A-lead dioxide

Figure 3. Active material of a lead dioxide electrode, charged on the left, discharged on the right. The charged state shows the typical "lump" structure in the discharged state lead sulfate crystals dominate [151-...

The lead storage cell (six of which make up the lead storage battery commonly used in automobiles) will be discussed as an example of a practical cell. The cell, pictured in Fig. 14-2, consists of a lead electrode and a lead dioxide electrode immersed in relatively concentrated H2S04 in a single container. When the cell delivers power (when it is used), the electrodes react as follows ... 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

Scheme 6.8. Oxidation of anisole to quinone at a lead dioxide anode in dilute sulphuric acid.

Reactions of phenols are frequently carried out at a lead dioxide anode in dilute aqueous sulphuric acid. A phenoxonium ion is the reactive intermediate. In dilute solution, the major reaction is para-hydroxylation of the phenol [100]. Phenols... 

Oxidation of methylpyridines in 60-80 % sulphuric acid at a lead dioxide anode leads to the pyridinecarboxylic acid [213]. The sulphuric acid concentration is critical and little of the product is formed in dilute sulphuric acid [214]. In these reactions, electron loss from the n-system is driven by concerted cleavage of a carbon-hydrogen bond in the methyl substituent. This leaves a pyridylmethyl radical, which is then further oxidised to the acid, fhe procedure is run on a technical scale in a divided cell to give the pyridinecarboxylic acid in 80 % yields [215]. Oxida-tionof quinoline under the same conditions leads to pyridine-2,3-dicarboxylic acid [214, 216]. 3-HaIoquino ines afford the 5-halopyridine-2,3-dicarboxylic acid [217]. Quinoxaline is converted to pyrazine-2,3-dicarboxylic acid by oxidation at a copper anode in aqueous sodium hydroxide containing potassium permanganate [218]. 

The oxidation of propargyl alcohol to the acid and of but-2-yne-l,4-diol to acetylene dicarboxylic acid is carried out on a technical scale at a lead dioxide anode in sulphuric acid [4, 5]. Electrochemical oxidation of acetylenic secondary alcohols to the ketone at lead dioxide in aqueous sulphuric acid [4], gives better results than the cliromic acid based process of Jones [6], Oxidation of aminoalkan-1-ols to the amino acid at a lead dioxide anode in sulphuric acid is achieved in 31 -73 % 5delds [7]. This route is applied to the technical scale production of (l-alanine from 3-aminopropanol in an undivided cell [8]. 

Aqueous periodic acid can be used to achieve glycol cleavage, combined with anodic oxidation of the iodate, which is formed, back to periodate [70]. Oxidation of iodate is catalysed at a lead dioxide anode [71] but at the potentials required, aldehydes are oxidised to the corresponding acids. Due to this further reaction, the redox-mediated cleavage of diols to form an aldehyde may be difficult to achieve witli a catalytic amount of periodic acid. Cleavage using a stoichiometric amount of periodic acid, followed by recovery of the iodic acid and then its electochemical oxidation, has been achieved [72]. 

Amines are stable to electrochemical oxidation in acid solution because the nitrogen lone pair is protonated and inaccessible for reaction. This is not the case for N-acetylamines, which are oxidisable at a lead dioxide anode in aqueous sulphuric acid [99]. The primary electron tiansfer step involves the amide function and leads to a radical-cation, which loses a proton from the carbon atom adjacent to nitrogen. Subsequent steps lead to an acylimmonium ion, which is trapped by water. N-acetylated primaiy amines are converted to the corresponding carboxylic acid. 

Anodic methoxylation of cyclohexyl isocyanide yields a mixture of six products whose formation is thought to be initiated by methoxy radicals 194). Cholesteryl acetate dibromide has been specifically hydroxylated at C25 in 85 to 93% yield at a lead dioxide anode 19 s. Silyl ethers have been prepared in 60 to 95 % yield by electrolysis of trialkylsilanes at a platinum electrode in ROH- Me4NI,... 

