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REDOX BEHAVIOR OF ETHYLAMINE AND NITROETHANE: REVIEW.

Byline: Inam-ul-Haque, Gulzar Fatima and Muqaddas Tariq

ABSTRACT: Catalytic synthesis of 2-aminonitroethane along with electrochemical behaviour of related ethylamine and nitroethane have been reviewed. Numerous electro-analytical techniques featuring a variety of electrode materials used for the redox investigations of the latter two compounds are summarized.

Synthesis of 2-aminonitroethanes

A direct one pot, three-component nitro-Mannich reaction of a nonenolizable aldehyde, aniline or one of its ring-substituted derivatives and nitromethane was carried out on silica gel surface. The products of the reaction, 2-aminonitroethane, were obtained in high yields. IR, 1H NMR, 13C NMR spectra and elemental analysis confirmed the structures of the products [1].

In the presence of tin(II) chloride, bromonitromethane reacts with imines derived from aromatic aldehydes and ammonia to yield 2-amino-2-aromatic substituted nitroethane derivatives via an addition reaction in good yields [2].

Oxidation of ethylamine

Cyclic voltammetry and Fourier transform infrared reflection-absorption spectroscopy were combined to analyze the oxidation of methylamine and ethylamine on platinum single crystal electrodes in an acidic medium. The oxidation of both amines on Pt(hkl) electrodes gives rise to the formation of adsorbed cyanide ad-layers, which were detected by in situ infrared spectroscopy.

In the case of Pt(111) the resulting adsorbed cyanide is rather stable but, on the contrary, the adsorbed CN-like species is highly reactive on Pt(100). It yields either adsorbed NO and CO2 when the electrode is polarized above 0.7 V or adsorbed CO below 0.4 V. The detection of adsorbed cyanide was difficult to achieve at the Pt(110) surface. In the case of ethylamine, these adsorbed species were present in lower coverage than for methylamine. The substitution of one hydrogen atom by a methyl group in the methylamine makes the amine molecule more stable. So, the behavior of ethylamine provides evidence for its lower electrochemical reactiity when compared with that of the methylamine molecule [3].

Linear scan (cyclic) voltammetry at the disk with simultaneous pulsed electrochemical detection at the ring of a rotated ring-disk electrode was demonstrated to be applicable for studies of the complex anodic behavior of ethylamine at gold electrodes in 0.10 M NaOH. The oxidation of ethylamine at the disk occurs during positive scans concomitantly with formation of surface oxide (Au - AuOH - AuO). However, the final oxide-covered surface (AuO) was inert for further ethylamine oxidation. Data obtained at the rotated ring-disk electrode demonstrate that the total ethylamine signal at the disk was composed of simultaneous contributions from: oxidative desorption of ethylamine pre-adsorbed at the oxide-free gold surface and oxidation of ethylamine transported to the disk simultaneously with oxide formation.

Munegumi et al. [5] investigated the oxidation of ethylamine to glycine in aqueous solution induced by KrF excimer laser irradiation. The electro-oxidation of isomeric butylamines at a gold electrode in contact with an alkaline electrolyte solution was studied by cyclic voltammetry. Differences in the electrochemical activity of the isomers were found. It was ascertained that the amine oxidation was catalysed by the gold oxide layer. Substrate molecules were adsorbed on the gold electrode surface in the potential range preceding the oxidation. The adsorptive behaviour of the substrate molecules was evaluated on the basis of changes in the differential capacitance of the double layer at the electrode/solution interface [6]. Oxidation of ethylamine also investigated at oxides-covered silver-bismuth composite electrodes [7].

Adsorption of amine

Pulsed electrochemical detection at the ring of a ring-disk electrode was applied to a study of amine adsorption at gold electrodes [8].

Glassy carbon electrodes coated with thin films of Nafion metalized with silver and lead species were investigated by cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy. Metalization of Nafion film was accomplished by dipping the coated electrodes in 3 mM AgNO3 + 3mM Pb(NO3)2 solution for 10 min. The resulting chemically modified electrodes were electrochemically characterized toward the oxidation of amino compounds in carbonate solutions buffered at pH 10. Under chronoamperometric experiments carried out at a constant applied potential of 0.95 V vs saturated calomel electrode, the linear range (r 2greater than0.995) was determined to be at least three decades and the limit of detection range from 26 mM (ethylamine) and 65 mM (tert-butylamine), for the investigated amino compounds.

