Dimethyl Sulfoxide (DMSO) is an organosulfur compound represented by the chemical formula (CH3)2SO. This molecule is one of the most widely used sulfoxides in both industrial and laboratory settings. Its function in a reaction is not singular; it can act as a simple medium, a dedicated reactant, or a protective agent in biological systems.
The Unique Properties of DMSO as a Solvent
The primary function of DMSO in a chemical reaction is often to serve as the solvent, or reaction medium, a role for which its chemical structure is ideally suited. The molecule possesses a sulfoxide functional group, featuring a double bond between sulfur and oxygen, which results in a highly polarized structure. This polarity arises from the oxygen atom drawing electron density away from the sulfur atom, creating a partial negative charge on the oxygen and a partial positive charge on the sulfur.
This strong dipole moment allows DMSO to dissolve an exceptionally broad range of compounds, including both highly polar substances and many nonpolar organic molecules. The solvent is completely miscible with water, yet it also dissolves many organic salts and polymers that are insoluble in common solvents like acetone or ethanol.
DMSO is classified as a polar aprotic solvent, meaning that while it is highly polar, it lacks the O-H or N-H bonds necessary to donate a hydrogen atom for hydrogen bonding. Protic solvents, such as water or methanol, tend to form strong hydrogen bonds with anionic reactants, effectively encasing them and reducing their reactivity.
In contrast, DMSO solvates the positive ions (cations) very well but leaves the negative ions (anions) relatively exposed and highly reactive. This phenomenon significantly increases the reaction rate for processes like nucleophilic substitution, where an anion attacks another molecule. This ensures that the core reaction proceeds much faster and with higher yields than in a protic medium.
DMSO’s Role as a Reactive Agent
While DMSO is most frequently used as an inert solvent, it can be purposefully consumed as a reactant under specific conditions. When activated by a co-reagent, DMSO acts as a mild oxidizing agent, capable of converting alcohols into their corresponding carbonyl compounds, such as aldehydes and ketones. This chemical transformation is valuable because it avoids the use of toxic heavy-metal oxidants traditionally employed in organic synthesis.
The mechanism for this reactivity involves the sulfur atom of DMSO being susceptible to electrophilic attack by an activating agent. This initial step forms a highly reactive intermediate, often a sulfonium salt, which then reacts with the alcohol being oxidized. The sulfur atom in the DMSO molecule functions as the site of chemical action, and the reaction ultimately reduces the DMSO to dimethyl sulfide as a byproduct.
The Swern Oxidation is a widely used example of DMSO’s function as an oxidant. This reaction is typically performed at low temperatures, such as \(-78^{\circ}\text{C}\), using oxalyl chloride as the primary activating agent. This process is favored in the synthesis of complex molecules because it is selective and operates under mild conditions, preserving sensitive functional groups elsewhere in the molecule.
Function in Cellular Protection and Penetration
Beyond its uses in pure chemistry, DMSO has unique physical interactions with biological materials, allowing it to serve as a cellular protective agent and a transport mechanism. The molecule’s small size and amphipathic nature—being both polar and nonpolar in character—allow it to readily pass through biological membranes, including cell walls and the skin. This membrane-penetrating property makes it highly effective as a vehicle to deliver pharmaceutical agents into tissues when applied topically.
One of the most recognized biological applications is its use as a cryoprotectant for the long-term preservation of cells, tissues, and organs. During the process of freezing, water inside and outside the cell can form sharp ice crystals that puncture the cell membrane, leading to irreversible damage. DMSO mitigates this damage by penetrating the cell and lowering the freezing point of the intracellular water.
By increasing the concentration of solutes inside the cell, DMSO limits the formation and growth of damaging ice crystals. Furthermore, it promotes vitrification, which is the transformation of the water into a glassy, non-crystalline solid state, reducing the physical stress on cellular structures. Used at concentrations typically between 5% and 10% in freezing media, DMSO ensures high cell viability after thawing, which is a foundational practice in biobanking and cell-based therapies.