Arylarsonic acids

Arylarsonic acids are most readily prepared by the Bart Reaction, in which a diazonium salt in aqueous solution is run into a solution of sodium arsenite in an excess of sodium carbonate. The addition of copper sulphate to the +. ... 

Arylarsonic acids have usually a very low solubility in cold water. They are however amphoteric, since with, for example, sodium hydroxide they form sodium salts as above and with acids such as hydrochloric acid they form salts of the type [CaHjAsCOHljlCl. Both types of salt are usually soluble in water, and to isolate the free acid the aqueous solution has to be brought to the correct pH for most arsonic acids this can be achieved by niaking the solution only just acid to Congo Red, when the free acid will usually rapidly separate. 

Z-Arginine hydrochloride, 12, 4 Arsanilic acid, 16, 85 Arsonoacetic acid, 10,108 Arylarsonic acid, preparation, IS, 59 y-Arylbutyric esters, 18, 25 Arylsulfur chlorides, preparation, 15, 45... 

This reaction was discovered by Bart in 1911 (see also Bart, 1922 a, 1922 b). The yields are highly dependent on the alkalinity of the system. Bart s claim (1922b) that arylarsonic acid anions are formed directly from (Z)-diazoates is, however, doubtful (see below). Various modifications with increased yields are described in the review by Hamilton and Morgan (1944). The reaction can also be carried out with heteroaromatic diazonium salts (Capps and Hamilton, 1938). 

Two examples of the synthesis of arylarsonic acids are included in Organic Syntheses (Bullard and Dickey, 1943 Ruddy and Starkey, 1955). 

It is possible to arylate arylarsonic acids a second time with an arenediazonium salt. The product Ar2As(0)0H is called diphenylarsinic acid. 

Arsenic oxide, 27, 48 Arsenious oxide, 26, 61 Arylarsonic acids, 26, 62 Asbestos fiber mat for Buchner funnel, 25, 57... 

Classical analysis by gravimetry (see Section 5) took a new turn when L. H. Tschugaeff reported the novel use of a new organic reagent, dimethylglyoxime, in 1905, but several decades passed before other reagents such as cupferron see Cupferron), 8-hydroxyquinoline, l-nitroso-2-naphthol, the alkyl- and arylarsonic acids, and so on, became firmly established. ... 

Historically, the Meyer reaction, which consists of alkylating metal arsenites with alkyl halides, is the most widely used preparative method for alkylarsouic acid salts (equation 135). Subsequent alkylation of the alkylarsonic acid salt gives the alkylarsinic acid salt (equation 136). Excellent yields of alkylarylarsinic acid salts are obtained upon alkylating an arylarsonic acid salt. Aryl halides do not give satisfactory yields of the arylarsonic acid salts. 

The Bart reaction, which consists of reacting Na3As03 (sodium arsenite) with aromatic diazouium salts iu buffered alkaliue solution (pH = 8.8-9.2), provides an established route to the arylarsonic acids (equation 137). Auseful variation consists of reacting the diazonium fluoroborates with AsCb in a nonaqueous solvent and then hydrolyzing the products to produce a mixture of arylarsonic and diarylarsinic acids. Anhydrous conditions favor the arsonic acid, while the presence of water favors the arsinic acid. 

Studies of the electrochemical reduction of RMO(OH)2 are scarce for M = Sb and Bi, while the reduction of arsonic acids, RAsO(OH)2, especially the arylarsonic acids, ArAsO(OH)2, has received considerable attention since 1930 . The electrochemical reductions of arsonic acids have been carried out almost exclusively by polarographic methods in buffered aqueous solution, and the work has focused on three main aspects (1) the effects of substituents on correlation of with pR, (2) mechanistic... 

TABLE 3. Half-wave potentials, , 2, for reduction of arylarsonic acids in buffered aqueous solution... 

Aliphatic arsonic acids have been investigated to a much smaller extent than the aromatic arsonic acids. A series of alkylarsonic acids with pK values in the range 3.6-4.2 have been studied in buffered aqueous solution and, like the arylarsonic acids, the reduction wave vanishes at higher pH values (> 2.0). The alkylarsonic acids have jy2 values which are in general lower than those found for aromatic arsonic acids, but which... 

In an early study of three arylarsonic acids the number of electrons consumed in the reduction process was estimated to be equal to six and, consequently, that the product was the corresponding arylarsine, ArAsHj. The estimation of the number of electrons was made by comparison of the polarographic wave height for the arsonic acid process with the wave heights of the two first processes observed for 3-nitrophenylarsonic acid at pH 0.9, which were attributed to the four-electron reduction of nitro to hydroxylamine and the two-electron reduction of hydroxylamine to amine, respectively . 

Both wave heights and half-wave potentials are dependent on pH in reduction of the arylarsonic acids, and it was found that a plot of 1,2 vs pH in the pH range 1.0-2.0 was linear for all of the derivatives except the nitro-substituted one with slopes equal to the slopes of the log [(i i — OA] vs E plots. This behaviour corresponds to the involvement of one proton prior to the irreversible electron transfer step . The initial step, equations 16 and 19, in the reduction process therefore includes the microscopic steps described by equations 23 and 24. 

Reaction of arenediazonium salts with arsenic trichloride gives arylarsonic acids in the presence of cuprous bromide (equation 328). ... 

The use of arenediazonium tetrafluoroborates has been reported to be useful in the preparation of arylarsonic acids (equation 329). ... 

It is a general observation that reduction of arsonic acids in aqueous solution only takes place under acidic conditions (otherwise, the reduction of water is the first electrochemical process taking place when the potential is scanned in the negative direction), and that the value of the half-wave potential is pH-dependent. In Table 3 the half-wave potentials for reduction of a number of arylarsonic acids are given. For several of the acids, values of E1/2 are given at more than one pH value in order to facilitate direct comparison within as large a number of substituted acids as possible. Data from older studies obtained at large concentrations of the substrate (> 10 mM) and under unbuffered conditions are not included. 

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