Hydrazine, anodic oxidation

The oxidation of hydrazine follows the change in surface completely since it oxidizes rapidly on bare nickel and again on the nickel(III) oxide surface but in the intermediate potential region, where the surface is covered with nickel(II) hydroxide, the anodic oxidation cannot occur (Fleischmann etal., 1972d). 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

Otherwise, lithium amides of secondary amines undergo anodic dimerization to form hydrazines in moderate yields [12]. Hydrazines are also generated, if secondary amines are electrochemically oxidized in the presence of an alkali hydroxide [27,28]. This reaction is mainly effective if the coupling takes place intramolecularly to give cyclic hydrazine derivatives [28]. If lithium amides of secondary amines are anodically oxidized in tetrahydro-furan (THF) solution in the presence of the free amine, 2-aminotetrahydrofurans are formed in reasonable yields. In contrast, the respective aminomagnesium bromides only gives A, A -coupling products [29]. 

The initial attack in the anodic oxidation of papaverine [75] probably involves a similar attack further oxidation and dimerization leads to the isolated product, 12,12 -bis-(2,3,9,10-tetramethoxyindolo[2,l-fl]isoquinolyl). An analogous reaction is the electrooxidation of a tetramethoxy-substituted 2-methyl-l-phenethyl-l,2,3,4-tetrahydroisoquinoline to a dibenzoquinolizinium derivative [76] and the oxidation of A,A -triphenyl-( -phenyle-nediamine to 9,10-diphenylphenazine [77]. Intramolecular Michel addition of nitrogen in a tetrahydroquinoline derivative to an o-quinone moity have resulted in the formation of a 5,6-dihydrodibenz[6,d]indolizine derivative [78]. A similar ring closure occurs during the oxidation of various catecholamines [79] and similar compounds [79] to indoles. Cyclic a-carbonylazo compounds, generated by anodic oxidation of the hydrazines, may be trapped by reaction with dienes to the expected heterocycles [80]. 

Aliphatic amines are mainly converted to a-substituted products [99,100], whereby especially the a-methoxylation leads to valuable reagents for synthesis. The intermediate iminium salts can be directly trapped by silyl enol ethers to form Mannich bases [108]. If the a-position is blocked or steric conditions favor it, N,N coupling to hydrazo or azo compounds occurs (Table 5, numbers 17-19). 1,1-Disubstituted hydrazines are dimerized to tetrazenes in fair to excellent yields (Table 5, numbers 20-24). The intermediate diaze-nium ions can attack enolizable carbonyl compounds to form aza-Mannich bases [109]. Arylazonaphthols undergo anodic oxidation, producing radical cations. These couple to biphenylbisazo compounds (up to 34%) or can be trapped by anisidine to form azodiphe-nylamines (up to 74%) [110a]. 

Nelsen and coworkers [562] detected conformational equilibria in eq, eq- and ax, eq-N, A -disubstituted cyclic hydrazines from their oxidation potentials. The anodic oxidation reactions of trans- and cw-l,3-diisopropy-l-2,4-bis(diisopropylamino)-cyclodiphospha(III)azanes are quite different [563] The trans isomer is reversibly oxidized at 0.53 V (SCE) forming a stable cation radical the c/5-isomer undergoes a completely irreversible oxidation at a more positive potential because an unstable radical cation is formed. Evans and coworkers studied structural changes associated with electron transfer reactions of W(// -C5(CH3)5)(CH3)4 and related compounds [564,565]. Yoshida and coworkers found a linear correlation on plotting the oxidation potentials of a-silylated ethers, where the rotation around the C-0 bond is restricted, against the HOMO energy-torsion angle (Si-C-O-C) curve estimated by MO calculation [566]. 

Equations 3 and 4 show that hydrazine is oxidized at the anode prior to Pu(III). Equation 6 is a possible side reaction increasing the efficiency for the hydrazine destruction. 

The variant described under (iv) has been the most thoroughly studied. Between pH 5 and 9 the anodic oxidation current of hydrazine at +100 mV vs SCE (in the Tafel region) depends linearly on OH concentration ... 