Bard, A. J., Flarshein, W. M. and Johnston, K. P. (1988) High-pressure electrochemical oxidation of benzene at a lead dioxide electrode in aqueous bisulfate solution at 25°C to 250°C. J. Electrochem. Soc. 135, 1939-1944. 

Lead-acid accumulator — (- Sinsteden 1854, - Plante 1859-60) A secondary - battery containing a lead dioxide positive electrode, a metallic lead negative electrode and a sulfuric acid aqueous electrolyte solution. The electrode reactions are... 

Lead-acid accumulator — secondary (electrically rechargeable) battery employing a lead and a lead dioxide electrode see - accumulator. 

The best known and most widely used secondary battery is the lead-acid battery, which consists of a lead anode and a lead dioxide cathode in a 25% solution of sulfuric acid. The reaction at the anode during discharge is... 

Fig. 6. An electrochemical reactor used for aniline oxidation on a lead dioxide packed-bed anode (A) lead anode (B) 1 mm lead pellets (C) stainless steel cathode (D) Na-fion 427 membrane (E) stainless steel retaining screen (F) glass beads (G) gasket (H) inlet (I) outlet (J) cathode gas vent. (Adapted from (27]).

Alkyl derivatives of 7r-electron-deficient heterocyclic compounds may be oxidized anodi-cally to carboxylic acids alkyl and arylalkyl pyridines can be oxidized to the corresponding acids in good yields at a lead dioxide anode in aqueous sulfuric acid [229, 230, 376-378]. 

This section covers the union of two aryl moieties, one of which is a fully alkylated phenol, and the other is either a free phenol or a phenol ether. Clearly, radical dimerization is not operative in such reactions. An early example is to be found in the work of Pitcher and Dietrich (1924) ° who showed that 3,3, 4,4 -tetramethoxybiphenyl was among the products of electrolytic oxidation of veratrole in sulfuric acid using a lead dioxide anode the biaryl was formed in about 20% yield based on reacted veratrole. 

For the electrolytic oxidation of soln. of chromic salts to chromic acid or the chromates, vide supra. According to E. Muller and M. Seller, a soln. of chrome-alum in i i-H2804 is not appreciably oxidized to chromic acid with a smooth platinum anode, but when traces of a lead salt are present in the soln., lead dioxide is deposited on the anode, and oxidation then occurs. About one-third of the oxidation which occurs with a lead dioxide anode occurs when an anode of platinized platinum is used. With a lead dioxide anode, the oxidation occurs quantitatively in fairly cone. soln. of chrome-alum, when the current density is not too high—about 0-005 amp. per... 

Li, L.J., neischmann, M. and Peter, L.M. (1989) Molecular level measurements of the kinetics of nucleation of a-lead dioxide on carbon microelectrodes. Electrochimica Acta, 34, 475. 

On a positive electrode soluble lead ions combine with water to produce a lead dioxide solid and protons during charge according to ... 

S. Ai, M. Gao, Y. Yang, J. Li, and L. Jin. Electrocatalytic sensor for the determination of chemical oxygen demand using a lead dioxide modified electrode. Electroanalysis, 16 404-409, 2004. 

The early attempts to use an undivided ceil were based on the concept of a sacrificia] anode reaction A lead dioxide anode was used and isopropanol was added as an anode depolarizer. The reaction ... 

One example of an indirect electrolysis-the methoxylation of the substituted furan-has already been described. The anode reaction was the oxidation of bromide ion to bromine which subsequently reacted with the furan. Here, two further examples will be presented. The first is the conversion of anthracene to anthraquinone via eicctrogenerated chromic add. A concentrated solution of chromiurnfm) in aqueous sulphuric acid is partly oxidized to chromic acid at a lead dioxide anode ... 

A two-dimensional mathematical model for a lead dioxide electrode has been presented by Dimpault-Darcy et al. [59] It contains all the features of the model by Gu et al. [58] According to Figure 9.22, its bounds are... 

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