dimethylmercury

Dimethylmercury is an extremely toxic organomercury compound with the formula ( CH₃)₂ Hg. A volatile, flammable, dense and colorless liquid, dimethylmercury is one of the strongest known neurotoxins. Less than 0.1 mL is capable of inducing severe mercury poisoning resulting in death.

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, Edward 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

Dimethylmercury is stable in water and reacts 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) and the 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 has few practical applications because of the risks involved. It has been studied for reactions involving bonding methylmercury cations to target molecules, forming potent bactericides, but methylmercury's bioaccumulation and ultimate toxicity has led to it being 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.

Around 1960, Phil Pomerantz, a man working at the Bureau of Naval Weapons, suggested that dimethylmercury be used as a fuel mix with red fuming nitric acid. This was never done, although it did lead to testing a red fuming nitric acid-unsymmetrical dimethylhydrazine rocket with elemental mercury being injected into the combustion chamber at the Naval Ordnance Test Station.

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. Since it is highly lipophilic, it absorbs through the skin and into body fat very easily and can permeate many materials, including many plastics and rubber compounds.

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 (OSHA) 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 easily absorbs into the body and 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 regiment to be successful. Methylmercury poisoning is also known as Minamata disease.

Incidents

* As early as 1865, two workers in the laboratory of Edward 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 awareness of the substance's extreme toxicity, and its ability to easily penetrate latex, compared to less porous materials such as nitrile. New OSHA material-handling guidelines were published, many institutions purged their supplies of the compound, and it became almost impossible to buy.
* Christoph Bulwin, a 40-year-old German database administrator for IG Bergbau, Chemie, Energie, claimed to have been attacked with a syringe-tipped umbrella on 15 July 2011 in Hanover, Germany. Bulwin, who died a year later from mercury poisoning, had said he confiscated the syringe, which was later found to contain dimethylmercury. According to a 2020 article in ''Forensic Science, Medicine and Pathology,'' police investigations revealed a syringe containing a typical mercury thallium compound in Bulwin's car, and mercury and thallium in thermometers at his workplace. Inconclusive antemortem and postmortem blood, urine, and tissue analysis cast doubts on the assault account. The absence of an identified assailant or motive, as well as the presence of different mercury compounds in Bulwin's car, led police to conclude that the intoxication was likely self-administered, thereby terminating the preliminary investigation. The ''Forensic Science, Medicine and Pathology'' account is contradicted by other reports, including the August 24, 2022 episode of the German flagship true crime TV program Aktenzeichen XY .. ungelöst that was co-edited by the Hannover criminal police and in which information was solicited from the public based on Bulwin's own umbrella scenario.

References

External links

ATSDR – ToxFAQs: Mercury


ATSDR – Public Health Statement: Mercury

ATSDR – MMG: Mercury

ATSDR – Toxicological Profile: Mercury

{{Use dmy dates, date=January 2021

Neurotoxins
Organomercury compounds
Methyl complexes