perylene are significantly lower than the largest precise value and are attributed to excited states. 

The electron affinities of organic halides and environmental pollutants are evaluated in Chapter 11 and those of biological molecules in Chapter 12. Many of the Ea measured in the gas phase are tabulated in the NIST tables [1], These are listed in the appendices according to molecules containing CHX, CHNX, CHOX, and CHONX with references. The ECD values for some of the aromatic hydrocarbons in NIST have not been updated. L. G. Christophorou s compilation includes Ea from half-wave reduction potentials and charge transfer complexes [2]. Some of these Ea will be revised and evaluated in this chapter based on gas phase measurements. 

TABLE 10.7 Electron Affinities (in eV) of Aromatic Hydrocarbons from Reduction Potentials [56]... 

Table 3.37. pA Values for Weakly Acidic Hydrocarbons Based on Reduction Potentials in DMF ... 

Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals." Benzene and naphthalene are examples. The EPR spectrum of the benzene radical anion was shown in Fig. 12.2a (p. 657). These reductions must be carried out in aprotic solvents, and ethers are usually used. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representitive compounds are given in Table 12.1. The potentials correlate with the energy of the LUMO as calculated by simple Huckel MO theory." A correlation that includes a more extensive series of compounds can be observed with the use of somewhat more sophisticated molecular orbital methods." ... 

The addition of a second electron is virtually irreversible, owing to the enhanced reactivity of the dinegative ions towards protons (31,32). Another feature which influences the reduction potential of the second step more than the first step is the enhanced ability of dinegative ions to cluster with the positive counter ions. This renders the reduction potential of the univalent ion sensitive to the concentration and to the nature of the solvent, so that the second reduction potential cannot be considered as a standard potential. In Table I the reduction potentials of a number of conjugated hydrocarbons are compiled. From the table it appears that the difference, Je, between the first and the second one-electron additions is of the order of a few tenths of a volt. 

In Table II the reduction potentials for a number of conjugated hydrocarbons, determined by potentiometric titrations, are given. If the Ae values... 

The properties of the intercalation compound, potassium graphite, KCg, have been detailed in several review articles.34/35 The bonding in potassium graphite is described in terms of the limiting structure, K+Cg", and it is believed that the anion forms as a result of the transfer of an electron from the alkali metal to the conduction band of graphite. Novikov and Volpin35 have noted a similarity between aromatic radical-anions and alkali metal-graphite intercalation compounds. Their observation was based on inspection of reduction potentials of aromatic hydrocarbons relative to biphenyl, Table 9.2 ... 

The overall result is a 1-electron reduction of the metal oxidant with concomitant formation of the substrate radical (R ) and is the same in both processes. The ease of electron transfer oxidation of hydrocarbons by a particular oxidant is related to their ionization potentials (Table VII). However, the ease of elec-... 

Table 11 shows some representative results from the cathodic reduction of some aromatic hydrocarbons. These include cases with Ei j2 near the cathodic limit or in the discharge region of the SSE (benzene, toluene) and cases with Ex j2 at considerably more positive potential (naphthalene, anthracene again we must anticipate the discussion of reactivity and refer to Table 21). Reactions nos. 1, 2, 6, and 7 immediately demonstrate one difficulty with such studies in that the catholyte of a divided cell becomes strongly basic as electrolysis progresses. In sufficiently basic medium, the initial product, a 1,4-dihydro derivative (cf. the Birch reduction Birch and Subba Rao, 1972), will rearrange to a conjugated system which, in contrast to the 1,4-dihydro derivative, is further reducible to the tetrahydro product (nos. 1 and 6). In a non-divided cell the acid production at the anode balances the base production and thus only a little rearrangement occurs. It is therefore not a trivial problem to find out if the tetrahydro product is formed from the conjugated dihydro product, formed directly or by rearrangement [eqn (78)].

The early applications of fast CV mainly focused on the measurement of the peak-potential separation, AEp, for the reduction or oxidation process of aromatic compounds, to obtain the pertinent standard heterogeneous rate constant k° from the relationship given in Table 2 [22]. The largest k° values of about 4 cm s were found for the reduction of aromatic hydrocarbons such as anthracene at a gold electrode in acetonitrile. The peak-potential separation increased from the 58 mV expected for a reversible process at low v to about 100 mV on going to v values of 10 kV s . This also shows that there is no real need for employing extremely large sweep rates in the determination of k° for the majority of compounds. Rather, it is important to ensure that the measurements at the lower sweep rates are not hampered by the contribution from spherical diffusion if a too small UME is used. 

Table 1 Redox Potentials E for the Reduction of Aromatic and Olefinic Hydrocarbons...

The electrochemical oxidation of nonbenzenoid unsaturated hydrocarbons is believed to include also the formation of the corresponding radical cation as the first step. However, radical cations derived from alkenes are only in rare cases sufficiently stable to allow for the observation of their reduction back to starting material during CV. Accordingly, the oxidation potentials, such as those listed in Table 2, have in most cases no thermodynamic significance. A notable exception is the oxidation of hydrocarbons of the carotenoid type, which are oxidized in two closely spaced reversible or quasireversible one-electron transfers to the corresponding dications [187,188], and even the quasireversible oxidation to the radical trication has been observed [206]. 

The product distribution is summarized in Table 4. The potentials given in the table are IR-free potentials. Under a nitrogen atmosphere, hydrogen was found to be the only product. This is due to the decomposition of methanol. On both electrodes, the main product was CO. Methyl formate was also formed. Insignificant amounts of hydrocarbons were produced. Comparing the two electrodes, InP has higher activity for COj reduction. 

Although main reduction product of CO2 on Ag under galvano/potentiostatic electrolysis is CO, CH4 was found to be effectively formed under the pulsed electrolysis at 17=-2.25V and V, =-0.6 -0.4V. The present result shows that the peak potential at which 77 (CH4) and 77 (CO) become a maximum shifts positively. A maximum faradaic efficiency of about 30% for hydrocarbonization reaction on Ag electrode was achieved as shown in Table 1. The yields of hydrocarbons increased while 77 (CO) decreased. 

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