General Description
Mercury dimethyl is a toxic environmental pollutant. It is found in polluted bottom sediments and in the bodies of fishes and birds. In the bodies of fishes and birds it occurs along with monomethyl mercury. The latter, as CH3Hg+ ion, is formed by microorganism-induced biological methylation of elemental mercury or agricultural fungicide mercury compounds that are discharged into the environment.
Mercury dimethyl is used in inorganic synthesis; and as a reference standard for Hg-NMR.
Reaction
Mercury dimethyl, unlike zinc dimethyl, is fairly stable at ordinary temperatures, and is not attacked by air or water.
Mercury dimethyl undergoes single replacement reactions with several metals such as alkali and alkaline earth metals, zinc, aluminum, tin, lead and bismuth forming their corresponding dialkyls.
Such reaction is a synthetic route to prepare many organometallic compounds. Thus, reaction with metallic zinc yields zinc dimethyl:
(CH3)2Hg + Zn → (CH3)2Zn + Hg
Toxicity
Mercury dimethyl is a highly toxic substance by all routes of exposure. Several cases of human poisoning are well documented. (Patnaik, P. 1999. A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 2nd ed. p. 574, New York: John Wiley & Sons.) The compound can accumulate in the brain and blood of humans. Intake of small quantities can cause death.
Description
The first indication of the extreme toxicity of dimethylmercury
(DMM) was documented in 1863 when two laboratory assistants
died of DMM poisoning while synthesizing DMM in the
laboratory of Frankland and Duppa. There are numerous reports
of people dying from alkyl mercury compounds including
a chemist who was preparing several thousand grams ofDMMin
his laboratory in 1974. The extreme toxicity was revisited in
1997, when Karen Wetterhahn, an internationally renowned
researcher of the carcinogenic effects of heavy metals on DNA
repair proteins, died within a few months after a single exposure
of less than a milliliter of DMM on her latex-covered hand.
DMM is extremely toxic and lethal at a dose of approximately
400 mg of mercury (equivalent to a few drops) or about
5mgkg-1 of body weight or as little as 0.1 ml
Chemical Properties
Dimethyl mercury is a volatile colorless liquid
with faint sweet odor.
Chemical Properties
colourless liquid
Uses
DMM has limited use because of its toxicity but can be used to
calibrate research equipment, as in its application as a standard
reference material for 199Hg NMR measurements.
Uses
Dimethylmercury is used as a reagent ininorganic synthesis, and as a reference standardfor mercury nuclear magnetic resonance(Hg NMR). It is an environmental pollutantfound in bottom sediments and also inthe bodies of birds and marine mammalssuch as whales and fishes. It occurs in fishesand birds along with monomethylmercury. Inhumans, its presence is attributed to the consumptionof pilot whale meat, cod fish, andother sea food.
Uses
As inorganic reagent.
Definition
ChEBI: Dimethylmercury is a methylmercury compound.
Health Hazard
All alkylmercury compounds are highly toxicby all routes of exposure. There are manyserious cases of human poisoning frommethylmercury (Lu 2003). Outbreaks ofmass poisoning from consumption of contaminatedfish occurred in Japan during the1950s, causing a severe neurological disease,so-called “Minamata disease,” whichresulted in hundreds of deaths. A similaroutbreak of food poisoning from contaminatedwheat caused several hundred deathsin Iraq in 1972. A tragic death from a singleacute transdermal exposure to dimethylmercury(estimated between 0.1 to 0.5 mL) thatpenetrated into the skin through disposablelatex gloves has occurred (Blayney et al.1997; The New York Times, June 11, 1997).The symptoms reported were episodes ofnausea and vomiting occurring three monthsafter the exposure followed by onset ofataxia, slurred speech (dysarthia), and loss ofvision and hearing 2 months after that. Thedeath occurred in about six months after theaccident.
Methylmercury can concentrate in certainfetal organs, such as the brain. Thetarget organs are the brain and the centralnervous system. It can cause death, miscarriage,and deformed fetuses. Unlike inorganicmercury compounds, it can penetrate throughthe membrane barrier of the erythrocyte,accumulating at about 10 times greater concentrationthan that in the plasma (WHO1976). Its rate of excretion on the bloodlevel is very slow. It gradually accumulatesin the blood. Such accumulation was found toreach 60% equilibrium at about 90 days, culminatedafter 270 days (Munro and Willes,1978). Skin absorption exhibits the symptomsof mercury poisoning. The toxic thresholdconcentration of mercury in the wholeblood is usually in the range 40 to 50 μg/L,while the normal range should be below10 μg/L.
