Dimethyl sulfoxide (DMSO) is the chemical compound with the formula (CH₃)₂SO.

This colourless liquid is an important polar aprotic solvent that dissolves both polar and non-polar compounds and is miscible in a wide range of organic solvents as well as water. It has a distinctive property of penetrating the skin very readily, allowing the handler to taste it. Its taste has been described as oyster- or garlic-like.

Dimethyl sulfoxide is a by-product of wood pulping. A supplier of DMSO is the Gaylord Chemical Corporation in the USA.

DMSO is an important polar aprotic solvent. It is less toxic than other members of this class such as dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, HMPA. Because of its excellent solvating power, DMSO is frequently used as solvent for chemical reactions involving salts, most notably Finkelstein reactions and other nucleophilic substitutions. Because DMSO is only weakly acidic, it tolerates relatively strong bases, and as such has been extensively used in the study and chemistry of carbanions. The work of the Bordwell group has provided a valuable set of non-aqueous pKa values (C-H, O-H, S-H and N-H acidities) for hundreds of organic compounds in DMSO.

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