Fig. 69. Schematic of the processes in an amperometric urea electrode based on the pH dependence of the anodic oxidation of hydrazine.

An amperometric urea sensor based on the pH dependence of the anodic oxidation of hydrazine (Kirstein, 1987) has been utilized in the Glukometer GKM 02 for hemodialysis monitoring. For urea concentration in dialyzate the following correlation was obtained with the Ber-thelot method ... 

A similar gas coulometer that uses hydrazine sulfate as an electrolyte is more accurate at low currents. In this case the hydrazine is oxidized to nitrogen at the anode (see Eqn. 4.6) so the gas mixture consists of nitrogen and hydrogen. 

In 2003, it was suggested to use this substance in a fuel cell with proton-conducting (i.e., an acid) membrane. Hydrazine was used as a 10% aqueous solution of hydrazine hydrate (N2H4-H20). In the aqueous solution, hydrazine, on account of its strong alkaline properties, dissociates into the ions N2H5+ and OH . The anodic oxidation of hydrazine can be written in terms of the equation ... 

Ozoemena [125] performed the anodic oxidation and amperometric sensing of hydrazine using a glassy carbon electrode modified with a cobalt(ll) phthalocya-nine-cobalt(n) tetraphenylporphyrin (CoPc-(CoTPP)4) supramolecular complex. This amperometric sensor displayed excellent characteristics for the determination of hydrazine in 0.2 M NaOH at low overpotential (+100 mV vs. Ag/AgCl), with very fast amperometric response time (1 s), linear range of 10-230 pmol LT, limit of detection of 1 pmol L and sensitivity of 0.0157 pA L pmol . ... 

Zhang WD, Chen H, Luo QM (2002) Anodic oxidation of hydrazine at carbon nanotube powder microelectrode and its detection. Talanta 58 529-534... 

Ozoemena KI (2006) Anodic oxidation and amperometric sensing of hydrazine at a glassy carbon electrode modified with cobalt (II) phthalocyanine-cobalt (II) tetraphenylporphyrin (CoPc-(CoTPP)4) supramolecular complex. Sensors 6 874—891... 

The anodic oxidation of fuels in low temperature cells, mainly on platinum metals, platinum metal alloys and alloys of platinum metals with other metals, is the subject of this chapter. Most oxidation studies were made on these metals because the efficiency of other electrocatalysts is too low. The mechanism for the oxidation of carbon monoxide, nlixtures of hydrogen and carbon monoxide, formic acid, methanol, higher alcohols, hydrocarbons, and hydrazine is discussed in separate sections. 

The anodic oxidation of hydrazine which is of interest as a non-carbonaceous fuel has been studied on different transition metals in acid and alkaline media in recent years [8, 129—138]. 

Oxygen is soluble in water to the extent of 9.4 ppm from air at 100 kPa and 20 °C, and 02 is the oxidant responsible for most metallic corrosion. Consequently, deaeration of water by purging with nitrogen or vacuum degassing may be desirable in some circumstances this should not be undertaken without circumspection, since deoxygenation may cause activation of otherwise passive metals or cause cathodic areas to become anodic (Chapter 16). At high temperatures, aqueous oxygen is consumed quite rapidly by hydrazine or sodium sulfite (Section 16.7). 

The 1,1-disubstituted hydrazine 53 a is rapidly oxidized in high yield to 54a at the nickel hydroxide electrode (Eq. (15), Table 17). The high reactivity of 53a can be seen from the dissolution of the black nickel oxide hydroxide deposit at the anode. 

A cathodic depolarizer is reduced in preference to solvent. For oxidation reoctions. anodic depolarizers include N2H4 (hydrazine) and NH2OH (hydroxylamine)... 

Similarly, the anode potential can be controlled by adding a suitable reducing agent. For example, if deposition of metallic lead at the cathode is desired, the anodic deposition of lead dioxide can be prevented by use of hydroxylamine or hydrazine in dilute hydrochloric acid. Lingane and Jones found hydrazine to be superior in keeping the anode potential at a lower value and in forming simpler oxidation products (nitrogen instead of mixtures of nitrous oxide, nitrate, and nitrite). 

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