1,721,148 research outputs found
Homogeneous reduction of haloacetonitriles by electrogenerated aromatic radical anions: Determination of the reduction potential of (CH2CN)-C-center dot
No full textThe mechanism of homogeneous reduction of XCH2CN (X) Cl, Br, I) by organic radical anions (D•-) has been investigated in DMF. All three haloacetonitriles undergo a concerted dissociative electron transfer with formation of a fragment cluster in the solvent cage. The interaction energy Dp of the fragment cluster has been determined by applying the “sticky” dissociative electron-transfer model to the kinetic data obtained for the reaction between each XCH2CN and a series of donors. The interaction energies lie in the range from 0.19 to 1.67 kcal/mol and decrease from Cl to Br and to I. Both the smallness of Dp values and their dependence on the bulkiness of X- confirm the electrostatic character of these interactions. The intermediate
radical stemming from the dissociative electron transfer to XCH2CN reacts with D•- either by radical coupling (kc) or by electron transfer(ket). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q) ket/(kc+ ket), as a function of E°D/D•- allows determination of the standard reduction potential of •CH2CN (E° = -0.69 V vs SCE) as well as the reorganization energy λ of the redox process. A significant contribution of internal reorganization to λ has been found, indicating a change of structure from •CH2CN to -CH2CN
Mechanism of the Electrochemical Carboxylation of Aromatic Ketones in Dimethylformamide
No full textThe mechanism of the electrochemical carboxylation of several benzophenones (X-C6H4COC6H5; X = 4-OCH3, 4-CH3, H, 3-Cl, 3-CF3, 4-CF3 and 4-CN) and several ring-substituted acetophenones (Y-C6H4COCH3; Y = 4-OCH3, H, 3-OCH3, 3-Cl, 3-CF3, 4-CF3, 3-CN and 4-CN) has been investigated by cyclic voltammetry in dimethylformamide. In the presence of CO2, all compounds exhibit a single irreversible peak representing a 2e- reduction process. The reaction mechanism has been analysed using the dependence of the peak potential Ep on various experimental parameters such as the concentrations of the reacting species, the scan rate and the temperature as a mechanistic tool. Also the kinetics of the electrocarboxylation reaction has been examined. The whole set of results has been carefully analysed in the framework of an ECE-DISP mechanism. It has been found that, under the experimental conditions employed, the electrocarboxylation reaction is always under a mixed ECE-DISP1 kinetic control. The first step of the reaction is an oxygen centred attack of the electrogenerated ketyl radical anion RR'CO•- at CO2. Further reduction of the carbonate-like adduct stemming from such an attack followed by a second carboxylation reaction gives a 2-arylcarboxylic acid
Absolute Potential of the Standard Hydrogen Electrode and the Problem of Interconversion of Potentials in Different Solvents
No full textThe absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H(g)• to H(g)+, and hydration of H+. The most up-to-dateliterature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. As a reference state for the electron, we have chosen the electron at 0 K, which is the one used in computational chemistry. Unlike almost all previous estimations of SHE, ∆G°aq(H+) was used instead of the real potential. This choice was
made to obtain a SHE value based on the chemical potential, which is the appropriate reference to be used
in theoretical computations of standard reduction potentials. The result of this new estimation is a value of
4.281 V for the absolute potential of SHE. The problem of conversion of standard reduction potentials (SRPs)
measured or estimated in water to the corresponding values in nonaqeuous solvents has also been addressed.
In fact, thermochemical cycles are often used to calculate SRPs in water versus SHE, and it is extremely important to have conversion factors enabling estimation of SRPs in nonaqueous solvents. A general equation relating E° of a generic redox couple in water versus the SHE to the value of E° in an organic solvent versus the aqueous saturated calomel electrode is reported
Evaluation of the standard reduction potentials of some electrochemical processes of primary importance
No full textThe definition and possible determination of an absolute standard reduction potential, SRP, is an intriguing subject, which continuously attracts the interest of researchers both from theoretical and experimental points of view. Experimental values of half-cell reduction potentials are generally anchored to the standard hydrogen electrode, SHE, in water to which the conventional value of exactly 0 V has been assigned. Thus, the experimentally measured values are relative SRPs and these are adequate for many applications of standard potentials. However, the growing development of computational efforts and the need of making comparisons between theoretical and experimental values set the question of absolute SRPs into relevant actuality.
