Carboxyl Cathodic

As brine concentration decreases, osmotic water transport through the membrane increases. The sulfonic anolyte layer of a composite membrane carries this increased flow easily, but the carboxylic cathode layer is more resistant. Osmotic pressure therefore builds up between the layers. This causes the formation of blisters or, in the extreme, complete delamination of the membrane. A lower limit of about 170 gpl NaCl on the brine concentration protects against these problems and incidentally prevents any loss in current efficiency due to low salt concentration. The value of 200 gpl mentioned above allows for some variation between electrolyzers and their individual cells. 

However, the hydride reduction of FeCp(arene)+ salts [124, 125] gives [FeCp(r 5-cyclohexadienyl)] complexes [125, 126] (via an ET mechanism [127] for the directing effect of substituents see Refs. [126, 128-130]. The electrochemical reduction of the carboxylic substituents at an Hg cathode in water leads to the primary alcohol [131-133] Eq. (39) ... 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

With 6-alkenoic acids the intermediate radical partially cyclizes to a cyclopentyl-methyl radical in a 5-exo-trig cycHzation [139] (Eq. 6) [138 a, 140] (see also chap. 6). To prevent double bond migration with enoic acids the electrolyte has to be hindered to become alkaline by using a mercury cathode. Z-4-Enoic acids partially isomerize to -configurated products. Results from methyl and deuterium labelled carboxylic acids support an isomerization by way of a reversible ring closure to cyclopropyl-carbinyl radicals. The double bonds of Z-N-enoic acids with N > 5 fully retain their configuration [140]. 

Methyl 2-furoate was dimethoxylated using methanol in sulfuric acid to give methyl-2,5-dihydro-2,5dimethoxy-2-furan carboxylate [70]. The reaction mechanism at the electrodes is not completely known. However, the anodic reaction is said to be the oxidation of methanol. A two-electron process is assumed and hydrogen production is observed at the cathode. 

Cathode Cathode potential Electrolyte (V) Quantity of electricity (C) Current efficiency (%) Concentration of carboxylic acid ( x 10 3 mol/dm3) ... 

On the other hand, it has been found that the electrochemical reduction is a very unique and useful tool in synthetic organic chemistry when magnesium is used as the material of the electrode. The cathodic reduction of 1,3-dienes with magnesium electrode gives very unique products, i.e. 3-cyclopentenol derivatives when it is carried out in the presence of a carboxylic acid ester (equation 23)17. 

Aliphatic carboxylic acids are difficult to reduce electroehemically. Reduction of a 10% oxalic acid in 10% H2SO4. at 15 °C at a mercury cathode (Refs. [494, 532] in Ref. [29]), a lead or amalgamated lead cathode (Ref. [495] in Ref. [29]) or at a sodium amalgam (Na(Hg) cathode (Ref. [497] in Ref. [29]) produces glyoxylic acid with a material yield of 88% and a current efficiency of 70%. The glyoxylic acid formed is stabilized by hydration [29]. 

G. Chimed and F. Masamichi, A lithium carboxylate ultrathin film on an aluminum cathode for enhanced electron injection in organic electroluminescent devices, Jpn. J. Appl. Phys., Part 2, 38 L1348-L1350 (1999). 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

A pH gradient is produced by incorporating a mixture of Ampholines with appropriate p/ values in either a polyacrylamide slab or column. An acid is used at the anode and an alkali at the cathode, the pH of these being approximately the same as the pi values of the two extremes of the Ampholine range. Phosphoric acid and sodium hydroxide are suitable for wide range separations while various amino or carboxylic electrolytes are used for intermediate pH ranges. 

Radicals are generated at the anode by oxidation of carbanions (Scheme lb), for example, alkoxides and carboxylates (see Chapter 5, 6), and at the cathode by reduction of protonated carbonyl compounds or onium salts (Scheme Ic) (see Chapter 7). Thereby, a wide choice of different radical structures can be mildly and simply... 

Cathodic 1,2-elimination of activated esters leads to the cleavage of the C—O bond and this method is useful for the deprotection of protected carboxyl groups (Scheme 9) [22, 23],... 

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. 

Scheme 23 Cathodic reduction of aromatic carboxylic acids to benzyl alcohols or benzaldehydes.

Scheme 24 Cathodic alkylation of carboxylic acids to esters R alkyl, aryl, R alkyl, yields 24-96%.

Scheme 25 Cathodic reduction of activated aliphatic carboxylic acids to aldehydes (R alkyl, yields 70-82%) and ketones (R benzyl, yields 66- 72%).

Scheme 26 Cathodic reduction of aliphatic carboxylic acids in the presence of triphenylphosphineto aldehydes R alkyl, aryl, yields 36 -100%.

Fig. 19 Cathodic carboxylation of perfluoroalkyl olefins with subsequent fluoride elimination [107].

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