Dimethyl sulfate is a chemical compound with formula (CH3O)2SO2. As the diester of methanol and sulfuric acid, its formula is often written as (CH3)2SO4 or even Me2SO4, where CH3 or Me is methyl. Me2SO4 is mainly used as a methylating agent in organic synthesis.
Sulfuric acid dimethyl ester; Me2SO4; DMSO4; Dimethyl ester of sulfuric acid, Methyl sulfate
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||126.13 g/mol|
|Appearance||Colorless, oily liquid|
|Density||1.33 g/ml, liquid|
|Melting point||−32 °C (−26 °F; 241 K)|
|Boiling point||188 °C (370 °F; 461 K) (decomposes)|
|Solubility||Methanol, dichloromethane, acetone|
|Vapor pressure||0.1 mmHg (20°C)|
|Main hazards||Extremely toxic, contact hazard, inhalation hazard, corrosive, environmental hazard, carcinogenic, mutagenic|
|R-phrases (outdated)||R45, R25, R26, R34,|
|S-phrases (outdated)||S53, S45, S30, S60, S61|
|NFPA 704 (fire diamond)|
|Flash point||83 °C; 182 °F; 356 K|
|Lethal dose or concentration (LD, LC):|
LC50 (median concentration)
|8.6 ppm (rat, 4 hr)|
75 ppm (guinea pig, 20 min)
53 ppm (mouse)
32 ppm (guinea pig, 1 hr)
LCLo (lowest published)
|97 ppm (human, 10 min)|
|NIOSH (US health exposure limits):|
|TWA 1 ppm (5 mg/m3) [skin]|
|Ca TWA 0.1 ppm (0.5 mg/m3) [skin]|
IDLH (Immediate danger)
|Ca [7 ppm]|
|Diethyl sulfate, methyl triflate, dimethyl carbonate|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Me2SO4 is a colourless oily liquid with a slight onion-like odour (although smelling it would represent significant exposure). Like all strong alkylating agents, Me2SO4 is extremely toxic. Its use as a laboratory reagent has been superseded to some extent by methyl triflate, CF3SO3CH3, the methyl ester of trifluoromethanesulfonic acid.
- 2 CH3OH + H2SO4 → (CH3)2SO4 + 2 H2O
- 2 CH3HSO4 → H2SO4 + (CH3)2SO4
Methyl nitrite and methyl chlorosulfonate also result in dimethyl sulfate:
- CH3ONO + (CH3)OSO2Cl → (CH3)2SO4 + NOCl
- (CH3)2O + SO3 → (CH3)2SO4
Dimethyl sulfate is best known as a reagent for the methylation of phenols, amines, and thiols. One methyl group is transferred more quickly than the second. Methyl transfer is assumed to occur via an SN2 reaction. Compared to other methylating agents, dimethyl sulfate is preferred by the industry because of its low cost and high reactivity.
Methylation at oxygen
- 2 (CH3)3COH + (CH3O)2SO2 → 2 (CH3)3COCH3 + H2SO4
- RO− Na+ + (CH3O)2SO2 → ROCH3 + Na(CH3)SO4
The methylation of sugars is called Haworth methylation.
Methylation at amine nitrogen
- C6H5CH=NC4H9 + (CH3O)2SO2 → C6H5CH=N+(CH3)C4H9 + CH3OSO3−
- CH3(C6H4)NH2 + (CH3O)2SO2 (in NaHCO3 aq.) → CH3(C6H4)N(CH3)2 + Na(CH3)SO4
Methylation at sulfur
- RS−Na+ + (CH3O)2SO2 → RSCH3 + Na(CH3)SO4
- p-CH3C6H4SO2Na + (CH3O)2SO2 → p-CH3C6H4SO2CH3 + Na(CH3)SO4
This method has been used to prepare thioesters:
- RC(O)SH + (CH3O)2SO2 → RC(O)S(CH3) + HOSO3CH3
Reactions with nucleic acids
Dimethyl sulfate (DMS) is used to determine the secondary structure of RNA. At neutral pH, DMS methylates unpaired adenine and cytosine residues at their canonical Watson-Crick faces, but it cannot methylate base-paired nucleotides. Using the method known as DMS-MaPseq, RNA is incubated with DMS to methylate unpaired bases. Then the RNA is reverse-transcribed; the reverse transcriptase frequently adds an incorrect DNA base when it encounters a methylated RNA base. These mutations can be detected via sequencing, and the RNA is inferred to be single-stranded at bases with above-background mutation rates.
Dimethyl sulfate can effect the base-specific cleavage of DNA by attacking the imidazole rings present in guanine. Dimethyl sulfate also methylates adenine in single-stranded portions of DNA (e.g., those with proteins like RNA polymerase progressively melting and re-annealing the DNA). Upon re-annealing, these methyl groups interfere with adenine-guanine base-pairing. Nuclease S1 can then be used to cut the DNA in single-stranded regions (anywhere with a methylated adenine). This is an important technique for analyzing protein-DNA interactions.
Although dimethyl sulfate is highly effective and affordable, its toxicity has encouraged the use of other methylating reagents. Methyl iodide is a reagent used for O-methylation, like dimethyl sulfate, but is less hazardous and more expensive. Dimethyl carbonate, which is less reactive, has far lower toxicity compared to both dimethyl sulfate and methyl iodide. High pressure can be used to accelerate methylation by dimethyl carbonate. In general, the toxicity of methylating agents is correlated with their efficiency as methyl transfer reagents.
Dimethyl sulfate is carcinogenic and mutagenic, highly poisonous, corrosive, and environmentally hazardous. Dimethyl sulfate is absorbed through the skin, mucous membranes, and gastrointestinal tract, and can cause a fatal delayed respiratory tract reaction. An ocular reaction is also common. There is no strong odor or immediate irritation to warn of lethal concentration in the air. The LD50 (acute, oral) is 205 mg/kg (rat) and 140 mg/kg (mouse), and LC50 (acute) is 45 ppm / 4 hours (rat). The vapor pressure of 65 Pa is sufficiently large to produce a lethal concentration in air by evaporation at 20 °C. Delayed toxicity allows potentially fatal exposures to occur prior to development of any warning symptoms. Symptoms may be delayed 6–24 hours. Concentrated solutions of bases (ammonia, alkalis) can be used to hydrolyze minor spills and residues on contaminated equipment, but the reaction may become violent with larger amounts of dimethyl sulfate (see ICSC). Although the compound hydrolyses, treatment with water cannot be assumed to decontaminate dimethyl sulfate.
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