Overview. A thiol is an organic compound that contains the functional group R–SH; the R stands for any alkyl or aryl fragment and the –SH is the sulfhydryl or thiol group. For a concise definition see general definition, and the structural motif is often written as R–SH. Thiols are commonly called mercaptans in industry and are formally related to alcohols by replacement of the oxygen atom in an alcohol (compare oxygen) with a sulfur atom.

Physical and chemical characteristics

Many low‑molecular‑weight thiols have strong, often unpleasant odors; this sensory property makes some of them useful as warning agents. Compared with alcohols, thiols are generally more acidic (their S–H protons are easier to remove) and are more easily oxidized. Typical reactions include oxidation to disulfides and deprotonation to give thiolate anions.

  • Odor: powerful and distinctive in many simple thiols.
  • Acidity: more acidic than alcohols; bases can remove the proton to form a thiolate (base).
  • Redox behavior: readily oxidized to disulfides or higher oxidation states (oxidation).
  • Nucleophilicity: thiolates are good nucleophiles and soft ligands (nucleophile).

Chemistry and common reactions

When deprotonated, a thiol becomes a thiolate anion that participates in many reactions. Thiolates perform nucleophilic attacks in substitution processes (nucleophilic substitution), add to activated double bonds, and coordinate to transition metals. Two thiol groups can be oxidized to form a disulfide bond (R–S–S–R), an important reaction in both laboratory and biological contexts.

Uses and typical examples

One practical application of volatile thiols is as odorants: small amounts are intentionally added to natural gas supplies so leaks can be detected by smell because pure methane is essentially odorless. Thiols are also employed in organic synthesis, polymer and rubber chemistry, and as ligands in coordination chemistry.

Biological significance

Thiols appear in biochemistry principally as the side chain of the amino acid cysteine and more broadly in many enzymes and cofactors. Cysteine residues can form disulfide bridges that stabilize protein structure, participate in catalytic mechanisms, and act as redox sensors. For background on amino acids see amino acids.

Safety, handling and notable distinctions

Because many thiols are malodorous at very low concentrations, small leaks are noticeable, but the smell does not imply low toxicity. Thiols can be flammable and should be handled with good ventilation and appropriate protective equipment. Chemically, they differ from alcohols in acidity, nucleophilicity and redox behavior, and they form characteristic sulfur-containing derivatives used across chemistry and biology.

Further reading and technical references: definitions and structural notes are available at R–SH functional group and comparative discussions at alcohols vs thiols. For applied examples see resources on gas odorization and basic organic reaction descriptions such as nucleophilic substitution and oxidation.

Additional quick links: chemical identity (definition), atom comparison (oxygen vs sulfur), reagent roles (bases, nucleophiles), and biological context (cysteine, amino acids).