Organomercury refers to the group of organometallic compounds that contain mercury. Typically the Hg–C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury cation, CH3Hg+; ethylmercury cation, C2H5Hg+; dimethylmercury, (CH3)2Hg, diethylmercury, and merbromin ("Mercurochrome"). Thiomersal is used as a preservative for vaccines and intravenous drugs.
The toxicity of organomercury compounds presents both dangers and benefits. Dimethylmercury in particular, is notoriously toxic, but has found use as an antifungal agent and insecticide. Merbromin and phenylmercuric borate are used as topical antiseptics, while Nitromersol is used as a preservative for vaccines and antitoxins.
Organomercury compounds are generated by many methods, including the direct reaction of hydrocarbons and mercury(II) salts. In this regard, organomercury chemistry more closely resembles organopalladium chemistry and contrasts with organocadmium compounds.
Mercuration of aromatic ring
- C6H5OH + Hg(O2CCH3)2 → C6H4(OH)–2-HgO2CCH3 + CH3CO2H
- C6H4(OH)–2–HgO2CCH3 + NaCl → C6H4(OH)–2-HgCl + NaO2CCH3
Addition to alkenes
- Hg(O2CCH3)2 + CH2=CHCO2CH3 → CH3OCH2CH(HgO2CCH3)CO2CH3
- CH3OCH2CH(HgO2CCH3)CO2CH3 + Br2 → CH3OCH2CHBrCO2CH3 + BrHgO2CCH3
This reaction is called the Hofmann-Sand Reaction.
Reaction of Hg(II) compounds with carbanion equivalents
A general synthetic route to organomercury compounds entails alkylation with Grignard reagents and organolithium compounds. Diethylmercury results from the reaction of mercury chloride with two equivalents of ethylmagnesium bromide, a conversion that would typically be conducted in diethyl ether solution. The resulting (CH3CH2)2Hg is a dense liquid (2.466 g/cm3) that boils at 57 °C at 16 torr. The compound is slightly soluble in ethanol and soluble in ether.
Similarly, diphenylmercury (m.p. 121–123 °C) can be prepared by reaction of mercury chloride and phenylmagnesium bromide. A related preparation entails formation of phenylsodium in the presence of mercury(II) salts.
Phenyl(trichloromethyl)mercury compounds can be prepared by generating dichlorocarbene in the presence of phenylmercuric chloride. A convenient carbene source is sodium trichloroacetate. This compound on heating releases dichlorocarbene:
- C6H5HgCCl3 → C6H5HgCl + CCl2
Organomercury compounds are versatile synthetic intermediates due to the well controlled conditions under which they undergo cleavage of the Hg-C bonds. Diphenylmercury is a source of the phenyl radical in certain syntheses. Treatment with aluminium gives triphenyl aluminium:
- 3 Ph2Hg + 2 Al → (AlPh3)2 + 3 Hg
As indicated above, organomercury compounds react with halogens to give the corresponding organic halide. Organomercurials are commonly used in transmetalation reactions with lanthanides and alkaline-earth metals.
Cross coupling of organomercurials with organic halides is catalyzed by palladium, which provides a method for C-C bond formation. Usually of low selectivity, but if done in the presence of halides, selectivity increases. Carbonylation of lactones has been shown to employ Hg(II) reagents under palladium catalyzed conditions. (C-C bond formation and Cis ester formation).
Due to their toxicity and low nucleophilicity, organomercury compounds find limited use. The oxymercuration reaction of alkenes to alcohols using mercuric acetate proceeds via organomercury intermediates. A related reaction forming phenols is the Wolfenstein-Boters reaction. The toxicity is useful in antiseptics such as thiomersal and merbromin, and fungicides such as ethylmercury chloride and phenylmercury acetate.
Thiol affinity chromatography
Thiols are also known as mercaptans due to their propensity for mercury capture. Thiolates (R-S−) and thioketones (R2-C=S) being soft nucleophiles form a strong coordination complex with mercury(II), a soft electrophile. As a result, organomercurial agarose gel or gel beads are used to isolate thiolated compounds (such as thiouridine) in a biological sample.
- Heavy metal poisoning
- Mercury poisoning
- Minamata disease
- Compounds of carbon with other elements in the periodic table:
|Compounds of carbon with other elements in the periodic table
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