# Organomercury chemistry

Organomercury chemistry refers to the study of organometallic compounds that contain mercury. Many organomercury compounds are highly toxic, but some are used in medicine, e.g., merbromin ("Mercurochrome") and the vaccine preservative thiomersal.

## Structure and bonding

Most organomercury compounds feature Hg(II), which is diamagnetic. They almost all adopt a linear C-Hg-X structure. They are neither Lewis basic or Lewis acidic. They are stable to oxygen and water, indicating the low polarity of the Hg-C bond.

## Toxicity

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.

## Synthesis

In part reflecting the strength of the C-Hg bond, organomercury compounds are generated by many methods. Indeed, mercury may adsorb onto laboratory glassware, such that laboratories performing mercury experiments may have difficulty avoiding C–Hg bond formation.
In some regards, organomercury chemistry more closely resembles organopalladium chemistry and contrasts with organocadmium compounds.

### From Hg

Metallic Hg reacts only slowly with methyl iodide to give dimethylmercury. With more electrophilic alkylating agents, the reaction is more efficient. Also, sodium amalgam react with organic halides to give diorganomercury compounds.

### Mercuration of aromatic rings

Electron-rich arenes, such as phenol, undergo mercuration upon treatment with Hg(O$_2$CCH$_3$)$_2$. The one acetate group that remains on the mercury atom can be displaced by chloride:
    C$_6$H$_5$OH + Hg(O$_2$CCH$_3$)$_2$ → C$_6$H$_4$(OH)–HgO$_2$CCH$_3$ + CH$_3$CO$_2$H
    C$_6$H$_4$(OH)–HgO$_2$CCH$_3$ + NaCl → C$_6$H$_4$(OH)–HgCl + NaO$_2$CCH$_3$
The first such reaction, including a mercuration of benzene itself, was first reported by Otto Dimroth in 1898.

### Addition to alkenes and alkynes

The Hg$^{2+}$ center binds to alkenes, inducing the addition of hydroxide and alkoxide. For example, treatment of methyl acrylate with mercuric acetate in methanol gives an α--mercuri ester:
    Hg(O$_2$CCH$_3$)$_2$ + CH$_2$=CHCO$_2$CH$_3$ → CH$_3$OCH$_2$CH(HgO$_2$CCH$_3$)CO$_2$CH$_3$
The resulting Hg-C bond can be cleaved with bromine to give the corresponding alkyl bromide:
    CH$_3$OCH$_2$CH(HgO$_2$CCH$_3$)CO$_2$CH$_3$ + Br$_2$ → CH$_3$OCH$_2$CHBrCO$_2$CH$_3$ + BrHgO$_2$CCH$_3$
This reaction is called the Hofmann–Sand reaction.
Internal alkynes undergo mercuration with incorporation of solvent:
    RC≡CR + Hg(OAc)$_2$ + ROH → R(AcOHg)C=CR(OR) + HOAc

### Reaction of Hg(II) compounds with C-heteroatom bonds

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 (CH$_3$CH$_2$)$_2$Hg is a dense liquid (2.466 g/cm$^3$) that boils at 57 °C at 16 torr. This extremely toxic 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.
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.
4-Chloromercuritoluene is obtained by the chloromercuration of sodium toluenesulfinite:
    CH$_3$C$_6$H$_4$SO$_2$Na + HgCl$_2$ → CH$_3$C$_6$H$_4$HgCl + SO$_2$ + NaCl

## Reactions

Organomercury compounds are versatile synthetic intermediates due to the well-controlled conditions under which Hg-C bonds undergo cleave.
Organomercurials are used in transmetalation reactions. For example diphenylmercury reacts with aluminium gives triphenyl aluminium:
    3 (C$_6$H$_5$)$_2$Hg + 2 Al → Al(C$_6$H$_5$)$_3$)$_2$ + 3 Hg
As indicated above, organomercury compounds react with halogens to give the corresponding organic halide. Phenyl(trichloromethyl)mercury 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:
    C$_6$H$_5$HgCCl$_3$ → C$_6$H$_5$HgCl + CCl$_2$
Cross coupling of organomercurials with organic halides is catalyzed by palladium. This approach 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).
One remarkable feature of organomercury compounds is the resilience of the C-Hg bond. This property is illustrated by the preparation of 4-chloromercuribenzoic acid by oxidation of 4-chloromercuritoluene using potassium permanganate.
Organomercury halides react with hydride sources to give organomercury hydrides. Exceptionally, those compounds have a weak C–Hg bond, and readily cleave to alkyl radicals.

## Applications

The toxicity of organomercury compounds notwithstanding, organomercury compounds have often proved useful catalysts.

### Hydration and related reactions of acetylene

Several Hg-catalyzed conversions of acetylene have been commercialized by Hoechst AG, BASF, and Chisso. Acetaldehyde is produced by hydration of acetylene:
    C$_2$H$_2$ + H$_2$O → CH$_3$CHO
The Hg-containing waste stream of the Chisso process led to the environmental catastrophe causing Minamata disease.
Ethylidene diacetate, a precursor to acetaldehyde, was also produced by a similar process. These routes, once dominant, have been significantly displaced by the Pd-catalyzed Wacker Process, a greener process that starts with ethylene. In general oxymercuration reactions of alkenes and alkynes using mercuric compounds proceed via organomercury intermediates. A related reaction forming phenols is the Wolffenstein–Böters reaction.

### Production of chlorocarbons

Mercury-based catalysis is woven throughout the history of chlorinated ethanes and ethylenes. Vinyl chloride is produced by the addition of HCl to acetylene using a mercury-carbon catalyst. Considerable effort is required to limit the contamination of the product with mercury.

### Medicinal

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.

### Thiol affinity chromatography

Thiols are also known as mercaptans due to their propensity for mercury capture. Thiolates (R-S$^−$) and thioketones (R$_2$C=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.