Definition
Thiols are organic compounds with the general structure R-SH. They are sulfur analogs of alcohols (R-OH). R can be aliphatic or aromatic. The simplest aliphatic representative is methanethiol, the simplest aromatic is thiophenol (analogue of phenol). Thiols are formally derived from hydrogen sulfide (H2S), in which a hydrogen atom has been replaced by a radical.
Nomenclature
The names of compounds are formed with the suffix -thiol. For some molecules and active ingredients, the obsolete suffix or prefix mercaptan is also common. The -SH group is called the sulfhydryl group. The name thiol is derived from sulfur.
Representative
Examples:
- Cysteine (amino acid)
- Ethanethiol
- Methanethiol
- Thiophenol
Properties
- Low molecular weight thiols often have an unpleasant odor, such as rotten eggs and decay.
- Skunks produce thiols.
- Due to the small difference in the electronegativity of sulfur and hydrogen, thiols are hardly polar and hardly form hydrogen bonds. Therefore, the boiling points are lower than comparable alcohols. Thiols are volatile (odor).
- For the same reason, the thiols are less soluble in water.
Thiols are stronger acids than alcohols. Their pKa is about 8 to 10:
- CH3-SH (methanethiol) + NaOH (sodium hydroxide) CH3–Na+ (methanethiolate) + H2O (water)
Reactions
Like alcohols, thiols can form esters called thioesters. Ethers with a sulfur atom are called thioethers and organic sulfides (suffix: -sulfide). Thiols can be oxidized to disulfides:
- R1-SH + R2-SH R1-S-S-R2
Proteins such as insulin and its derivatives may contain disulfide bridges between cysteines.
In pharmaceuticals
Some pharmaceutical agents are thiols, for example, the ACE inhibitor captopril or mercaptamine.