Chlorine reduction

Samples of pure CIO2 for measurement of physical properties can be obtained by chlorine reduction of silver chlorate at 90°C ... 

Titanium tetrachloride is produced on an industrial scale by the chlorination of titanium dioxide-carbon mixtures in reactors lined with silica. During the reactor operation, the lining comes into contact not only with chlorine but also with titanium tetrachloride. There appears to be no attack on silica by either of these as the lining remains intact. However, the use of such a reactor for chlorinating beryllium oxide by the carbon-chlorine reduction chlorination procedure is not possible because the silica lining is attacked in this case. This corrosion of silica can be traced to the attack of beryllium chloride on silica. The interaction of beryllium chloride with silica results in the formation of silicon tetrachloride in accordance with the reaction... 

The efficiency of nitrobenzene photoreduction may be increased remarkably in 2-propanol/hydrochloric acid mixtures. In 50% 2-propanol/water containing 6 moles l i HCl, acetone and a complex mixture of chlorinated reduction products are formed i ). Both HCl and 2-propanol (as hydrogen source) are needed. When sulfuric acid is substituted for HCl, enhanced photoreduction does not occtu . When using mixtures of HCl and LiCl to maintain a constant chloride concentration (6 M) and vary [H+], a constant disappearance quantum yield 366 =0.15 is found within the [H+]-range 0.05—6 moles l i. This strongly suggests that chloride ions play an essential role, probably via electron transfer to 3(n, tt )-nitrobenzene i > [Eq. (1)], but it is also evident from the data presented that the presence of add is probably important in subsequent steps, [Eq. (3)]. 

This scheme implies formation of one mole of acetone per mole of nitrobenzene being consumed, which agrees with the experimental results. Nitrosobenzene is an attractive intermediate, since it had been shown independently to undergo chlorination/reduction to the product pattern given above in a dark reaction 18,34). 

Sensing of chlorine is possible with a phthalocyanine-based optode that is elec-trochemically reset [101]. Also a direct electrochemical Clark-type sensor employing carbon electrodes has been investigated [102]. For this type of sensor, the various types of carbon gave different responses and the edge-plane sites of graphitic electrodes were identified as electrochemically active. Both chlorine reduction and chlorine evolution were studied and the effects of the trichloride anion, Ch", were highlighted. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

In a mixture of THF and H2O (1 1) pentachloropyridine is reduced at a silver cathode to 2,3,5,6-tetrachloropyridine and further to 2,3,5-trichloropyridine [482]. 3,4,5,6-Tetrachloro-2-picolinic acid may similarly be stepwise reduced [483] and chlorine reductively removed from 3-chlorobenzothiophene [484]. 

As it was pointed out by the authors [77] themselves, a serious limitation of the chlorination / reduction method is its extreme sensitivity to moisture both at the chlorination and reduction steps resulting in time-consuming and labor-intensive procedures. Additionally, it is necessary to point out the potential difficulties associated with the presence of chemisorbed aluminum in reduced silicas. 

Acetylation, chlorination, reduction (aldehyde - alcohol), Benzoin condensation, Wittig-, (jrignard-, Aldol reaction, Suzuki-Miyaura cross-coupling, directed ortho-metalation [91]. 

The groups on the phenyl rings appear to have some effect on the reductive dechlorination of the trichloroethane. Thus, o,p -DDT is de-chlorinated reductively to o,p -DDD by mechanisms and rates similar to the reductive dechlorination of p,p -DDT 17, 25). In contrast, Mendel et al. 17) were unable to obtain reductive dechlorination when the phenyl chlorine atoms of p,p -DDT were replaced by ethyl or methoxy groups. It is interesting, however, that they were unable to recover completely the methoxychlor. This indicates anaerobic degradation of this compound by some other route. 

In a model synthesis <81CC524>, a nitro-Michael addition of the readily available nitroalkyl pyrrole 36 to mesityl oxide was used to introduce a geminally dimethylated structural element into an AD component rac-39 for the desired chlorin. Reduction of the nitro function in rac-37 leads to the desired AD dimer rac-38 which is combined in the presence hydrobromic acid with the well known a-bromo-a -bromomethyl dipyrromethene 40 after acid induced ester clevage and decarboxylation to yield the tetrapyrrolic biline rac-41. In the final step the linear tetrapyrrole rac-41 undergoes oxidation and cyclization in the presence of copper(II) acetate to give the copper chlorin. The cyclization occurs via the enamine tautomer of rac-41 by nucleophilic attack of the enamine structure on the bromo imine part of the linear tetrapyrrole. 

Titanium is stable in chlorine-containing brine solutions. However, it is prone toward crevice corrosion in the sealing areas and gaps between welded titanium structures, as titanium can be anodically oxidized to TiOj either from Ti or UCh or to the corresponding cathodic reaction being the chlorine reduction reaction or the hydrogen... 

A promising approach to selective recovery of chlorine from the inerts in the vent gas is to electrochemically reduce chlorine to chloride at the cathode and oxidize the ion to chlorine at the anode. The selectivity arises because of the fast kinetics of the chlorine reduction reaction compared to that of the oxygen and carbon dioxide reduction... 

At high overpotential electrodes, e.g. vitreous carbon, chlorine reduction is at more negative potentials (0.85 V vs. NHE). In the presence of CIO2, the reduction wave for chlorine is not observed. This is explained by an EC catalytic mechanism in which the C102 formed electrochemically is re-oxidised near the electrode by chlorine in solution ... 

We consider the reaction scheme of eq. (3.55), that is frequently encountered in organic syntheses, e.g., in partial oxidations, chlorinations, reductions, etc., of compounds that can be converted further. In principle, conditions must be found where the desired reaction has a much higher rate than the undesired one. Also, in general an excess of reactant B will be applied to avoid the undesired conversion of the product particularly towards the end of the process. 

An asymmetric chlorination/reduction reaction for the synthesis of p-chloroalcohols has been developed using MacMillan s resolved catalyst and NCS as the chlorine source (Scheme 7.17) [31]. The reaction was highly selective and enantiomeric ratios of >95 5 were common. 

The data on chlorine reduction and on the electrode capacity which will be given below play an important role in the choice between different mechanisms of the process. 

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