Fire Hazard
It is a flammable liquid; flash point 38°C
(101°F). The flammability of this compound,
its ease of oxidation and the energy of decomposition is relatively lower than
the alkyls of lighter metals. It is mildly
endothermic. The heat of formation, △H°f
is ＋75.3 kJ/mol (Bretherick 1995). Unlike
most other metal alkyls formed by elements
of lower atomic numbers, dimethylmercury
does not pose any serious fire or explosion
hazard. Although it does not ignite in air,
the compound is easily inflammable. It dissolves
in lower alcohols without any violent
decomposition. Heating with oxidizing substances
can cause explosion. Violent explosion
is reported with diboron tetrachloride at
－63°C (－81°F) under vacuum (Wartik et al.
1971).
Safety Profile
Suspected carcinogen.
Highly toxic. Mutation data reported. Easily
flammable. When heated to decomposition
it emits toxic fumes of Hg.
Potential Exposure
Dimethyl mercury has been
used as seed disinfectants and for fungicides. It has
also been used in organic synthesis.
Environmental Fate
DMM is a colorless liquid that is volatile at room temperature
(vapor pressure 62.3 mmHg) and is slightly soluble in water
(water solubility 8860 mg l
-1). There are no reports on the
partition behavior of DMM but it is known to readily evaporate
and is thus rarely found in sediment or soil. No reports were
found on the environmental persistence of DMM. While DMM
vaporizes, no studies were found on long range transport. The
lipophilicity ofDMMresults in its accumulation inadipose tissue,
plasma proteins, and brain. DMM has not been found in fish.
Shipping
UN2025 Mercury compounds, solid, n.o.s.,
Hazard Class: 6.1; Labels: 6.1-Poisonous materials,
Technical Name Required.
Toxicity evaluation
In contrast to the white crystalline solids of the pure forms of
methylmercury (MMM) and phenylmercury, DMM exists as
a colorless liquid at room temperature with high volatility.
These physical qualities enable high concentrations of the
substance to be absorbed by exposure pathways of the skin and
lungs that circumvent first-pass elimination. Effectively, this
prolongs the systemic circulation of DMM, and extends its
residence time in the body.
The additional alkyl group flanking the mercury imparts
DMM with lipophilicity that exceeds its monoalkylated
counterpart, and allows DMM to be sequestered in lipid-rich
depots. The metabolic delay allows the neurotoxicity of DMM
to remain latent for months.
The gradual conversion into MMM results in the release of
DMM from depots such as lipid-rich tissues and plasma
proteins, and permits its movement through barriers such as
the blood–brain and placenta. A cysteine complex of the
monomethylated metabolite penetrates the endothelial cells of
the blood–brain barrier by mimicking methionine and using
the large neutral amino acid transporter.
Thus, the toxicity of DMM is mediated by its dealkylation.
Cleavage of the carbon–mercury bond generates MMM
metabolites, which can form covalent bonds with cellular
ligands with amphiphilic properties. The mercury center reacts
with sulfur and sulfur-containing thiol groups of enzymes
and thereby inhibits them, resulting in cellular dysfunction.
The metal center of DMM acts as a soft acid, and binds tightly
to polarizable donor atoms in soft bases. An additional
mechanism of adverse effect is the disruption of the prooxidant–
antioxidant balance, causing oxidative damage to
biomolecules resulting cellular damage. Within cells, mercury
may interact with a variety of proteins, particularly microsomal
and mitochondrial enzymes. Recent studies demonstrated that
the combined administration of the antioxidants N-acetyl
cysteine, zinc, and selenium mitigated DMM acute and chronic
toxicity by reducing enzymatic and cellular dysfunction.
Incompatibilities
Incompatible with oxidizers (chlorates,
nitrates, peroxides, permanganates, perchlorates, chlorine,
bromine, fluorine, etc.); contact may cause fires or explosions. Keep away from alkaline materials, strong bases,
strong acids, oxoacids, epoxides. May be sensitive to light.
