Abstract

1-Octanethiol (C₈H₁₈S), systematically named octane-1-thiol, is a straight-chain aliphatic thiol compound characterized by a terminal sulfhydryl group. This organosulfur compound exhibits a density of 0.843 g/mL at 25°C, melting point of -49°C, and boiling point range of 197-200°C. As a member of the alkanethiol homologous series, 1-octanethiol demonstrates characteristic thiol properties including nucleophilicity, metal coordination capability, and radical reactivity. The compound finds extensive application in surface chemistry as a self-assembled monolayer forming agent, particularly on gold and other noble metal surfaces. Industrial uses include its role as a chemical intermediate, polymerization modifier, and odorant compound. The distinctive sulfur-based reactivity and amphiphilic nature of 1-octanethiol make it valuable in materials science, nanotechnology, and synthetic chemistry applications.

Introduction

1-Octanethiol represents a significant member of the alkanethiol family, organic compounds characterized by an alkyl chain terminated with a thiol functional group. Classified systematically as an organosulfur compound, 1-octanethiol possesses the molecular formula C₈H₁₈S and the IUPAC name octane-1-thiol. The compound's historical significance stems from its role in early studies of sulfur-containing organic compounds and their distinctive chemical behavior compared to oxygen analogs.

Industrial interest in 1-octanethiol emerged during the mid-20th century with developments in polymer chemistry, where it served as a chain transfer agent in free radical polymerization processes. The compound gained further prominence in the 1980s with the advent of self-assembled monolayer technology, where its ability to form ordered structures on metal surfaces became extensively utilized in surface science and nanotechnology.

Structurally, 1-octanethiol consists of an eight-carbon alkyl chain with a terminal thiol group, creating an amphiphilic molecule with both hydrophobic and hydrophilic regions. This structural arrangement enables diverse chemical behavior including surface activity, molecular self-assembly, and participation in various organic transformations.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of 1-octanethiol follows expected patterns for straight-chain alkanethiols. The carbon skeleton adopts an extended zig-zag conformation with typical C-C bond lengths of 1.54 Å and C-C-C bond angles of approximately 112°. The thiol group exhibits a C-S bond length of 1.82 Å, consistent with single bond character. According to VSEPR theory, the sulfur atom in the thiol group demonstrates sp³ hybridization with bond angles approaching tetrahedral geometry (approximately 109.5°).

Electronic structure analysis reveals that the sulfur atom possesses a formal charge of approximately -0.2, while the hydrogen atom in the SH group carries a partial positive charge of +0.15. The highest occupied molecular orbital (HOMO) primarily localizes on the sulfur atom, with significant contribution from sulfur's 3p orbitals. This electronic distribution explains the compound's nucleophilic character and its propensity for oxidation reactions. The lowest unoccupied molecular orbital (LUMO) distributes across the alkyl chain, indicating potential sites for electrophilic attack.

Chemical Bonding and Intermolecular Forces

Covalent bonding in 1-octanethiol follows typical patterns for alkanes with the addition of the C-S and S-H bonds. The C-S bond dissociation energy measures 272 kJ/mol, while the S-H bond requires 366 kJ/mol for homolytic cleavage. These values compare to C-H bond energies of approximately 413 kJ/mol in the alkyl chain.

Intermolecular forces in 1-octanethiol include London dispersion forces along the hydrocarbon chain and weak hydrogen bonding capabilities through the thiol group. The sulfur atom's electronegativity of 2.58 creates a modest dipole moment of 1.52 D for the molecule. While thiol groups can participate in hydrogen bonding, these interactions are significantly weaker than those involving oxygen or nitrogen, with S-H···S hydrogen bond energies measuring approximately 4-8 kJ/mol compared to 20-40 kJ/mol for O-H···O bonds.

The compound's amphiphilic nature results in unique intermolecular interactions in different environments. In nonpolar solvents, 1-octanethiol tends to form dimers through weak S-H···S hydrogen bonding, while in polar environments, the molecules may adopt more extended conformations to maximize solvation of the polar thiol group.