The perm-selective properties of the Nafion film with respect to anion species were investigated toward the electro-oxidation of ethylamine in pres ence of large concentration of chloride ions. The X-ray photoelectronspectroscopy analysis revealed heterogeneous distribution of the catalytic species dispersed in the metalized Nafion film. Thus, a comparison of the spectra of silver 3d and lead 4f acquired at various take-off angles, indicated an increase in the atomic ratio silver:lead and a notable enrichment of lead oxide species in the outer surface of the film when compared with the bulk membrane coated electrode [9].

Ethylamine at concentrations lower than 0.5 him in N,N-dimethylformamide gives a diffusion-controlled anodic wave due to the reversible oxidation of the mercury to a 1:2 complex of mercury(II) with the amine. At higher concentrations, another anodic wave appears at a potential less positive than the main wave; this was attributed to the adsorption of the oxidation product on the mercury electrode. The adsorption prewave was analyzed on the basis of Brdicka's theory, in which the Langmurian adsorption was assumed. It was shown that the nature of the prewave can be well interpreted on the additional assumption that the adsorption coefficient of the adsorbed complex decreases exponentially as the electrode potential becomes more positive [10].

Table 1: Electrochemical oxidation of ethylamine

Technique###Reaction Type###Electrode###Media###Ref.

Cyclic voltammetery,###Oxidation of ethylamine###Platinum###Acid media###[3]

Fourier transform

infrared

reflection-absorption

spectroscopy###

Cyclic voltammetery,###Oxidation of ethylamine###Rotated

pulsed electrochemical###ring-disk

detection###gold electrode###0.1 M NaOH###[4]###

KrF excimer###Oxidation of ethylamine###Aqueous media###[5]

laser irradiation###

Cyclic voltammetery,###Oxidation of butylamine###Gold###Alkaline media###[6]

Voltammetry###Oxidation of ethylamine###Silver-bismuth

###composite electrode###[7]

Pulsed Voltammetry###Amine adsorption###Gold###[8]

Cyclic voltammetery,###Glassy carbon###Carbonate soln.###

Chronoamperometry###coated with

and X-ray###Nafion,

photoelectron###metalized with

spectroscopy###Oxidation of amino compound###silver:lead###PH=10###[9]

Ring disc electrode###Oxidation of ethylamine###Anodized

###Silver-lead

###eutectic alloy

###electrodes###Alkaline

###media###[11]

Ammonia in alkaline media was oxidized to NO3- at anodized silver-lead eutectic alloy electrodes (2.4% silver by weight). The anodic signal is diminished for pH less than ca. 8, and this attenuation was attributed to the protonation of ammonia to form NH4+. Protonation of ammonia was concluded to prevent adsorption of the NH3 at silver sites in the electrode surface as the initial step in the electrocatalytic oxidation mechanism. For pH greater than ca. 10, the anodic signal decreases with time because of the loss of NH3 by volatilization. The heterogeneous rate constant for oxidation of ammonia to NO3- was significantly smaller than that for oxidation of ethylamine to acetaldehyde and ammonia (kapp,NH3/kapp,EA = ~ 0.2). Hence, ammonia was concluded to be a product of ethylamine oxidation at a rotated disk electrode whereas acetaldehyde and NO3- were the final products of the exhaustive electrolysis of ethylamine [11].

Electrochemical oxidation of ethylamine using various techniques is given in Table 1.

Reduction of ethylamine

Some compounds readily form [M+46]+ adduct ions during positive ion electrospray ionization mass spectrometry analysis. These [M+46]+ ions were characterized as [M+CH3CH2NH2+H]+ by accurate mass determination. Ethylamine involved in the adduct was proposed to be the reduction product of acetonitrile and this was confirmed using deuterated acetonitrile. Other nitrile-containing compounds tested, including isobutyronitrile and benzonitrile, also formed the adduct ions of the respective amine forms under positive ion electrospray ionization mass spectrometry conditions. Hydrogen/deuterium exchange experiments demonstrated that the reductive hydrogen originated from water. Reduction of nitriles (R-CN) to their respective amines (R-CH2NH2) under positive ion electrospray ionization mass spectrometry conditions expands the ability to identify nitrile-containing chemical unknowns [12].

