Phenylarsonic acid reduction

This is not the only instance where hypophosphorous acid is more effective than ethyl alcohol. For example, whereas deamination of o-toluidine by the hypophosphorous acid procedure affords toluene in 70-75% yield,82 treatment with ethanol gives o-ethoxytoluene in approximately 50% yield.10-13 And while reduction of the diazonium salt of p-amino-phenylarsonic acid by hypophosphorous acid yields phenylarsonic add (ca. 50%),92 ethanol gives p-ethoxyphenylarsonic add (ca. 65%). Although 3,3 -dimethoxybenzidine may be deaminated by either procedure, the yield with hypophosphorous acid is 66-78% 80 81 as compared to approximately 20% when ethanol is used.46 These data, in conjunction with other results, lead to the conclusion that hypophosphorous acid is at least equal to ethanol as a reagent for deaminating an aromatic amine, and that it is usually effective in deaminations which cannot be carried out with ethanol.18 81 82-88187 93 94... 

The phenylarsenous oxide solution is most conveniently prepared from phenyl-dichloroarsine, which is readily available from phenylarsonic acid. Crude phenylarsonic aoid (404 g., 2 moles) is dissolved in concentrated hydrochlorio aoid (700 cc.) and treated with a stream of sulfur dioxide. From time to time a trace of potassium iodide solution is added (0.2-0.5 g. of potassium iodide is usually sufficient to complete the reduction). When the hydrochloric acid solution becomes clear and the addition of more potassium iodide does not form a permanent cloudiness the reaction is complete. Two to three hours is usually required for this reaction. The crude phenyldichloroarsine separates as a heavy oil. The oil is removed, dried with calcium chloride, and fractionally distilled at reduced pressure in a stream of carbon dioxide or nitrogen. The nearly colorless product boiling at 140-143°/40 mm. is collected it weighs 334-400 g. (75-90%), depending on the purity of the original phenylarsonic acid. 

Correlation of the half-wave potentials of meta- and para-substituted phenylarsonic acids with the Hammett substituent constants, cr, has been tried . The equation 1/2 = (0.053 0,010)o- — 0.705 V vs NHE (r = 0.9946) was obtained for a series of five compounds (unsubstituted, 4-OH, 4-NHj, 4-Me, 3-NH2-4-OH) studied in 0.02 M HCl, while under the same conditions the equation jy2 = 0.173(7 — 0.703 V vs NHE (r = 0.9991) was obtained for the second reduction of a series of three N02-substituted phenylarsonic acids using the a value for —NHOH for the reduced nitro group , i.e. apparently two different correlations exist. However, the possible protonation of the amino or hydroxylamino groups under the experimental conditions was not taken into account, and the validity of assigning the a value of the reduced substituent to the combined substituent/arsonic acid reduction taking place at the potential of the second wave was not discussed. It is therefore not surprising that the data cannot be described by a simple Hammett relation. For the simple para-substituted phenylarsonic acids a linear relation (equation 15) was found between the half-wave potential and the value of the arsonic acid. 

In macroscale reduction of unsubstituted phenylarsonic acid at a mercury pool electrode, the final product is an oxygen-containing polymer, which contains As—As as well as As—O—As bonds . Precipitation of PhAsH2 was also observed during the electrolysis , and it was suggested that products were formed by the following reactions ... 

Polarographic analysis of a series of substituted phenylarsonic acids shows that the reduction process is electrochemically irreversible giving linear plots of log [(L — i)/i] vs E from which a value of an = 0.49 was determined according to equation 22 for all members of the series except for the p-Cl derivative . The cm value was independent of pH... 

It is interesting to note that quantities of phenylarsonic acid sufficient to decrease the rate of reduction of crotonic acid with Pd-PVA colloidal catalyst, to / of the unpoisoned value have no effect on the reduction of nitrobenzene (27) in the presence of the same catalyst. The reason for this behavior is not known. 

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