Physical Properties

Phase Behavior and Thermodynamic Properties

1-Octanethiol exists as a colorless to pale yellow liquid at room temperature with a characteristic strong odor typical of thiol compounds. The melting point occurs at -49°C, while boiling takes place between 197°C and 200°C at atmospheric pressure. The compound exhibits a density of 0.843 g/mL at 25°C, decreasing linearly with temperature according to the relationship ρ = 0.863 - 0.00078T g/mL, where T is temperature in Celsius.

Thermodynamic properties include an enthalpy of vaporization of 45.2 kJ/mol at the boiling point and heat capacity of 298 J/mol·K at 25°C. The compound demonstrates a vapor pressure described by the Antoine equation: log₁₀P = 4.123 - 1582/(T + 203.5), where P is vapor pressure in mmHg and T is temperature in Celsius. The surface tension measures 29.8 mN/m at 20°C, reflecting the compound's amphiphilic character.

1-Octanethiol shows limited solubility in water (approximately 0.05 g/L at 25°C) but is miscible with most organic solvents including ethanol, diethyl ether, chloroform, and benzene. The refractive index is 1.453 at 20°C and sodium D-line wavelength.

Spectroscopic Characteristics

Infrared spectroscopy of 1-octanethiol reveals characteristic vibrations including the S-H stretch at 2570 cm⁻¹, C-H stretches between 2850-2960 cm⁻¹, and C-S stretch at 670 cm⁻¹. The S-H stretching frequency appears as a weak, sharp band due to the small change in dipole moment during vibration.

Proton NMR spectroscopy shows distinctive signals: the SH proton appears as a broad singlet at δ 1.3 ppm, the methylene group adjacent to sulfur resonates at δ 2.5 ppm (triplet, J = 7.3 Hz), the terminal methyl group appears at δ 0.9 ppm (triplet, J = 6.9 Hz), and the remaining methylene protons produce a complex multiplet between δ 1.2-1.4 ppm. Carbon-13 NMR displays signals at δ 13.7 ppm (CH₃), δ 22.6-31.9 ppm (CH₂ groups 2-7), and δ 24.3 ppm (C-1 adjacent to sulfur).

Mass spectrometric analysis shows a molecular ion peak at m/z 146 with characteristic fragmentation patterns including loss of SH radical (m/z 113), cleavage at the C-S bond (m/z 41, 105), and McLafferty rearrangement products. UV-Vis spectroscopy reveals no significant absorption above 210 nm, consistent with the absence of extended conjugation in the molecule.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

1-Octanethiol exhibits reactivity characteristic of primary alkanethiols, participating in numerous chemical transformations. The compound demonstrates nucleophilic behavior through the sulfur atom, with a nucleophilicity parameter (N) of 5.0 in the Swain-Scott scale. This nucleophilicity enables reactions with alkyl halides to form thioethers, with second-order rate constants typically ranging from 10⁻³ to 10⁻¹ M⁻¹s⁻¹ depending on the electrophile.

Oxidation represents a significant reaction pathway for 1-octanethiol. Mild oxidation with iodine or hydrogen peroxide produces the disulfide, octyl disulfide, with a reaction rate constant of 0.15 M⁻¹s⁻¹ in ethanol at 25°C. Stronger oxidizing agents such as potassium permanganate or nitric acid convert the thiol to sulfonic acid derivatives through sulfinic acid intermediates.

The compound participates in radical reactions, particularly as a chain transfer agent in free radical polymerization. The chain transfer constant for 1-octanethiol in styrene polymerization measures 0.85 at 60°C, indicating efficient hydrogen atom donation from the S-H group to growing polymer radicals.

Acid-Base and Redox Properties

1-Octanethiol functions as a weak acid with a pKa of 10.3 in water at 25°C, making it significantly more acidic than corresponding alcohols (pKa ~16) but less acidic than hydrogen sulfide (pKa ~7). This acidity enables salt formation with strong bases, producing water-soluble thiolate anions. The thiolate conjugate base demonstrates enhanced nucleophilicity compared to the neutral thiol.

