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. Methanesulfonic acid is used as an acid catalyst in organic reactions because it is 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 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 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. 

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