Reduction of nitroethane

The first stage in the reduction of 1,1-dinitroethane and fluorodinitromethane in strongly acidic solution was a two-electron C-N bond rupture process, the subsequent stages of the reaction being determined by the relative rates of the competing processes of protonation, nitrosation, and Neff reaction [13]. The studies conducted by Petrosyan et al [14]. in previous years on polarography of polynitro compounds and their derivatives made it possible to go over to a study of whether nitro compounds of variable structures can be obtained by electrolysis method at a controlled potential. Fluorodinitromethane is prepared by electrochemical denitration of fluorodinitromethane.

In polarographic study of a-chloro-, bromo-, and iodopolynitroalkanes, it has been found that cleavage of the C-Hal bond occurs primarily, despite the well-known ease of reduction of nitro group. The reduction of fluoropolynitroalkanes was investigated. Fluorotrinitromethane, flourodinitromethane, and 1-fluoro-1,1-dinitroethane were selected as the objects of investigation [15].

Addition of nitromethane to aromatic and heteroaromatic aldehydes has been carried out in nitromethane. The reaction was catalyzed by superoxide produced by cathodic reduction of dioxygen, which serves as an electrogenerated base. The initially formed products were nitroalcohols which undergo dehydration, either in situ or in a subsequent chemical dehydration, to give 1-nitroalkenes. The addition of nitroethane to benzaldehyde, thiophene-2-carboxaldehyde, and furan-2-carboxaldehyde was also investigated, and good yields of diastereomeric mixtures of the nitroalcohols were obtained. The electrochemical reduction of seven of the 1-nitroalkenes prepared by stated method and earlier work was studied by cyclic voltammetry and controlled potential

Table 2: Electrochemical reduction of nitroethane

Technique###Reaction Type###Electrode###Media###Ref.

Polarography###Reduction of nitroethane###Dropping mercury electrode###Acid media###[13]

Controlled

potential###Electrochemical denitration

Electrolysis###of fluorotrinitromethane###Graphite, Platinum and Mercury###-###[14]

Polarography###Reduction of fluoropolynitroalkanes###Dropping mercury electrode###Both acidic and alkaline uffers.###[15]

Thermal,

photochemical and

electrochemical

treatment.###Reduction of nitroalkanes###Metal-modified carbon electrodes###Acid media###[17]

Electrochemical###

pulse methods###Adsorption###Platinum###-###[18]

voltamperometry###Adsorption of nitroalkanes###Graphite###Phosphate buffer###[19]

Cyclic Voltammetry Reduction of nitroethane###Nickel-Copper nanowire###Aqueous electrolyte###[21]

coulometry. The reduction was thought to proceed by initial formation of the radical anion which subsequently dimerizes.

However, in many cases, the reduction is accompanied by oligomerization of the starting material, leading to coulometric n-values that were much less than one [16].

Several metals, silver, copper, lead, mercury and gold, were electrodeposited on carbon fibre electrodes after cation exchange of their salts with the acidic functional groups of oxidized fibres. Oxidation of carbon fibres was performed by thermal, photochemical and electrochemical treatment. The uptake of the metal cations is greater in the case of anodically oxidized and partially re-reduced carbon fibres, since this procedure leads to the formation of functional groups not only on the carbon surface, as in the case of thermal or photochemical oxidation, but also in the bulk of the fibres. The above-mentioned metals are deposited on the carbon support in a highly dispersed state, which decreases the hydrogen overvoltage and catalyses the reduction of nitroalkanes. These reactions take place on the metal-modified carbon electrodes at much more positive potentials than on carbon and at slightly more positive potentials than on the respective plain metallic electrodes [17].

The fundamental data on the adsorption of various organic substances on platinum, obtained by electrochemical methods, were discussed. It was shown that electrochemical pulse methods make it possible to determine the degree of adsorption directly under the steady-state conditions of an electrocatalytic process and also to establish in many instances the nature of the adsorbed complex on the surface of the catalyst electrode [18].

It was also observed that there is a correlation between the volt-ampere characteristics of nitroalkanes adsorbed on graphite and the values of the thermodynamic parameters calculated from the adsorption isotherm [19].