Redox properties include a standard reduction potential of -0.3 V for the RSSR/2RSH couple at pH 7. The compound exhibits reversible oxidation at approximately +0.8 V versus standard hydrogen electrode in acetonitrile, corresponding to the one-electron oxidation to sulfenium cation. Electrochemical studies show that 1-octanethiol forms stable self-assembled monolayers on gold electrodes with surface coverage of 4.6×10⁻¹⁰ mol/cm².

The compound demonstrates stability across a pH range of 2-12, with decomposition occurring under strongly acidic or basic conditions through various pathways including hydrolysis, oxidation, and disproportionation reactions.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Several laboratory methods exist for the synthesis of 1-octanethiol. The most common approach involves the reaction of 1-bromooctane with thiourea, followed by alkaline hydrolysis of the resulting isothiouronium salt. This two-step process proceeds with overall yields of 75-85% and provides high purity product after distillation.

Alternative synthetic routes include the displacement of halide by hydrosulfide anion in high-pressure reactions, although this method often produces dialkyl sulfide byproducts. More modern approaches utilize thiol-disulfide exchange reactions or reduction of disulfides with lithium aluminum hydride or other reducing agents.

Purification typically involves fractional distillation under reduced pressure, with the pure compound distilling at 98-100°C at 20 mmHg. Analytical purity assessment employs gas chromatography, with commercial samples typically exhibiting purity greater than 98%.

Industrial Production Methods

Industrial production of 1-octanethiol primarily utilizes the catalytic addition of hydrogen sulfide to 1-octene over acidic catalysts at elevated temperatures and pressures. This process operates at 150-200°C and 20-50 bar pressure using catalysts such as silica-alumina or acidic resins. The reaction follows Markovnikov addition, providing high selectivity for the primary thiol product.

Alternative industrial routes include the high-temperature reaction of octanol with hydrogen sulfide over tungsten or cobalt-molybdenum catalysts, though this method typically produces lower yields due to competing dehydration reactions. Annual global production estimates range between 1000-5000 metric tons, with primary manufacturing facilities located in the United States, Germany, and China.

Process optimization focuses on catalyst development to improve selectivity and reduce energy consumption. Environmental considerations include efficient recovery and recycling of hydrogen sulfide and implementation of closed-system operations to minimize odor emissions characteristic of thiol compounds.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection or mass spectrometric detection serves as the primary method for identification and quantification of 1-octanethiol. Capillary columns with non-polar stationary phases such as DB-1 or HP-1 provide excellent separation from potential impurities and decomposition products. Retention indices typically range from 980-990 on methyl silicone stationary phases.

Spectroscopic methods including FT-IR and NMR provide complementary identification through characteristic functional group vibrations and proton/carbon chemical shifts. The weak S-H stretching vibration at 2570 cm⁻¹ in infrared spectroscopy offers specific identification, though its low intensity requires careful baseline correction.

Quantitative analysis typically employs internal standard methods with compounds such as decane or dodecane that elute near but separate from 1-octanethiol. Detection limits for gas chromatographic methods approach 0.1 ppm, while NMR methods demonstrate detection limits near 100 ppm for routine analysis.

Purity Assessment and Quality Control

Purity assessment of 1-octanethiol focuses on determination of common impurities including dialkyl sulfides, disulfides, and octanol residues. Gas chromatographic methods with sulfur-specific detection such as flame photometric detection or atomic emission detection provide sensitive measurement of sulfur-containing impurities at levels below 0.01%.

Water content represents another critical quality parameter, typically determined by Karl Fischer titration with acceptable limits below 0.05%. Commercial specifications generally require minimum purity of 98%, with individual impurity limits established for specific applications.

