Sulfoxides are organic compounds with a sulfur atom bonded to one oxygen atom and two carbon-based groups. They occur naturally and are also synthesized. Their unique chemical properties lead to diverse roles in biological systems, scientific research, industrial processes, and medical applications.
The Chemistry of Sulfoxides
A sulfoxide molecule has a central sulfur atom double-bonded to one oxygen atom and singly bonded to two carbon atoms. This arrangement gives the sulfur atom a pyramidal shape, with a lone pair of electrons. This specific bonding creates a polar sulfoxide group, where the sulfur carries a partial positive charge and the oxygen a partial negative charge.
Sulfoxides can be compared with related sulfur compounds. Sulfides are less oxidized, with a sulfur atom bonded only to two carbon groups and no oxygen. Sulfones are more oxidized, with sulfur bonded to two oxygen atoms and two carbon groups. Sulfoxides therefore sit as an intermediate oxidation state between sulfides and sulfones.
Sulfoxides in Nature and Industry
Sulfoxides are found in various natural sources and are also produced synthetically for industrial use. In nature, a notable example is alliin, a sulfoxide present in fresh garlic. Alliin is a derivative of the amino acid cysteine and remains stable within intact garlic cloves. When garlic is crushed, an enzyme called alliinase converts alliin into allicin, which is responsible for garlic’s pungent aroma and recognized benefits.
In industry, dimethyl sulfoxide (DMSO) is a prominent synthetic sulfoxide. DMSO is a colorless liquid, primarily a byproduct of the kraft process in paper manufacturing, derived from wood pulp. It is also synthesized by oxidizing dimethyl sulfide. It is recognized for its solvent properties, making it valuable across industrial applications.
Applications and Uses
The unique chemical attributes of sulfoxides lead to a range of practical applications, particularly in the fields of chemistry and medicine. Dimethyl sulfoxide (DMSO) is a highly versatile polar aprotic solvent, dissolving both polar and nonpolar substances without donating protons. This property makes it a common solvent for many chemical reactions, especially those involving salts, such as nucleophilic substitutions. Industrially, DMSO is used in various processes, including as a cleaning agent, in paint stripping, and in the manufacturing of polymers.
Beyond its solvent role, DMSO can penetrate biological membranes, including human skin. This allows it to act as a carrier for transdermal delivery of pharmaceutical compounds. The U.S. Food and Drug Administration (FDA) has approved DMSO for the symptomatic relief of chronic interstitial cystitis, a painful bladder condition. While its use in other medical contexts, such as an anti-inflammatory or topical analgesic, is explored, scientific evidence supporting these broader applications remains limited.
The sulfoxide functional group is also incorporated into several pharmaceutical drugs. Modafinil, a sulfoxide-containing medication, is used as a wakefulness-promoting agent. It improves wakefulness in patients with excessive daytime sleepiness from conditions like narcolepsy, obstructive sleep apnea, or shift work sleep disorder. Another example is esomeprazole, a widely used proton-pump inhibitor. It is the S-enantiomer of omeprazole and reduces stomach acid in conditions like gastroesophageal reflux disease (GERD), peptic ulcer disease, and Zollinger-Ellison syndrome.
Health and Safety Considerations
When handling sulfoxides, particularly dimethyl sulfoxide (DMSO), health and safety considerations are important due to its unique properties. DMSO is rapidly absorbed through the skin. This means any substances dissolved in DMSO and applied to the skin can also be carried into the body, posing a risk if impurities or harmful chemicals are present. Therefore, ensuring the purity of DMSO and thoroughly cleaning skin before use is recommended.
Common side effects of DMSO exposure, particularly after topical application, include minor skin irritation, itching, or a burning sensation. Many individuals also report a distinctive garlic-like taste in their mouth or breath, a common and harmless byproduct of its metabolism. For pharmaceutical sulfoxides like esomeprazole, their safety profiles are extensively studied for their approved medical uses under professional supervision. However, like all medications, they can have potential side effects, which are typically outlined in their prescribing information.