Organomercury

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, and Merbromin ("Mercurichrome").

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 is used as a topical antiseptic.

Synthesis
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 rings
Electron-rich arenes undergo direct "mercuration" upon treatment with Hg(O2CCH3)2. The one acetate group that remains on mercury can be displaced by chloride:


 * C6H5OH +  Hg(O2CCH3)2  &rarr;  C6H4(OH)-2-HgO2CCH3  +  CH3CO2H
 * C6H4(OH)-2-HgO2CCH3 +  NaCl  &rarr;   C6H4(OH)-2-HgCl  +  NaO2CCH3

Addition to alkenes
The Hg2+ center binds to alkenes, inducing the addition of hydroxide and alkoxide. For example, treatment of methylacrylate with mercuric acetate in methanol gives an α--mercuri ester:


 * Hg(O2CCH3)2 +  CH2=CHCO2CH3  &rarr;  CH3OCH2CH(HgO2CCH3)CO2CH3

The resulting Hg-C bond can be cleaved with bromine to give the corresponding alkyl bromide:
 * CH3OCH2CH(HgO2CCH3)CO2CH3 +  Br2  &rarr;  CH3OCH2CHBrCO2CH3  +  BrHgO2CCH3

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 phenyl sodium in the presence of mercury(II) salts.

Other methods
Hg(II) can be alkylated by treatment with diazonium salts in the presence of copper metal. In this way 2-chloromercuri-naphthalene has been prepared.

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 &rarr;  C6H5HgCl  +  CCl2

Reactions
Organomercury compounds are versatile synthetic intermediates due to the well controlled conditions that 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:
 * Ph2Hg +  2 Al  &rarr;  (AlPh3)2  +  3 Hg

As indicated above, organomercury compounds react with halogens to give the corresponding organic halide.

Applications
Due to its 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.