Methanesulfonic acid

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Methanesulfonic acid
Structural formula Ball-and-stick model
CAS number 75-75-2 YesY
PubChem 6395
ChemSpider 6155 YesY
UNII 12EH9M7279 YesY
EC number 200-898-6
ChEBI CHEBI:27376 YesY
Jmol-3D images Image 1
Molecular formula CH4O3S
Molar mass 96.11 g mol−1
Density 1.48 g/cm3
Melting point 17 to 19 °C (63 to 66 °F; 290 to 292 K)
Boiling point 167 °C (333 °F; 440 K) at 10 mmHg, 122 °C/1 mmHg
Acidity (pKa) -1.9[1]
EU classification Harmful (Xn), Corrosive (C)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Methanesulfonic acid is a colorless liquid with the chemical formula CH3SO3H. It is the simplest of the alkylsulfonic acids. Salts and esters of methanesulfonic acid are known as mesylates (or methanesulfonates, as in ethyl methanesulfonate). It is hygroscopic in its concentrated form. Methanesulfonic acid may be considered an intermediate compound between sulfuric acid (H2SO4), and methylsulfonylmethane ((CH3)2SO2), effectively replacing an -OH group with a -CH3 group at each step. This pattern can extend no further in either direction without breaking down the -SO2 group. Methanesulfonic acid can dissolve a wide range of metal salts, many of them in significantly higher concentrations than in hydrochloric or sulphuric acid.[2]


Methanesulfonic acid is used as an acid catalyst in organic reactions because it is a non-volatile, strong acid that is soluble in organic solvents. Methanesulfonic acid is convenient for industrial applications because it is liquid at ambient temperature, while the closely related p-toluenesulfonic acid (PTSA) is solid. However, in a laboratory setting, solid PTSA is more convenient.

Methanesulfonic acid can be used in the generation of borane (BH3) by reacting methanesulfonic acid with NaBH4 in an aprotic solvent such as THF or DMS, the complex of BH3 and the solvent is formed.[3]

Methanesulfonic acid is considered a particularly suitable supporting electrolyte for electrochemical applications, were stands as an environmentally friendly alternative to other acid electrolytes used in plating processes.[2] Methanesulfonic acid is also the electrolyte of choice in zinc-cerium (see cerium(III) methanesulfonate) and lead-acid (methanesulfonate) flow batteries.


  1. ^ Guthrie, J. P. Hydrolysis of esters of oxy acids: pKa values for strong acids. Can. J. Chem. 1978, 56, 2342-2354.
  2. ^ a b Gernon, M. D.; Wu, M.; Buszta, T.; Janney, P. (1999). "Environmental benefits of methanesulfonic acid: comparative properties and advantages". Green Chemistry 1 (3): 127–140. doi:10.1039/a900157c. 
  3. ^ Lobben, Paul C.; Leung, Simon Shun-Wang; Tummala, Srinivas (2004). "Integrated Approach to the Development and Understanding of the Borane Reduction of a Carboxylic Acid". Org. Proc. Res. Dev. 8: 1072. doi:10.1021/op049910h.