Question

What are the products obtained by oxidation of (a) thiols and (b) thioethers, with mild oxidizing agents?

Short answer

Thiols oxidize to disulfides; thioethers oxidize to sulfoxides under mild conditions.

Step-by-step solution

Understanding Thiols

Thiols are compounds containing a sulfhydryl group (-SH). When thiols are oxidized, they typically form disulfides as products. This is a common reaction under mild oxidation conditions.

Oxidation of Thiols

When thiols (R-SH) are subjected to mild oxidation, they link together to form disulfides (R-S-S-R). For instance, the oxidation of two molecules of methanethiol (CH₃-SH) would form dimethyl disulfide (CH₃-S-S-CH₃).

Understanding Thioethers

Thioethers, also known as sulfides, are compounds where a sulfur atom is bonded to two hydrocarbon groups (R-S-R'). Under mild oxidizing conditions, thioethers can be oxidized to form sulfoxides.

Oxidation of Thioethers

When thioethers (R-S-R') undergo mild oxidation, they can form sulfoxides (R-S(=O)-R'). For example, the oxidation of dimethyl sulfide (CH₃-S-CH₃) can lead to the formation of dimethyl sulfoxide (CH₃-S(O)-CH₃).

Key concepts

Thiols

Thiols are organic compounds characterized by the presence of a sulfhydryl group (-SH), analogous to alcohols in structure but with sulfur replacing the oxygen. They are often referred to as mercaptans and are known for their strong and often unpleasant smells.
Interestingly, thiols are fundamental in biochemistry, playing crucial roles in enzyme function due to their ability to form disulfide bonds. These bonds are essential in determining the structure and function of proteins.

• Thiols are relatively reactive, largely due to the presence of the sulfur atom.
• The sulfur atom in thiols is less electronegative than oxygen, which makes thiols more nucleophilic and prone to oxidation.

Understanding the basic nature of thiols helps in predicting their behavior during chemical reactions such as oxidation.

Thioethers

Thioethers, also known as sulfides, contain a sulfur atom bonded to two organic groups (R-S-R'). These differ from ethers only by the replacement of the oxygen atom with sulfur, adding unique chemical properties.
Due to the presence of sulfur, thioethers display distinct physical and chemical characteristics compared to their oxygen counterparts, known as ethers.

• Thioethers are less polar than alcohols and ethers.
• They generally have higher boiling points compared to ethers of similar molecular weight, due to the larger atomic mass of sulfur.

Thioethers can be oxidized under mild conditions to form specific products like sulfoxides, highlighting their versatility in organic synthesis.

Oxidation

Oxidation in organic chemistry refers to the process where a molecule loses electrons, typically involving the gain of oxygen or loss of hydrogen atoms.
For thiols and thioethers, oxidation under mild conditions involves specific changes in their molecular structure.

• Thiols are oxidized to form disulfides, connecting two sulfur atoms from separate molecules.
• Conversely, thioethers undergo oxidation to produce sulfoxides, introducing a sulfanyl group (=O) to the sulfur atom.

These transformations are crucial in organic synthesis and practical applications, such as the stabilization of pharmaceutical compounds.

Disulfides

Disulfides are the result of the oxidation of thiols, characterized by a linkage between two sulfur atoms (\[ R-S-S-R \]). This bond formation is an important protective mechanism for thiols, often stabilizing their structure.
Disulfide bonds are crucial in biological systems, particularly in protein structures, where they provide stability and influence protein folding.

• They are less reactive than thiols, as the sulfur atoms are already oxidized.
• The formation of disulfide bonds is reversible, which is utilized in biochemistry to modulate protein activities.

In chemical manufacturing, disulfides indicate the mild oxidation of thiols and represent stable end-products under controlled conditions.

Sulfoxides

Sulfoxides are produced by the oxidation of thioethers. The sulfur atom in sulfoxides has a double bond with oxygen (\[ R-S(=O)-R' \]), which contributes to its unique properties.
Sulfoxides are polar molecules, often used as solvents in chemical reactions due to their ability to dissolve a wide range of substances.

• Dimethyl sulfoxide (DMSO) is a well-known example, frequently used in laboratories for its excellent solvency properties.
• Sulfoxides' oxygen-sulfur double bond facilitates their participation in further chemical reactions, often leading to the formation of sulfones.

Sulfoxides exemplify the intermediate oxidation state of sulfur in organic compounds, bridging the chemical properties of thioethers and sulfones.