The influence of the chemical composition and structure of nickel base alloys on their catalytic activity was examined using the electrochemical reduction of 1-nitropropane as a test reaction. The electrocatalytic activity of cathodes consisting of crystalline and amorphous Ni-alloys, respectively, with systematically varying composition was analyzed. Aluminum, iron, and palladium were added as alloying elements. The effects of the nonmetallic components of the amorphous alloys, i.e., boron and silicon, which are required as glass formers, were also assessed. The results are discussed in terms of the effects of alloy composition and structure of the cathode materials on electrocatalytic hydrogenation and the electron transfer/protonation process at the cathode as a test reaction [20].

The electrochemical reductions of nitroethane and hydrogen peroxide were investigated at composite nickel-copper nanowire electrode arrays and an extraordinary high catalytic activity of composite nickel-copper nanowire electrode arrays was observed. By using template synthesis method, well-defined composite nickel-copper nanowire electrode arrays were prepared. Homogeneous nanoparticles of Ni in composite nickel-copper nanowire electrode arrays show extraordinary high catalytic activity, which was discussed in terms of the size effect of nickel nanoparticles. Cyclic voltammograms were measured in a saturated aqueous electrolyte of nitroethane at 0.12 M at pH 4.5 but only the reduction branch was evaluated. The initial potential was set to 20.2 V, which was negative to the reduction potentials of copper and nickel oxides. Two reduction waves could be discerned at nickel nanowire electrode arrays and nickel-copper nanowire electrode arrays, which correspond to the two reduction steps of nitroethane [21].

The adsorption of nitroethane was studied on skeletal Rhodium-Ruthenium catalysts containing 5, 10, 20, 50, 60, 90 and 95 at % ruthenium which had preliminary pretreatment under various conditions. At 40-60 degC, a decrease of stationary potential was observed with nitroethane adsorption. It was explained by an increase in temperature in sulphuric acid, resulting in the formation of active electrochemical phases, which affect the poten- tial. A maximum rise of potential of 10 mV was found on Rh in ethylamine [22].

Techniques with reaction media and electrode materials for the electroreduction of nitroethane are summarized in Table 2.

Oxidation of nitroethane

Electrochemical approaches to the synthesis of 2,2-dinitropropanol was described, and the potential for pilot-plant scale synthesis was discussed. The anode of the electrochemical cell replaces the chemical oxidants used in the conventional synthesis for the purpose of reducing secondary waste and the consequent disposal cost. The common starting material, nitroethane was used in electrosynthesis reactions. The synthesis of the end-product

Table 3: Reaction types catalyzed by skeletal catalysts

Electrode/ catalyst###Reaction Type###Raw materials###Products###Ref.

Platinum###Electrooxidation of nitroethane###Nitroethane###2,2-dinitropropanol###[23]

Rhodium###Electroreduction of nitroalkanes###Nitroethane###Ethylamine###[25]

Platinum###Hydrogenation###Nitroethane###Ethylamine###[26]

Copper###Electrolytic method###Nitromethane###N-methylhydroxyl-amine ydrochloride [29]

2,2-dinitropropanol involves two steps: (1) electrochemical oxidative nitration (addition of a geminal NO2 group); and, (2) condensation with formaldehyde. Electrochemical oxidation of nitroethane was first attempted by direct oxidation on a Pt electrode surface resulting in low yield and significant generation of undesirable by-product. Alternatively, two different mediators were employed resulting in a dramatic improvement of yield for the oxidative nitration step. The two different mediators used, Ag+/Ag0 and Fe(CN)6-3/-4, were derived from the chemical oxidants known to perform the oxidative nitration. The Fe(CN)6-3/-4 mediator demonstrated the best promise for scale-up and industrial production due to the lower cost of the mediator and the solubility of the mediator lending it to greater ease-of-use in conventional electrochemical cell designs [23].

According to the general differential kinetic equation of several types of reversible reactions and the thermokinetic transformation equation, the characteristics parameter method for reversible reactions, with which the rate constants of forward and backward reactions and equilibrium constant can be obtained simultaneously from a single thermogram, were proposed. The thermokinetics of the reactions of nitroethane with ammonia at 15 oC and 25 oC and with trihydroxymethyl aminomethane at 15 oC and 30 oC had been studied with this method respectively. The validity of this method had been proved by the experimental results [24].

