Arsenic-carbon bonds, formation

When pathways (a) and (b) are followed, the electrons in the arsenic-carbon bond are displaced in an opposite direction in the two mechanisms. In alkene formation displacement of electrons is away from the arsenic atom, and in epoxide formation displacement of electrons is towards the arsenic atom. The change in pathway, depending upon the nature of the substituents at arsenic, could be associated with this, for electron-donating substituents on arsenic should assist displacement of the electrons away from the arsenic and favour alkene formation as observed . For similar reasons electron-withdrawing substituents on the ylidic carbon atom should favour alkene formation. 

Edmonds, J.S. (2000) Diastereoisomers of an arsenomethionine -based structure from Sargassum lacerifolium the formation of the arsenic-carbon bond in arsenic-containing natural product. Bioorg. Med. Chem. Lett., 10, 1105-1108. 

Two quaternary arsoniosugars (diastereoisomers), previously isolated as an unresolved mixture (see Fig. 2, structure 14, epimeric at the carbon bonded to the carboxyl group) from the brown alga Sargassum lacerifolium, were separated by HPLC and their structures proposed on the basis of NMR spectral data (17). Electrospray mass spectrometric analysis of the unresolved mixture supports the proposed structures by demonstrating a [M + H]+ molecular species with m/z 569 (Pedersen and Francesconi, unpublished results). The presence of these two compounds in S. lacerifolium formed the basis of a more general scheme for the formation of naturally occurring arsenic compounds (17). 

HG in its simplest form allows the speciation of inorganic As and As For the speciation of samples containing more than these two hydride-active species, a combination with GC is necessary. Every arsenic species, with the exception of those carrying four carbon bonds, can be transformed into arsines by borohydride. Most of the arsines are volatile and can be separated by GC. Depending on the pH used during hydride formation a distinction between tri-and pentavalent arsenic is in most cases possible. Trivalent arsenic species form hydrides already at pH 7, whereas pentavalent ones are only reactive at pH 1 (Table 2). Measuring the same sample with both pHs... 

Only a few reports deal with reactions of arsenic and antimony compounds with HFA. Several reports describe insertion of HFA into As—H bonds 43, 72, 155). In contrast to the heavier group IV elements, insertion leads to the formation of 2-arsanoperfluoropropanols 87. This difference can be explained by assuming nucleophilic attack by the arsenic lone pair on the highly electrophilic carbonyl carbon. 

The application of SPE has partially eliminated the above problems, particularly in the case of arsenic concentration in water samples [123]. SPE delivers better selectivity than LEE because it can be used to sequentially elute compounds from the activated carbon bed, and to separate inorganic compounds of As(III) and As (V), as well as the phenyl (PAS) and dimethyl (DMA) derivatives of arsenic (V) acid [124]. The extraction process is short, which is why it is possible to directly coimect the SPE module with the ICP MS detector [125]. Whenever modified silica is used, arsenic recovery is low (even below 50 %) owing to the formation of hydrogen bonds between the substances being separated and silanol groups [114, 126, 127]. 

We have essentially exhausted all of the directly measured enthalpies of formation of compounds containing carbon-arsenic, -antimony and -bismuth bonds. However, let us now make use of other thermochemical data and see what can be derived using some plausible estimates. And barring that, let us see what new enthalpies of formation would become available if only some new measurement were made. 

As Ip increases above values as low as 8.5, and we enter the upper-left portion of Fig. 3.4, the bonding between core cation and associated oxygen or hydroxyl is even stronger and largely covalent. The result is the formation of oxyanionic species such as silicate, selenate, borate, carbonate, arsenate, and sulfate, which, because of their relatively low charge densities as oxyanions, form rather weak bonds with cations and are soluble. 

It is worth noting that arsenic trichloride and diphenylamine produce phenarsazine chloride (Section II, A, 10) in which arsenic is attached to carbon atoms in the aromatic rings rather than to the nitrogen. Thus the formation of As—bonds by this method appears to be a function of the base strength of the amine. 

In addition to iodonium, sulfonium and selenonium compounds, onium salts of bromine, chlorine, arsenic, and phosphoras are also stable and can act as sources of cation radicals as well as Bronsted acids, when irradiated with light. Performance of diaryl chloronium and diaryl bromonium salts was studied by Nickers and Abu. Also, aryl ammonium and aryl phosphonium, and an alkyl aryl sulfonium salt were investigated. It appears that the general behavior of these materials is similar to diphenyl iodonium and triphenyl sulfonium salts. These are formations of singlet and triplet states followed by cleavages of the carbon-onium atom bonds and in-cage and out of cage-escape reactivity. The anions of choice appear to be boron tetrafluoride, phosphorus hexafluoride, arsenic hexafluoride, and antimony hexafluoride. 

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