Dimethylmercury

Dimethylmercury (( C H₃)₂ Hg) is an extremely toxic organomercury compound. A highly volatile, reactive, flammable, and colorless liquid, dimethylmercury is one of the strongest known neurotoxins, with a quantity of less than 0.1 mL capable of inducing severe mercury poisoning resulting in death, and is easily absorbed through the skin. Dimethylmercury is capable of permeating many materials, including plastic and rubber compounds. It has a slightly sweet odor.

Synthesis, structure, and reactions

The compound was one of the earliest organometallics reported, reflecting its considerable stability. The compound was first prepared by George Buckton in 1857 by a reaction of methylmercury iodide with potassium cyanide:
: 2 CH₃HgI + 2 KCN → Hg(CH₃)₂ + 2 KI + (CN)₂ + Hg
Later, Frankland discovered that it could be synthesized by treating sodium amalgam with methyl halides:
: Hg + 2 Na + 2 CH₃I → Hg(CH₃)₂ + 2 NaI
It can also be obtained by alkylation of mercuric chloride with methyllithium:
: HgCl₂ + 2 LiCH₃ → Hg(CH₃)₂ + 2 LiCl
The molecule adopts a linear structure with Hg–C bond lengths of 2.083 Å.

Reactivity and physical properties

An unusual feature of this compound is its low reactivity towards proton sources, being stable in water and reacting with mineral acids at a significant rate only at elevated temperatures, whereas the corresponding organocadmium and organozinc compounds (and most metal alkyls in general) hydrolyze rapidly. The difference reflects the high electronegativity of Hg (Pauling EN = 2.00) low affinity of Hg(II) for oxygen ligands. The compound undergoes a redistribution reaction with mercuric chloride to give methylmercury chloride:
: (CH₃)₂Hg + HgCl₂ → 2 CH₃HgCl
Whereas dimethylmercury is a volatile liquid, methylmercury chloride is a crystalline solid.

Use

Dimethylmercury currently has few applications because of the risks involved. As with many methyl-organometallics, it is a methylating agent that can donate its methyl groups to an organic molecule; however, the development of less toxic nucleophiles such as dimethylzinc, trimethylaluminium, and Grignard reagents (organomagnesium halides), has essentially rendered this compound obsolete in organic chemistry. It was also studied for reactions involving bonding methylmercury cations to target molecules, forming potent bactericides, but methylmercury's bioaccumulation and ultimate toxicity has led it to be largely abandoned in favor of the less toxic ethylmercury and diethylmercury compounds, which perform a similar function without the bioaccumulation hazard.

In toxicology, it still finds limited use as a reference toxin. It is also used to calibrate NMR instruments for detection of mercury (δ 0 ppm for ¹⁹⁹Hg NMR), although diethylmercury and less toxic mercury salts are now preferred.

Safety

Dimethylmercury is extremely toxic and dangerous to handle. Absorption of doses as low as 0.1 mL can result in severe mercury poisoning. The risks are enhanced because of the compound's high vapor pressure. Medicinal chemist Derek Lowe called it "deadly and hideous" in a 2013 article.

Permeation tests showed that several types of disposable latex or polyvinyl chloride gloves (typically, about 0.1 mm thick), commonly used in most laboratories and clinical settings, had high and maximal rates of permeation by dimethylmercury within 15 seconds. The American Occupational Safety and Health Administration advises handling dimethylmercury with highly resistant laminated gloves with an additional pair of abrasion-resistant gloves worn over the laminate pair, and also recommends using a face shield and working in a fume hood.

Dimethylmercury is metabolized after several days to methylmercury. Methylmercury crosses the blood–brain barrier easily, probably owing to formation of a complex with cysteine. It is not quickly eliminated from the organism, and therefore has a tendency to bioaccumulate. The symptoms of poisoning may be delayed by months, resulting in cases in which a diagnosis is ultimately discovered, but only at a point in which it is too late or almost too late for an effective treatment regimen to be successful. Methylmercury poisoning is also known as Minamata disease.

Incidents

As early as 1865, two workers in the laboratory of Frankland died after exhibiting progressive neurological symptoms following accidental exposure to the compound.

Karen Wetterhahn, a professor of chemistry at Dartmouth College, died in 1997, ten months after spilling only a few drops of dimethylmercury onto her latex gloves. This incident resulted in improved safety procedures for chemical-protection clothing and fume hood use.

On 15 July 2011, a German man was stabbed with an umbrella in the city of Hanover, Germany. The man, who died a year later, had managed to take the syringe from the umbrella, which was later analyzed to contain dimethylmercury; the reported cause of death was mercury poisoning.

See also

* Diethylmercury
* Mercury poisoning
* Minamata disease
* Methylmercury

References

External links

ATSDR – ToxFAQs: Mercury

ATSDR – Public Health Statement: Mercury

ATSDR – MMG: Mercury

ATSDR – Toxicological Profile: Mercury


* ttps://www.chemistryworld.com/opinion/the-dangers-of-dimethylmercury-/3010064.article Chemistryworld – Death of Dr. Wetterhahn and resultant improvements in security practices

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Neurotoxins
Organomercury compounds
Methyl complexes