Thioalkohols

Thioalkohols, commonly called thiols or mercaptans, are organic compounds that contain a sulfhydryl group (-SH). They are the sulfur analogues of alcohols, with the general structure R-SH, where R is an alkyl or aryl group. Thiols are categorized as primary, secondary, or tertiary based on the degree of substitution at the carbon attached to the sulfur atom.

Nomenclature and examples: IUPAC names convert the -ol suffix of alcohols to -thiol (for example, methane to methanethiol, ethane to ethanethiol). Common names use the alkyl mercaptan form (methyl mercaptan, ethyl mercaptan). Thiols range from simple, low-molecular-weight compounds to complex, odoriferous molecules in natural products.

Properties: Many thiols have strong, often unpleasant odors, even at trace levels. They are somewhat more acidic than corresponding alcohols, with pKa values around 10 in water. Sulfur’s size and polarizability give thiols distinctive nucleophilic behavior at sulfur. Thiols readily oxidize to disulfides (R-S-S-R) and can be deprotonated to thiolate anions, which are powerful nucleophiles. Boiling points increase with molecular weight, but are generally influenced by hydrogen bonding and sulfur’s properties.

Preparation: Common laboratory routes include nucleophilic substitution of alkyl halides by thiolate sources (generated from NaSH or potassium thiolates), and thiourea-mediated methods that convert alkyl halides to thiols after hydrolysis. Thiols can also be introduced by hydrothiolation of alkenes under appropriate conditions.

Reactions and applications: Thiols are versatile nucleophiles, forming thioethers (R-S-R') upon alkylation. They oxidize to disulfides and participate in various coupling and protective-group strategies in organic synthesis. In biology, thiol groups (as in cysteine) are essential in protein structure and enzymatic function. Industry uses include odorants for natural gas detection and various applications in polymers and catalysis.

Safety and occurrence: Many thiols are malodorous and can be toxic in higher concentrations. They occur widely in nature, including garlic and onions, where sulfur-containing thiols contribute characteristic aromas.