Stability testing indicates that 1-octanethiol maintains acceptable purity for at least two years when stored under nitrogen atmosphere in amber glass containers at temperatures below 25°C. Decomposition primarily occurs through oxidation to disulfide compounds, which can be minimized through addition of stabilizers such as hydroquinone or butylated hydroxytoluene at concentrations of 50-100 ppm.

Applications and Uses

Industrial and Commercial Applications

1-Octanethiol finds extensive application as a surface modification agent in materials science. The compound forms self-assembled monolayers on gold, silver, and other metal surfaces through strong Au-S bonds with bond energies of approximately 160 kJ/mol. These monolayers serve as model systems for studying surface phenomena and provide platforms for creating functionalized surfaces in sensor technology and microelectronics.

In polymer chemistry, 1-octanethiol functions as a chain transfer agent in free radical polymerization processes, particularly for styrene, acrylate, and methacrylate monomers. The compound effectively controls molecular weight and polydispersity through hydrogen atom transfer to growing polymer radicals, with chain transfer constants typically ranging from 0.6-1.2 depending on monomer and temperature.

Additional industrial applications include use as a chemical intermediate in synthesis of more complex organosulfur compounds, as a corrosion inhibitor for metals, and as an odorant in natural gas and propane for leak detection. The compound's strong odor, detectable at concentrations as low as 0.0001 ppm, makes it valuable for safety applications in fuel systems.

Research Applications and Emerging Uses

Research applications of 1-octanethiol span numerous fields including nanotechnology, surface science, and materials chemistry. The compound serves as a standard reference material in studies of self-assembled monolayer formation kinetics and structure. Research focuses on understanding the relationship between molecular structure, packing density, and monolayer properties on various substrates.

Emerging applications include use in nanoparticle synthesis and functionalization, where 1-octanethiol provides surface stabilization and controls particle growth through coordination to metal surfaces. The compound finds increasing use in creating patterned surfaces through microcontact printing and other lithographic techniques for biosensor and microfluidic device fabrication.

Ongoing research explores the potential of 1-octanethiol derivatives in molecular electronics, where the well-defined electronic structure of the gold-thiol interface enables creation of molecular-scale electronic devices. Studies also investigate the compound's role in creating responsive surfaces that change properties in response to external stimuli such as light, electric fields, or chemical environment.

Historical Development and Discovery

The history of 1-octanethiol parallels the development of organosulfur chemistry in the late 19th and early 20th centuries. Early investigations of thiol compounds focused on lower molecular weight analogs, with systematic study of higher homologs such as 1-octanethiol emerging in the 1920s as synthetic methods improved.

Significant advancement occurred with the development of the thiourea route to thiols by Marvel and colleagues in the 1930s, which provided reliable access to pure alkanethiols including 1-octanethiol. This methodological improvement enabled detailed physical and chemical characterization of the compound and established its place as a reference material in organosulfur chemistry.

The modern era of 1-octanethiol research began in the 1980s with the pioneering work of Nuzzo, Allara, and others on self-assembled monolayers. Their demonstration that alkanethiols including 1-octanethiol form highly ordered monolayers on gold surfaces revolutionized surface science and opened new avenues for molecular-level surface engineering.

Conclusion

1-Octanethiol represents a chemically versatile organosulfur compound with significant scientific and industrial importance. Its well-defined molecular structure, characterized by an eight-carbon alkyl chain terminated with a thiol group, enables diverse chemical behavior including surface activity, nucleophilicity, and participation in radical processes. The compound's physical properties, including its amphiphilic nature and relatively high boiling point, make it suitable for various applications ranging from polymer chemistry to nanotechnology.

Ongoing research continues to explore new applications for 1-octanethiol and its derivatives, particularly in emerging fields such as molecular electronics, nanomedicine, and advanced materials. Challenges remain in developing more sustainable synthetic routes and improving understanding of the fundamental interactions that govern its behavior at interfaces. The compound's established role as a model system in surface science ensures its continued importance in both fundamental research and technological applications.