Electroreduction acids

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

The electrochemical results suggested to explore the possibility of creating a C-C bond between the electrogenerated a-carbanion fi and carbon nucleophiles. Results of practical importance have hitherto been obtained upon electroreduction of 2-bromoisobutyramides in acetonitrile at Hg or Pt cathodes, in the presence of carbon dioxide and an alkylating agent. The enolate-amide fi undergoes quantitative carboxy-alkylation, to yield ester amides of 2,2-dimethylmalonic acid (ref. 16). 

Bouroushian M, Kosanovic T, Loizos Z, SpyreUis N (2000) On a thermodynamic description of Se(IV) electroreduction and CdSe electrolytic formation on Ni, Ti and Pt cathodes in acidic aqueous solution. Electrochem Commun 2 281-285... 

At the cathode, oxygen electroreduction occurs according to the overall reaction (in acid medium) ... 

Jiang R, Chu D. 2000. Remarkably active catalysts for the electroreduction of O2 to H2O for use in acidic electrolyte containing concentrated methanol. J Electrochem Soc 147 4605-4609. 

Chu D, Gilman S. 1994. The influence of methanol on O2 electroreduction at a rotating Pt disk electrode in acid electrolyte. J Electrochem Soc 141 1770-1773. 

Zinola CF, AM Castro Luna, Arvia AJ. 1994. Temperature dependence of kinetic parameters related to oxygen electroreduction in acid solutions on platinum electrodes. Electrochim Acta 39 1951-1959. 

Applications As the basic process of electron transfer at an electrode is a fundamental electrochemical principle, polarography can widely be applied. Polarography can be used to determine electroreductible substances such as monomers, organic peroxides, accelerators and antioxidants in solvent extracts of polymers. Residual amounts of monomers remain in manufactured batches of (co)polymers. For food-packaging applications, it is necessary to ensure that the content of such monomers is below regulated level. Polarography has been used for a variety of monomers (styrene, a-methylstyrene, acrylic acid, acrylamide, acrylonitrile, methylmethacrylate) in... 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

Potential applications of vitamin Bi2 in electrocatalytic degradation of dibromide and a-haloacetic acid pollutants has been demonstrated in aqueous buffers303,304 and in surfactant-stabilized emulsions.305 Electroreductive dehalogenations in water and microemulsions were also efficiently catalyzed by a vitamin Bi2 derivative grafted onto a polylysine-coated electrode.306... 

Electroreduction of esters (R COOR ) is classified into three types of reaction. The first type of reaction is the reductive cleavage of a bond between oxygen and R. This type of reaction is similar to the cathodic reduction of alcohols (Sect. 7.2) and ethers (Sect. 7.3). That is, activated esters such as allylic and benzylic esters are electrochemically reduced to the corresponding hydrocarbons and acids (Schemes) [1,16,19-21]. 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

Although some electroreduction of aliphatic acids are reported in the older... 

Appropriate activation of carboxyl groups enables reduction of aliphatic carboxylic acids to the corresponding aldehydes. The electroreduction of iminium salts prepared from aliphatic carboxyKc... 

The acyl radical equivalents formed by the electroreduction of acyl phosphonium salts have an interesting reactivity and the electroreduction of nonconjugated olefinic acids yields intramolecular coupling products (Scheme 28) [11, 54, 55]. 

Unlike oxygen derivatives, thioesters of phosphor(III) acids (S-ethyldiphenylphos-phinite, S,S-diethylphenylphosphonite and triethylthiophosphite) undergo electroreduction atDME (dropping mercury electrode) in DME or AN [215]. An irreversible two-electron process results in the P—S bond scission. 

The electroreductive hydrogenation of pyridazine-3-ones performed at the first wave, in acidic or basic medium, takes place at the 4,5-double bond. A further reduction of 4,5- dihydropyridazin-3-ones in basic media, affords the corresponding tetrahydro derivatives (Scheme 139) [252]. 

Imines can be stereoselectively hy-drodimerized to 1,2-diamines with rac/ meso rahos of 0.9 to 1.1 similar to the cathodic coupling of carbonyl compounds to pinacols in an acidic medium [304]. With 4,6-dimethylpyrimid-2-one only the meso-diamine was obtained [305]. Electroreduction of diimines prepared from... 

The electroreductive dehalogenation of a-haloacetic acids has been achieved with cobalamin [387]. The hydrophobic vitamin B12 Co complex immobilized on a glassy carbon electrode (252) may catalyze the electrochemical carbon-skeleton rearrangements of... 

A polypyrrole film electrochemically deposited on gold electrodes from an MeCN-liCl04/Co(OAc)2 solution shows electrocatalytic activity in dioxygen reduction [404]. The catalytic electroreduction of dithio dipropionic acid (PSSP) with the water-soluhle cohalt(II I)tetrakis(4-trimethyl-ammonium phenyl) porphyrin (CoTMAP) has heen studied. The Co catalyst adsorbed on the glassy carbon electrode plays a major role in the electroreductive cleavage of the S—S bond [405]. 

Titanium ions can also he used as redox catalysts for the indirect cathodic reduction of nitro compounds (417). The electroreduction is carried out in an H20-H2S04/Ti(S04)2-(Pb/Cu) system at 45 80°C under 5 20Am . Nitrobenzene, dinitrobenzene, nitrotoluene, 2,4-dinitrotoluene, 2-nitro-m-xylene, nitro-phenol, 2,4-dinitrophenol, nitrophenetole, o-nitroanisole, 4-nitrochlorotoluene, ni-trobenzenesulfonic acid, and 4,4 -dinitro-stilbene-2,2 -disulfonic acid can all be reduced by this procedure to the corresponding amino compounds (418) in good yields (Scheme 146) [513-516]. Tin... 

An electroreductive Barbier-type allyla-tion of imines (434) with allyl bromide (429) also occurs inaTHF-PbBr2/Bu4NBr-(Al/Pt) system to give homoallyl amine (436) (Scheme 151) [533]. The combination of Pb(II)/Pb(0) redox and a sacrificial metal anode in the electrolysis system plays a role as a mediator for both cathodic and anodic electron-transfer processes. The metals used in the anode must have a less positive anodic dissolution potential than the oxidation potentials of the organic materials in order to be present or to be formed in situ. In addition, the metal ion plays the role of a Lewis acid to form the iminium ion (437) by associating with imine (435) (Scheme 151). 

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