Paired synthesis

Electrochemical processes are particularly well suited for the manufacture of fine chemicals in view of their high sjjecificity (almost comparable to that offered by enzymes), the smaller number of steps required, adoption of milder conditions, lack of scale-up problems, avoidance of effluents, etc. The ease with which oxidation and/or reduction can be carried out with the practically mass-free clean electrons makes electrochemical processes well suited for such jobs, including paired synthesis in effect, we use electricity as a reagent . Consider a standard chemical oxidant like manganic or chromic sulphate. Here, a stoichiometric amount of the reduced salt will be formed the disposal of which can be a serious problem. If we adopt an electrochemical process, then the reduced salt is converted into the desired oxidized salt. 

Paired synthesis can be academically very elegant and industrially interesting because the cost of the process is shared between two products and the production per cell is increased [57]. 

Fig. 3. Process flow for the paired synthesis of cysteic acid and L-cysteine. Partial and overall reactions [57]...

Sanchez-Cano et al. have proposed paired synthesis for obtaining L-cysteic acid and L-cysteine from L-cystine which greatly improves the economical parameters [57], The global process-flow for the paired synthesis, with L-cystine and water as starting materials is shown in Fig. 3. Table 2 compares the results for the paired (B) and the individual syntheses (A, C). 

Direct production of benzoquinone (BQ) from benzene is one of the targets in industrial chemistry. Considerable efforts have been made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process [40]. Benzene in aqueous emulsions containing sulfuric acid (1 1 mixture of benzene and 10% aqueous H2S04) forms, at the anode, p-benzoquinone which can be reduced cathodically to yield hydroquinone in a paired synthesis. A divided cell with Pb02 anodes is used. 

Paint technologies, 18 54-55, 56 Paint viscosity, measuring, 18 69 Paired comparison test, 11 512 Paired synthesis, of phthalide and 4.4-butylbenzaldehyde, 9 680-681 PAI resins, properties of, 10 215t Pair production process, 21 313 Palatinit, 12 44... 

Propylene Oxide Propylene BASF (West Germany) others in UK and West Germany Past pilot-plant Past pilot-plant Paired synthesis... 

Hydroquinone or Quinone Benzene Several Past pilot-plant Paired synthesis or anodic oxidation + chemical reduction... 

Quinolinic acid (133) was prepared by methods similar to those described for the monocarboxylic acids.182,183,189-191,196-202 In many cases the resulting diacid was decarboxylated to nicotinic acid (126). Quinoline (130) was simultaneously oxidized to the diacid (133) and reduced to tetrahy-droquinoline (134) in one of the rare reports of paired synthesis of pyridine compounds (Scheme 44).189 An attempt was made to delineate some of the electrode processes for the diacid (133).200... 

Stamicarbon217> claims the use of anion exchange membranes in divided cells for the electrochemical oxidation of alkylpyridines to the corresponding pyridinecarboxylic acids. Finally, Takeda 218) has used the principle of paired synthesis in a divided cell for the simultaneous generation of synthetic intermediates. 

Tatapudi and Fenton [69] explored the synthesis of ozone in a proton exchange membrane (PEM) electrochemical flow reactor as part of an overall scheme to study the paired synthesis of ozone and hydrogen peroxide in the same PEM reactor. A mixture of commercially available lead dioxide powder and Teflon , deposited on a Nafion 117 membrane, was used as the anode. Current efficiencies ranged from 2.5% at an applied potential of 3.0 V to 5.5% at 4.0 V. The low current efficiencies were attributed to inefficient reactor design. A decrease in ozone concentrations, observed at higher applied potentials (> 4.0 V) was attributed to the disintegration of lead dioxide at high anodic potentials. 

Fig. 18. Membrane cell for the paired synthesis of hydrogen peroxide and sodium chlorate showing electrode conditions and transport processes. (Adapted from [75]).

Anodic oxidation may be used instead of the chemical oxidation the reaction may be run as a paired synthesis. 

Example of paired synthesis in aqueous HBr solutions / divided cells... 

Paired synthesis. Paired electrochemical syntheses are processes in which both the anode and cathode reactions simultaneously contribute to the formation of the final products. The classic example is the simultaneous production of chlorine and sodium hydroxide in the chlor-alkali industry. Paired electrosynthesis can be generally classified in terms of the following ... 

The examples illustrate the diversity as well as the common features of a paired electrosynthesis. One can start with one or two substrates to generate one or two products. Electrode processes can be mediated or direct. Undivided and divided cells are employed in paired electrosyntheses. But as in the BASF phthahde example, it is crucial for the synthesis of glyoxylic acid, sorbitol, and methyl ethyl ketone that the cathodic process is the reduction of the substrate and not the reduction of protons because in these cases protons are generated at the anode and the electrolysis takes place in aprotic solvent. Therefore effects that minimize the overpotential of hydrogen have to be omitted. Reaction control is important in all described examples, and consequently the cell and the setup have to fit for each case. Work-up and product isolation are significant for a successful synthesis and can be even more challenging in a paired synthesis. 

Sanchez-Cano et aL have proposed paired synthesis for obtaining L-cysteic acid... 

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