Another important redox process for which reliable SRPs are not available is the reduction of the halogen atoms. EoX•/X- plays a crucial role in the evaluation of the SRPs of many alkyl halides that are involved in several important processes such as reductive dehalogenation of recalcitrant pollutants, atom transfer radical polymerization and various other processes of synthetic importance.
This paper reports on the calculation of the SRPs of H+, X• and RX. The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H2(g) atomization, ionization of H•(g) to H+(g) and hydration of H+. The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e- and H+ are fermions. Unlike almost all previous estimations of SHE, Goaq(H+) was used instead of the real potential, aq(H+). This choice was made to obtain an SHE value based on chemical potential, which is the appropriate reference to be used in theoretical computations of SRPs.
More complicated thermochemical cycles were used for the calculation of EoX•/X (vs SHE) in water as well as in MeCN and DMF. Last, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other processes such as pollution abatement have been calculated in MeCN and DMF. This has been done by using a thermochemical cycle involving gas phase homolytic dissociation of the C-X bond, solvation of RX, R• and X•, and reduction of X• to X- in solution
Electrocatalytic carboxylation of benzyl chlorides at silver cathodes in acetonitrile
No full textSilver exhibits powerful electrocatalytic activities towards
the reductive carboxylation of benzyl chlorides (RCl): in
CO2-saturated CH3CN, reduction of RCl occurs at potentials
that are about 0.6 V more positive than those of the
same process at Hg or carbon electrodes and gives carboxylic acids in good to excellent yields
Electrochemical synthesis of cyanoacetic acid from chloroacetonitrile and carbon dioxide
No full textThe electrochemical carboxylation of chloroacetonitrile was investigated in dimethylformamide (DMF) and acetonitrile(MeCN) by cyclic voltammetry and controlled-potential electrolysis. Both direct electroreduction and mediated reduction of the halide in CO2-saturated solvents were used to achieve the electrocarboxylation process. Also the effects of cathode material and cell type (divided or undivided with dissolving anode) were examined. In DMF the electrolyses performed in the divided cell resulted in low to moderate yields of NCCH2CO2H (25-45%), independent of the electrode material and catalyst type. The process is
remarkably more efficient in MeCN, in which acid yields of ca. 60% were obtained under similar conditions. Very good results were obtained in both solvents when an undivided cell with aluminum sacrificial anode was used. In this case, the acid yield increased to 73 and 93% in DMF and MeCN, respectively
Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization
No full textThe redox properties of some alkyl radicals, which are important in atom transfer radical polymerization
both as initiators and mimics of the propagating radical chains, have been investigated in CH3CN by an
indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and
electrogenerated aromatic or heteroaromatic radical anions(D•−). Dissociative electron transfer between
RX and D•− yields an intermediate radical(R•), which further reacts with D•− either by radical coupling or
by electron transfer. Examination of the competition between these reactions, which depends on E◦D/D•−,
allows determination of the standard reduction potential of R• as well as the self-exchange reorganization
energyR•/R−. The standard reduction potentials obtained for the radicals •CH2CN, •CH2CO2Et and
•CH(CH3)CO2Me are −0.72±0.06, −0.63±0.07 and −0.66±0.07V vs. SCE, respectively. Quite high values
ofR•/R− (from 122 to 164kJmol−1) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R•
Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment
No full textIn this paper, nitrogen and sulfur doped or co-doped mesoporous carbons (N-MC, S-MC and N,S-MC) were prepared
according to a hard template approach and employed for the in situ production of H2O2. N-MC and to a
lesser extent S-MC showed catalytic activity towards oxygen reduction reaction with high selectivity, up to
80%, for the production of H2O2. The possible application of doped MCs for the in situ generation of H2O2 in
water treatments was confirmed by the degradation of methyl orange, which is a benchmark for degradation
of pollutants, in potential controlled electrolysis in an undivided electrochemical cell, resulting in the complete
degradation of the organic dy
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