Reduction of nitroethane to ethylamine was also investigated via electro-reduction and hydrogenation at rhodium [25] and platinum [26] respectively.

The results of potentiometric and voltammetric studies of the interfaces between electrolyte solutions in water and nitroethane were presented. The Galvani potentials and distribution constants of several electrolytes were measured and from these data the ionic components were evaluated. The basic electrochemical properties of the water greater thannitroethane system were compared with those of the water greater thannitrobenzene system [27].

Six solvents acetonitrile, nitromethane, nitroethane, sulpholane, propylene carbonate and methylene chloride containing a tetrafluoroborate as the inert electrolyte, were shown to have extensive anodic potential ranges and half wave potential data were reported for a series of hydrocarbons in each of these solvents. In nitroethane, electrolysis of propylene was shown to lead to polypropylene. In addition, products from the anodic oxidation of propylene in the solvents containing 2% acetic acid were compared [28].

An industrial electrolytic cell was designed for the electrochemical synthesis of N-methylhydroxyl-amine hydrochloride. Copper was used as the cathode, graphite as the anode, and a cation membrane as the separator. The results show that N-methylhydroxyl-amine hydrochloride with a high purity of 99% can be electrosynthesized directly from nitromethane in HCl solution. Under a constant current of 1000-2500A*m[?]2 in the temperature of 30-50degC, the average yield, current efficiency, and reaction selectivity were 65%, 70%, and 99%, respectively.

Graphite electrode and membrane material can be used continuously in the preparative electrolysis for 5000h. Moreover, the effects of the electrode and membrane materials, current intensity, electrolyte temperature, and other associated parameters on the electrosynthesis results were investigated. The direct current power consumption was 8151.3 kW*h*(1000 kg N-methylhydroxyl-amine hydrochloride)[?]1. This method is a simple separation process with limited contamination and hence, is a new green synthesis method for the industrial production of N-methylhydroxyl-amine hydrochloride [29].

Summary of skeletal catalysts and the types of reactions catalyzed by them are given in Table 3.

Reduction of nitroamine

N-nitroso compounds are potent carcinogens. Reliable methods for the analysis of volatile carcinogenic N-nitroso compounds are well established; however selective and sensitive methods for routine analysis of thermally unstable, ionic or non-volatile N-nitroso compounds are still needed. For this purpose, a method based on micellar electrokinetic chromatography with laser induced fluorescence detection was described for the simultaneous determination of a broad range of N-nitroso compounds. According to the procedure, the nitroso group is photolytically cleaved from the N-nitroso compounds to yield the corresponding amine. The amines are then derivatized with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, identified and quantified using micellar electrokinetic chromatography - laser induced fluorescence. For the standard mixture of N-nitroso compounds, this method has good sensitivity and a large dynamic range. The detection limit provided by the method is 9 ppb for N-nitrosopyrrolidine [30].

The electrochemical reduction of 5-nitroamino- and 2-methyl-5-nitroaminotetrazoles at the first wave potential consumes six electrons, while the reduction at the second wave potential consumes eight or nine electrons. The preparative reduction of 2-methyl-5-nitroaminotetrazole at the second wave potential leads to 2-methyl-5-aminotetrazole and ammonia. A mechanism was proposed for these reactions [31].

A molecule containing a nitroamine redox center (2'-amino-4-ethynylphenyl-4'-ethynylphenyl-5'-nitro-1-benzenethiol) was used in the active self-assembled monolayer in an electronic device. Current-voltage measurements of the device exhibited negative differential resistance and an on-off peak-to-valley ratio in excess of 1000:1 [32].

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J-263, Defense Housing Authority Lahore-54792 Pakistan

Department of Chemistry University of Engineering and Technology Lahore 54890 Pakistan

Presently at Department of Pure and Applied Chemistry, University of Oldenburg, D-26111 Oldenburg, Germany 0.7 +- 0.1 monolayer for 10 to 60 mM ethylamine. Of this coverage, ca. 75% corresponds to ethylamine coadsorbed reversibly with OH[?] and 25% to ethylamine adsorbed irreversibly by a mechanism concluded to be chemisorptions [4].
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