DMSO is not a strong nucleophile. It is primarily classified as a polar aprotic solvent, and its main importance in organic chemistry is how it makes other nucleophiles stronger, not how it acts as one itself. That said, DMSO does have weak nucleophilic character, and under certain conditions it can participate in reactions as a nucleophile, sometimes causing unwanted side reactions.
Why DMSO Is Classified as a Solvent, Not a Nucleophile
DMSO (dimethyl sulfoxide) belongs to a family of solvents called dipolar aprotic solvents, alongside DMF (dimethylformamide), acetone, and acetonitrile. These solvents share a key trait: they have high polarity (DMSO’s dielectric constant is 47.2, remarkably high for an aprotic solvent) but cannot donate hydrogen bonds the way water or alcohols can. DMSO also has a large dipole moment of 3.96 D, which makes it excellent at dissolving salts and stabilizing cations.
Its primary role in synthesis is as a reaction medium. When chemists choose DMSO as a solvent, they’re doing so because of what it does to the nucleophiles dissolved in it, not because DMSO itself attacks anything.
How DMSO Makes Other Nucleophiles Stronger
This is the concept most organic chemistry courses are really getting at when DMSO comes up. In a protic solvent like ethanol or water, nucleophiles get surrounded by a cage of hydrogen bonds. These hydrogen bonds stabilize the nucleophile and lower its energy, which means it’s less eager to react. The nucleophile is effectively “held back” by the solvent molecules clinging to it.
DMSO can’t donate hydrogen bonds. So when you dissolve a nucleophile like chloride or fluoride in DMSO, those ions are poorly solvated. They’re essentially “naked” and far more reactive. This is why SN2 reactions run dramatically faster in DMSO than in ethanol. The nucleophile hasn’t changed, but the solvent is no longer getting in its way. DMSO also stabilizes the positive counterion (like sodium or potassium), which helps keep the salt dissolved while leaving the nucleophilic anion free to attack.
One striking consequence: the nucleophilicity order of halides actually reverses when you switch from a protic solvent to DMSO. In water, iodide is the best nucleophile among the halides because it’s large and easily sheds its solvation shell. In DMSO, fluoride becomes far more reactive because it’s no longer buried under hydrogen bonds. Research at the University of Houston established the nucleophilicity order in DMSO as: thiophenoxide > hydroxide > methoxide > fluoride > phenoxide > azide > chloride > bromide > iodide.
DMSO’s Own Nucleophilic Character
Although DMSO is a weak nucleophile compared to common reagents, it does have lone pairs available for donation. The molecule is best described as a resonance hybrid between two forms: one with a true S=O double bond and one with a charge-separated S⁺–O⁻ single bond. This means both the sulfur atom (which carries a soft lone pair) and the oxygen atom (which carries partial negative charge) can potentially act as nucleophilic sites.
In practice, DMSO can attack electrophilic carbon through either atom. The sulfur end can displace a leaving group to form S-alkyl sulfonium salts, while the oxygen end can attack to form O-alkyl derivatives. Which atom reacts depends on the hardness or softness of the electrophile and the reaction conditions.
Reactions Where DMSO Acts as a Nucleophile
The most well-known example is the Kornblum oxidation, first reported in 1959. In this reaction, a primary tosylate is heated to 150°C, and the oxygen of DMSO performs an SN2 displacement on the carbon bearing the leaving group. The result, after a subsequent elimination step, is an aldehyde. Here, DMSO is deliberately used as both the solvent and the nucleophilic oxidant.
Several other named oxidation reactions (Swern, Pfitzner-Moffatt) similarly exploit DMSO’s ability to act as a mild oxygen nucleophile to convert alcohols into aldehydes or ketones. In each case, DMSO must first be activated by another reagent because on its own it’s too weak a nucleophile to react with most substrates at room temperature.
When DMSO Causes Side Reactions
The flip side of DMSO’s weak nucleophilicity is that it can sometimes interfere with the reaction you actually want. Researchers have observed decomposition of both substrates (like alkyl tosylates) and products (like alkyl halides) when reactions are run in DMSO, particularly at elevated temperatures or with highly reactive electrophiles. The sulfur end of DMSO displaces the leaving group to form sulfonium byproducts, or the oxygen end attacks to form unwanted ethers.
This is generally a concern only with very reactive electrophiles (primary alkyl halides with good leaving groups, for example) or at high temperatures. For most standard SN2 reactions at moderate temperatures, DMSO behaves as an innocent bystander. But if you’re seeing unexpected byproducts in a DMSO-based reaction, the solvent itself may be competing as a nucleophile.
DMSO Compared to True Strong Nucleophiles
To put DMSO’s nucleophilicity in perspective, consider the nucleophilicity order measured in DMSO itself. Thiophenoxide, hydroxide, methoxide, and even fluoride all rank well above DMSO as nucleophiles. Common strong nucleophiles like cyanide, thiolates, and iodide (in protic solvents) react orders of magnitude faster with electrophilic carbon than DMSO does.
DMSO is best thought of as a solvent that happens to have borderline nucleophilic properties. It needs activation or forcing conditions (high temperature, highly reactive substrates) to participate in nucleophilic reactions. Under normal synthetic conditions, its role is to boost the reactivity of whatever actual nucleophile you’ve added to the flask.