Question

Disulfides are compounds that have $S-$ S bonds, like peroxides have $O-O$ bonds. Thiols are organic compounds that have the general formula $R-SH,$ where $R$ is a generic hydrocarbon. The SH ${}^{-}$ is the sulfur counterpart of hydroxide, OH ${}^{-}.$ Two thiols can react to make a disulfide, $\mathrm{R}-\mathrm{S}-\mathrm{S}-\mathrm{R}$ (a) What is the oxidation state of sulfur in a thiol? (b) What is the oxidation state of sulfur in a disulfide? (c) If you react two thiols to make a disulfide, are you oxidizing or reducing the thiols? (d) If you wanted to convert a disulfide to two thiols, should you add a reducing agent or oxidizing agent to the solution? (e) Suggest what happens to the H's in the thiols when they form disulfides.

Short answer

(a) The oxidation state of sulfur in a thiol is -1. (b) The oxidation state of sulfur in a disulfide is 0. (c) When reacting two thiols to make a disulfide, you are oxidizing the thiols. (d) To convert a disulfide to two thiols, you should add a reducing agent to the solution. (e) When thiols react to form disulfides, the hydrogen atoms (H's) combine to form molecular hydrogen gas (H₂), which is released in the reaction process.

Step-by-step solution

(a) Oxidation state of sulfur in a thiol

To find the oxidation state of sulfur in a thiol, we first look at its formula, which is $R-SH$. In this case, $R$ is a generic hydrocarbon and has an oxidation state of 0. Hydrogen has an oxidation state of +1, and sulfur (S) is directly bonded to it, so it has an oxidation state of -1 to balance the oxidation state of hydrogen.

(b) Oxidation state of sulfur in a disulfide

To find the oxidation state of sulfur in a disulfide, we look at its formula, which is $\mathrm{R}-\mathrm{S}-\mathrm{S}-\mathrm{R}$. The oxygen states of the two R-groups are 0. The two sulfur atoms are bonded to each other. So, each sulfur atom is in an oxidation state that gets their combined charges balanced to 0. In this case, each sulfur atom has an oxidation state of 0.

(c) Oxidizing or reducing thiols when forming a disulfide

In the formation of a disulfide, we see that the oxidation state of sulfur changes from -1 (in thiols) to 0 (in disulfide). Since the oxidation state is increasing (becoming less negative), the process is considered an oxidation, and the thiols are being oxidized.

(d) Converting a disulfide to two thiols - Reducing or oxidizing agent

To convert a disulfide ($\mathrm{R}-\mathrm{S}-\mathrm{S}-\mathrm{R}$) to two thiols ($2\times \mathrm{R}-\mathrm{S}-\mathrm{H}$), we must reverse the oxidation process we saw in part (c) and change the oxidation state of sulfur from 0 to -1. This process requires a reducing agent.

(e) What happens to the H's in the thiols during disulfide formation

When two thiols ($2\times \mathrm{R}-\mathrm{S}-\mathrm{H}$) react to form a disulfide ($\mathrm{R}-\mathrm{S}-\mathrm{S}-\mathrm{R}$), the two hydrogen atoms are removed from the sulfur atoms. These hydrogen atoms can combine to form molecular hydrogen gas (${\mathrm{H}}_2$), which is then released in the reaction process.

Key concepts

Understanding Disulfides

Disulfides are fascinating sulfur compounds marked by their unique sulfur-sulfur bonds, symbolized as $S-S$. Imagine two sulfur atoms connecting like best friends holding hands. These bonds are analogous to the well-known oxygen-oxygen bonds seen in peroxides. Disulfides can be seen in structures like proteins, where they play a crucial role in maintaining the protein's shape.

In terms of their chemical structure, disulfides are often formed through the reaction of two thiols. This forms this stable $R-S-S-R$ bond. Here, "R" represents a hydrocarbon group that can vary and make each disulfide unique. Once formed, disulfides are relatively stable, making them less reactive and more challenging to break without specific conditions like the presence of a reducing agent.

Exploring Thiols

Thiols, also known as mercaptans, are sulfur-containing organic compounds with the functional group $R-SH$. Picture thiols as the sulfur equivalents of alcohols, but instead of the typical $OH$ group, they have an $SH$ group. This thiol group gives them distinct properties, often with a strong odor.

In these compounds, sulfur has an oxidation state of -1. This is due to the sulfur atom being bonded to hydrogen, which has an oxidation state of +1. The hydrocarbon part, $R$, typically contributes an oxidation state of 0, which keeps everything balanced. Despite their odor, thiols are essential in many chemical reactions, such as in the forming and breaking of disulfide bonds.

• Thiols can act as nucleophiles due to the lone pairs on sulfur, making them highly reactive.
• The SH bond can dissociate in certain conditions, leading to the formation of disulfides or other compounds.

Oxidation and Reduction Reactions: The Special Role of Thiols and Disulfides

Oxidation and reduction, often termed as redox reactions, are crucial for understanding the behavior of thiols and disulfides in chemical processes. When thiols ($R-SH$) combine to form disulfides ($R-S-S-R$), an oxidation reaction occurs. Here, the oxidation state of sulfur changes from -1 in thiols to 0 in disulfides, showcasing sulfur's shift towards losing electrons.

This increase in oxidation state implies that the thiols are oxidized. In the reverse process, where disulfides are converted back to thiols, a reduction takes place. This requires the presence of a reducing agent that helps to reintroduce electrons to sulfur, reverting its oxidation state back to -1. Such redox transformations are fundamental in biological systems, like protein folding where disulfide bonds provide stability and resilience.

• Oxidation denotes an increase in oxidation state (loss of electrons).
• Reduction denotes a decrease in oxidation state (gain of electrons).
• Conversion of thiols to disulfides is an oxidation process.
• The reverse process, converting disulfides back to thiols, is a reduction.