DMSO, or dimethyl sulfoxide, is a small organosulfur compound with the molecular formula C₂H₆OS, written more descriptively as (CH₃)₂SO. In organic chemistry, it serves primarily as a polar aprotic solvent that dramatically accelerates certain reactions, but it also plays active chemical roles as an oxidant and as a precursor to reactive intermediates. Its combination of high polarity, low toxicity, and ability to dissolve both organic and ionic compounds makes it one of the most versatile solvents in the organic chemist’s toolkit.
Structure and Physical Properties
DMSO consists of a central sulfur atom bonded to two methyl groups and one oxygen atom. The sulfur-oxygen bond has significant polar character, giving the molecule a large dipole moment. This polarity is reflected in its dielectric constant of 46.7, one of the highest among common organic solvents. The geometry around sulfur is roughly pyramidal because sulfur retains a lone pair of electrons, which matters for both its solvent behavior and its chemistry as a ligand.
DMSO is a colorless liquid at room temperature with a melting point of about 19 °C, meaning it can freeze on a cool lab bench. It boils at 189 °C, which makes it useful for high-temperature reactions but a headache to remove afterward. It mixes completely with water and with most organic solvents, and it dissolves a remarkably wide range of salts, reagents, and organic substrates.
Why It Matters as a Polar Aprotic Solvent
The classification “polar aprotic” captures the two features that define DMSO’s behavior in reactions. “Polar” means it stabilizes charged species through strong electrostatic interactions. “Aprotic” means it has no O-H or N-H bonds, so pure DMSO cannot donate hydrogen bonds the way water or alcohols can. It can only accept hydrogen bonds through its oxygen atom.
This distinction has enormous consequences for nucleophilic substitution reactions. In a protic solvent like methanol or water, nucleophilic anions (chloride, acetate, cyanide) become surrounded by a tight shell of hydrogen bonds from the solvent. That shell acts like a cage, lowering the anion’s energy and making it less reactive. DMSO solvates cations aggressively through its electronegative oxygen, but it interacts only weakly with anions. The result is that anions in DMSO are relatively “naked” and far more nucleophilic than they would be in water or an alcohol.
This effect produces dramatic rate increases for SN2 reactions. Switching from a protic solvent to DMSO can accelerate a bimolecular substitution by several orders of magnitude. The practical takeaway for synthetic chemists is straightforward: if an SN2 reaction is sluggish in ethanol, running it in DMSO often solves the problem. The same logic applies to other reactions that depend on strong nucleophiles or basic anions, including Williamson ether synthesis and alkylation of enolates.
DMSO as a Reagent: The Swern Oxidation
Beyond its role as a solvent, DMSO participates directly as a chemical reagent in several important transformations. The most famous is the Swern oxidation, which converts primary and secondary alcohols to aldehydes and ketones under extremely mild conditions.
In the Swern oxidation, DMSO is first activated by oxalyl chloride at around −78 °C. This activation produces a highly electrophilic sulfur species along with carbon monoxide and carbon dioxide as gaseous byproducts. The alcohol then attacks this activated intermediate, forming a new bond between the alcohol oxygen and sulfur. Treatment with a base (triethylamine) strips a proton from the carbon adjacent to oxygen, causing the sulfur to leave as dimethyl sulfide and generating the carbonyl product.
The Swern oxidation is prized because it avoids heavy metal oxidants like chromium, works at low temperatures that protect sensitive functional groups, and reliably stops at the aldehyde stage without over-oxidizing to a carboxylic acid. The chief drawback is the foul smell of the dimethyl sulfide byproduct.
The Corey-Chaykovsky Reaction
DMSO also connects to the Corey-Chaykovsky reaction, introduced in 1960, which builds three-membered rings: epoxides from aldehydes and ketones, cyclopropanes from enones, and aziridines from imines. The key reagent is a sulfonium or sulfoxonium ylide generated by deprotonating a trimethylsulfoxonium salt with a strong base like sodium hydride, often in DMSO as the solvent.
The ylide acts as a one-carbon donor. It adds to the electrophilic carbonyl carbon, and the resulting intermediate closes to form a small ring as the sulfur-containing group departs. Published syntheses report yields as high as 90–96% for epoxide and cyclopropane formation under optimized conditions. This reaction is a workhorse in natural product synthesis, where building strained three-membered rings with precise stereochemistry is frequently required.
DMSO as a Ligand in Metal Chemistry
DMSO is an ambidentate ligand, meaning it can bind to a metal through either its sulfur atom or its oxygen atom. Which coordination mode it adopts depends on the metal. Soft metals like ruthenium(II), osmium(II), rhodium(I), and platinum(II) generally prefer binding through sulfur. Harder or more electropositive metals tend to bind through oxygen. Palladium(II) sits in an interesting middle ground: its complexes have been characterized with both S-bound and O-bound DMSO, and some palladium species contain both types simultaneously. In the complex [Pd(DMSO)₄]²⁺, for instance, two DMSO ligands bind through sulfur and two through oxygen. Oxygen-bound DMSO ligands are more easily displaced, making them kinetically labile and useful in catalytic cycles where an open coordination site needs to be generated quickly.
Practical Challenges in the Lab
DMSO’s high boiling point (189 °C) makes it notoriously difficult to remove from reaction mixtures. You cannot simply evaporate it on a rotary evaporator under normal conditions. The standard approach is to dilute the reaction mixture with a large volume of water, then extract the product into a nonpolar solvent like ethyl acetate or dichloromethane. A useful rule of thumb: for every 5 mL of DMSO, wash the organic layer with at least five 10 mL portions of water to strip it out completely.
Keeping DMSO dry presents its own challenges. Its high dielectric constant makes it strongly hygroscopic, readily absorbing moisture from the air. Even small amounts of water can reduce the nucleophilicity-boosting effects that make DMSO valuable in the first place. Research published in the Journal of Organic Chemistry found that the best drying method is fractional distillation (discarding the first 20% of distillate) followed by storage over 4Å molecular sieves. Basic desiccants like calcium hydride work but can trigger unwanted exchange reactions with DMSO’s slightly acidic methyl protons, complicating the drying process.
Safety Considerations
DMSO penetrates intact human skin rapidly and carries dissolved substances with it into the bloodstream. This transdermal penetration is its most significant safety concern. A spill containing DMSO mixed with a toxic reagent can deliver that toxin directly through your skin before you notice. Gloves rated for DMSO exposure are essential, and nitrile gloves alone may not provide adequate protection for prolonged contact. A characteristic sign of skin exposure is a garlic-like taste in the mouth within minutes, caused by DMSO metabolites reaching the tongue through the bloodstream. This property demands careful attention to what else is dissolved in the DMSO you are handling.