Organomercury chemistry

Group of chemical compounds containing mercury

Organomercury chemistry refers to the study 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(II) cation, CH3Hg+; ethylmercury(II) 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 found use as an antifungal agent and insecticide. Merbromin and phenylmercuric borate are used as topical antiseptics, while thimerosal is safely 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.

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 (melting point 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.

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 Wolffenstein–Böters reaction. The toxicity is useful in antiseptics such as thiomersal and merbromin, and fungicides such as ethylmercury chloride and phenylmercury acetate. Mercurial diuretics such as mersalyl acid were once in common use, but have been superseded by the thiazides and loop diuretics, which are safer and longer acting, as well as being orally active.

Thiols are also known as mercaptans due to their propensity for mercury capture. Thiolates (R S−) and thioketones (R2C=S), being soft nucleophiles, form strong coordination complexes with mercury(II), a soft electrophile. This mode of action makes them useful for affinity chromatography to separate thiol containing compounds from complex mixtures. For example, organomercurial agarose gel or gel beads are used to isolate thiolated compounds (such as thiouridine) in a biological sample.