A thiol, also called a mercaptan, is an organic compound in which a sulfur atom forms a single bond with a carbon atom. Its defining feature is the sulfhydryl (–SH) group, represented by the general formula R–SH, where R is an organic fragment such as an alkyl or aryl group. For example, CH₃–SH, or methanethiol, is the simplest alkyl thiol. Thiols are often considered the sulfur analogs of alcohols, with sulfur replacing the oxygen atom in the hydroxyl (–OH) group.

Thiols play important roles in both chemistry and biology. For instance, tert-butylthiol is added to natural gas as a safety odorant because thiols have a strong, distinctive smell detectable even at very low concentrations, alerting people to gas leaks. In proteins, the sulfhydryl groups of cysteine residues can form covalent disulfide bonds, which are critical for stabilizing protein structure and maintaining proper function.

The Lewis structure of a thiol is shown below.

Nomenclature

Thiols are named using either IUPAC rules or common names: ^[1]

1. IUPAC Naming

The suffix –thiol is added to the name of the parent hydrocarbon. The carbon atom bonded to the –SH group is numbered to provide the lowest possible position.

Example:

• CH₃CH₂SH → Ethanethiol
• CH₃CH(SH)CH₃ → 2-Propanethiol

2. Common Names

Thiols are sometimes named as derivatives of the corresponding alcohol, replacing “–ol” with “mercaptan”.

Example:

• CH₃SH → Methyl mercaptan
• C₆H₅CH₂SH → Benzyl mercaptan

A few examples of thiol compounds are shown below.

Structure and Bonding ^[8]

• Bond lengths: The C–S bond (~180 pm) in thiols is ~40 pm longer than a C–O bond in alcohols.
• Bond angles: The C–S–H angle is nearly 90°, smaller than the C–O–H angle.
• Intermolecular forces: Thiols exhibit weak hydrogen bonding and are generally more volatile than alcohols. The primary cohesive forces are van der Waals interactions between polarizable sulfur atoms.
• Bond strength: The S–H bond (366 kJ/mol) in CH₃–SH is weaker than the O–H bond (440 kJ/mol) in CH₃–OH.
• Radical formation: Hydrogen abstraction gives a thiyl radical (RS•)

Characteristics ^[2,9]

• Odor: Most thiols have strong, pungent odors reminiscent of rotten eggs, garlic, or skunk spray. Their presence is noticeable even in tiny amounts, making them ideal for use as natural gas odorants.
• Boiling point: Thiols generally have lower boiling points than their alcohol counterparts due to weaker hydrogen bonding.
• Solubility: Thiols are less soluble in water than alcohols of similar molecular weight. Solubility increases with shorter alkyl chains and decreases with longer or bulkier groups. 

Preparation ^[6]

1. Industrial Methods

a. From Alcohols: Methanethiol forms by reacting methanol with hydrogen sulfide in the presence of an acid catalyst:

CH₃OH + H₂S → CH₃SH + H₂O

b. From Alkenes: Hydrogen sulfide adds to alkenes under acidic conditions or UV light. For example, ethylene reacts with H₂S to produce ethanethiol:

CH₂=CH₂ + H₂S →  CH₃CH₂SH (in presence of UV light)

c. From Alkyl Halides: Alkyl halides react with sodium hydrosulfide (NaSH) to form thiols. For instance, thioglycolic acid is prepared from chloroacetic acid:

ClCH₂​COOH + NaSH → HSCH₂​COOH + NaCl

2. Laboratory Methods ^[7]

a. Thiourea Process: Alkyl halides react with thiourea to form an isothiouronium salt, which hydrolyzes to yield the thiol. Example for ethyl thiol:

CH₃CH₂Br + SC(NH₂)₂ → [CH₃CH₂SC(NH₂)₂]Br

[CH₃CH₂SC(NH₂)₂]Br + NaOH → CH₃CH₂SH + OC(NH₂)₂ + NaBr

This approach is most efficient for primary halides. Secondary and tertiary thiols are more difficult to obtain this way.

b. Bunte Salt Method: Alkylation of sodium thiosulfate produces a thiosulfonate (Bunte salt), which hydrolyzes to the thiol. Example for thioglycolic acid:

ClCH₂CO₂H + Na₂S₂O₃ → Na[O₃S₂CH₂CO₂H] + NaCl

Na[O₃S₂CH₂CO₂H] + H₂O → HSCH₂CO₂H + NaHSO₄

c. Organometallic Routes: Grignard (RMgX) or organolithium (RLi) reagents react with elemental sulfur to form thiolate salts, which upon hydrolysis give thiols:

RLi + S → RSLi

RSLi + HCl → RSH + LiCl

Reactions ^[2]

1. Reaction with Alkyl Halides

Thiols act as nucleophiles, reacting with alkyl halides to form thioethers (sulfides). Dimethyl sulfide can be prepared in this way:

 CH₃SH + CH₃I → CH₃–S–CH₃ + HI

2. Oxidation

a. To Disulfides – Mild oxidants like bromine or iodine convert thiols to disulfides (e.g., diethyl disulfide):

2 C₂H₅SH + I₂ → C₂H₅–S–S–C₂H₅ + 2 HI₂

b. To Sulfonic Acids – Strong oxidizing agents such as sodium hypochlorite or hydrogen peroxide fully oxidize thiols to sulfonic acids:

C₂H₅SH + 3 H₂O₂ → C₂H₅SO₃H + 3 H₂O

3. Coordination with Heavy Metals

Thiols form stable metal thiolate complexes. For example, ethanethiol reacts with silver nitrate:

C₂H₅SH + AgNO₃ → C₂H₅S–Ag + HNO₃

Thiols are highly significant due to their strong odors, versatile reactivity, and ability to form disulfides and metal complexes. These properties make them essential in safety applications, organic synthesis, and biological processes, highlighting their broad impact in chemistry and everyday life.