AlkylthiolsEdit

Alkylthiols are a class of organosulfur compounds characterized by a sulfhydryl group (-SH) bonded to an alkyl moiety. They are the sulfur analogs of alcohols and occupy an important niche in organic and industrial chemistry because of the distinctive reactivity of the S–H bond. In common usage, these compounds are often referred to as mercaptans, a name that reflects their history in early sulfur chemistry. Within this broad category, the simplest members include methane thiol (methanethiol, CH3SH) and ethane thiol (ethanethiol, C2H5SH), with larger alkyl groups yielding higher-boiling, often more malodorous substances. For many alkylthiols, the combination of low odor thresholds and high reactivity has made them useful in both practical applications and as intermediates in synthesis. mercaptan alkylthiol

Alkylthiols are distinguished from arylthiols, where the sulfur atom is bonded to an aryl ring rather than an alkyl group. In nomenclature, alkylthiols can be described as alkane-1-thiols in IUPAC terms, with the thio group replacing the terminal hydrogen of an alkane to form a thiol. The behavior of the sulfhydryl group is central to the chemistry of these compounds: the S–H bond is comparatively acidic for a hydrocarbon, allowing deprotonation to the thiolate anion (RS−) under basic conditions, and the resulting thiolate is a soft, highly nucleophilic species that participates in a variety of bond-forming processes. The sulfur center also strongly influences physical properties, including volatility, boiling point, and odor. thiol disulfide thiolate

Nomenclature and classification - Primary, secondary, and tertiary alkylthiols: The classification refers to the carbon atom attached to the sulfur. Primary alkylthiols (R–SH where R is a primary alkyl group) tend to be more straightforward to oxidize to disulfides and to undergo thiol-ene reactions under mild conditions. Secondary and tertiary alkylthiols exhibit different steric and reactivity patterns, particularly in substitution and oxidation chemistry. For broader context, see alkylthiol and disulfide. - Alkyl vs aryl thiols: Alkylthiols (R–SH with alkyl R) differ from arylthiols such as thiophenol (Ph–SH) in acidity, nucleophilicity, and in the kinds of substrates they react with. See arylthiol for comparison. - Related species: In discussions of alkylthiols, it is common to encounter references to mercaptans as a general term. See mercaptan for historical and usage notes. alkylthiol arylthiol mercaptan

Synthesis Alkylthiols are typically prepared by indirect routes that install the thiol functionality after building the carbon skeleton. Two widely used approaches are: - Thiourea-mediated conversion of alkyl halides: A classic laboratory method converts an alkyl halide (R–X) to the corresponding thiol (R–SH) through a thiourea intermediate. The general sequence involves formation of a thioamide after reaction with thiourea, followed by hydrolysis to yield the free thiol upon acidic workup. This route is favored for its relatively broad substrate compatibility and straightforward workup. See the discussion of thiourea-based thiolation in organic synthesis. thiourea alkylhalide mercaptan - Functional-group interconversion starting from alcohols or other precursors: In practice, chemists convert an alcohol to a suitable leaving group (e.g., chloride or tosylate) and then substitute with a sulfur source that ultimately yields the thiol after hydrolysis or oxidative workup. This family of routes can involve several steps but is valuable for accessing primary, secondary, and tertiary alkylthiols from readily available starting materials. See alkyl halide and thiol-ene click chemistry for related transformations. alkyl halide thiol-ene click chemistry mercaptan

Reactions and properties - Acidity and thiolate formation: The S–H bond in alkylthiols is more acidic than the O–H bond in alcohols, with pKa values typically around 10 for many simple alkylthiols, though substituents can shift this value. This makes RS− formation under basic conditions facile and underpins many subsequent reactions. See pKa for general acid–base concepts. thiolate pKa - Disulfide formation: Oxidation of two RS−H units yields a disulfide (RSSR) with the concomitant release of protons. Disulfide formation is a common route for storing and transferring sulfur equivalents in biological and synthetic contexts. See disulfide for a broader treatment. disulfide - Nucleophilic and ligand behavior: The thiolate anion and neutral thiol can act as soft nucleophiles and ligands, readily binding to soft metal centers such as those of transition metals. This makes alkylthiols important in organometallic chemistry and catalysis, including metal–thiolate complexes. See thiolate and organometallic chemistry. - Thiol-ene chemistry: Alkylthiols participate in thiol-ene click reactions, a versatile and widely used method for forming carbon–sulfur bonds and constructing sulfur-containing polymers. See thiol-ene click chemistry for details. thiol-ene click chemistry - Applications as protecting and blocking groups: The thiol moiety can be used in various protective strategies or as a handle for selective transformations, including in polymer and surface chemistry. See protecting group in related contexts. protecting group

Applications - Odorization of natural gas: Some alkylthiols, particularly certain branched mercaptans, are deliberately added to odorless natural gas to provide a detectable odor for leak detection. This application relies on the strong, characteristic smell of many low-molecular-weight alkylthiols and the very low odor thresholds of some members. See natural gas and odorant for context. natural gas odorant - Polymers and coatings: In polymer chemistry, alkylthiols are used in thiol-ene click reactions to form networks and coatings with controlled architecture. They also participate in thiol–acrylate and related crosslinking chemistries that impart desirable mechanical and chemical properties to polymers and adhesives. See polymer chemistry and thiol-ene click chemistry. polymer chemistry thiol-ene click chemistry - Organic synthesis: Alkylthiols serve as versatile intermediates and reagents in organic synthesis, enabling the formation of thioethers (R–S–R') through substitution and coupling, as well as serving as precursors to more complex sulfur-containing motifs. See thioether for related chemistry. thioether - Biological context: In biology, thiol groups are found in cysteine residues of proteins and play critical roles in redox biology and structure, including the formation and breaking of disulfide bonds that affect protein folding and activity. See cysteine and disulfide for related biological chemistry. cysteine disulfide

Safety and environmental aspects - Odor and toxicity: Many alkylthiols have very low odor thresholds and can be irritants or toxic in higher concentrations. Proper handling, adequate ventilation, and appropriate personal protective equipment are standard in laboratory and industrial settings. See general toxicology resources and material safety data sheets for specific compounds. toxicology safety data sheet - Flammability and reactivity: Alkylthiols are generally volatile and flammable under typical laboratory and industrial conditions. They can form dangerous mixtures with oxidizers and other reactive quenchers, and oxidation to disulfides can be exothermic in certain contexts. See flammability for broader safety concepts. flammability disulfide

History and terminology - Mercaptans: The term mercaptan has historical roots in early sulfur chemistry and reflects the distinctive reactivity of these compounds. See mercaptans for historical background and usage notes. mercaptans

See also - thiol - mercaptan - disulfide - thiol-ene click chemistry - cysteine - organosulfur chemistry