Water organomercury compounds

Abstraction of H+ from the protonated enol by water gives an organomercury compound. 

The solubility of organomercury compounds depends primarily on the nature of the X group nitrates and sulfates tend to be salt-like and relatively water-soluble, whereas chlorides are covalent, nonpolar compounds of low water solubility. Methyl mercury compounds tend to be more volatile than other organomercury compounds. 

Agemian and Chau [55] have described an automated method for the determination of total dissolved mercury in fresh and saline waters by ultraviolet digestion and cold vapour atomic absorption spectroscopy. A flow-through ultraviolet digester is used to carry out photo-oxidation in the automated cold vapour atomic absorption spectrometric system. This removes the chloride interference. Work was carried out to check the ability of the technique to degrade seven particular organomercury compounds. The precision of the method at levels of 0.07 pg/1, 0.28 pg/1, and 0.55 pg/1 Hg was 6.0%, 3.8%, and 1.00%, respectively. The detection limit of the system is 0.02 pg/1. 

Ealy [ 75 ] also used conversion to alkyl mercury iodides for the gas chromatographic determination of organomercury compounds in benzene extracts of water. The iodides were then determined by gas chromatograph of the benzene extract on a glass column packed with 5% of cyclohexane-succinate on Anakron ABS (70-80 mesh) and operated at 200 °C with nitrogen (56 ml min-1) as carrier gas and electron capture detection. Good separation of chromatographic peaks was obtained for the mercury compounds as either chlorides, bromides, or iodides. The extraction recoveries were monitored by the use of alkylmer-cury compounds labelled with 203 Hg. 

Other applications of gas chromatography to the determination of organomercury compounds in non saline water are reviewed in Table 15.17. 

Mercury contaminated foodstuffs and water supplies are a concern because of the extreme toxicity of the element and its compounds. Elemental mercury is used in the production of chlorine gas, and organomercury compounds formerly found use as pesticides and fungicides. Alkyl mercury compounds are of greatest concern since they do not degrade readily, and methyl mercury compounds concentrate in fish lipid tissue [9]. Pregnant women are at greatest risk since methyl mercury readily crosses the placenta, affecting the fetus [6]. 

Mercuric acetate, Hg(02CCH3)2 Adds to alkenes in the presence of water, giving a-hydroxy organomercury compounds that can be reduced with NaBH4 to yield alcohols. The overall effect is the Markovnikov hydration of an alkene (Section 7.4). 

Organometallic compounds of less active metals and metalloids (e.g., silicon," antimony, and bismuth, are quite inert to water. Organomercury compounds (RHgX or R2Hg) can be reduced to RH by H2, NaBITj, or other reducing agents." The reduction with NaBH4 takes place by a free-radical mechanism." Alkyl-Si... 

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (Figure 7.5). Nucleophilic attack of water, followed by loss of a proton, then yields a stable organomercury addition product. The final step, reaction of the organomercury compound with sodium boro-hydride, is not fully understood but appears to involve radicals. Note that... 

Several organomercury compounds of formula CH3HgX are water-soluble, which makes them very poisonous to organisms [20], They dissociate according to Eq. (6). 

Organomercury compounds derive their toxicity from their solubility in both aqueous and lipophilic systems. They primarily affect the central nervous system. The reversible ionic/covalent bonding in organomercury compounds distributes them in the body. Thus, water-soluble species such as 3 are converted in the stomach into lipophilic 5 (cf. Eq. 8) X = [N03]-, etc., where they are then absorbed. 

Organomercury compounds have only slight reactivity. They are also completely indifferent to atmospheric oxygen and to water under normal conditions and can thus be prepared and used under conditions that are not permissible for the more reactive organometallic compounds discussed above. 

The dangers of mercury (Hg) and its derivatives, especially organomercury compounds, have been well documented for almost half a century. Significant concerns remain regarding mercury contamination in aqueous ecosystems. The US EPA has advanced water quality criteria [1] for the protection of organisms native to water environments. The criterion for mercury in fi esh water ecosystems is 12 nanogram/liter (ng/1), and the mercury chronic criterion for salt water is 25 ng/1. These extremely low criteria present significant demands for the analyst, and atomic fluorescence spectroscopy provides a viable option for the measurements. 

---