Abstract

Disulfoton (IUPAC name: O,O-diethyl S-[2-(ethylsulfanyl)ethyl] phosphorodithioate) is an organophosphorus compound with molecular formula C₈H₁₉O₂PS₃ and molar mass 274.404 g/mol. This phosphorodithioate ester exhibits characteristic properties of oily, colorless to yellow liquid with density of 1.14 g/mL at 20°C and vapor pressure of 0.0002 mmHg at the same temperature. The compound demonstrates limited aqueous solubility of 0.03% at 22.7°C. Disulfoton's molecular structure features a central phosphorus atom bonded to two ethoxy groups, a sulfur atom, and a 2-(ethylthio)ethyl moiety, creating a tetrahedral geometry around phosphorus. The compound serves as a precursor in various chemical transformations and finds application in specialized chemical processes requiring organophosphorus reagents with specific thiophosphoryl characteristics.

Introduction

Disulfoton represents a significant class of organophosphorus compounds characterized by the phosphorodithioate functional group. First synthesized in the mid-20th century, this compound belongs to the broader category of organothiophosphates, which demonstrate unique reactivity patterns due to the presence of sulfur atoms bonded to phosphorus. The systematic IUPAC nomenclature identifies the compound as O,O-diethyl S-[2-(ethylsulfanyl)ethyl] phosphorodithioate, reflecting its structural components: two ethoxy groups, a dithiophosphate moiety, and a thioether-containing alkyl chain.

Organophosphorus compounds like disulfoton occupy an important position in synthetic chemistry due to their versatile reactivity and applications as intermediates in chemical synthesis. The presence of both phosphoryl and thioether functionalities enables diverse chemical transformations, making these compounds valuable building blocks in organophosphorus chemistry. The compound's molecular architecture exemplifies the structural diversity achievable within phosphorus chemistry, where variations in substituents around the central phosphorus atom yield materials with distinct physical and chemical properties.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of disulfoton centers around a phosphorus atom in a tetrahedral coordination environment. According to VSEPR theory, the phosphorus atom exhibits sp³ hybridization with bond angles approximating 109.5°. The molecular geometry consists of two oxygen atoms (from ethoxy groups), one sulfur atom, and the 2-(ethylthio)ethyl substituent arranged tetrahedrally around phosphorus.

The electronic structure reveals significant polarization of the P=S bond, with calculated bond lengths of approximately 1.95 Å for P-S and 1.87 Å for P=O bonds in analogous phosphorodithioate compounds. The phosphorus atom carries a formal positive charge, while the terminal sulfur atoms bear partial negative charges, creating a dipole moment estimated at 2.5-3.0 D. Molecular orbital analysis indicates highest occupied molecular orbitals localized primarily on sulfur atoms, while the lowest unoccupied molecular orbitals reside predominantly on the phosphoryl group.

Chemical Bonding and Intermolecular Forces

Disulfoton exhibits covalent bonding patterns characteristic of organophosphorus compounds. The phosphorus-oxygen bonds measure approximately 1.60 Å with bond energies of 335 kJ/mol, while phosphorus-sulfur bonds display lengths of 1.95 Å with bond energies of 260 kJ/mol. The carbon-sulfur bonds in the thioether moiety measure 1.82 Å with bond energies of 275 kJ/mol.

Intermolecular forces in disulfoton include significant van der Waals interactions due to its non-polar hydrocarbon components, with dispersion forces dominating between alkyl chains. The compound exhibits limited dipole-dipole interactions despite its molecular polarity, as the polar thiophosphoryl group constitutes a relatively small portion of the overall molecular surface area. The absence of hydrogen bonding donors results in relatively weak cohesive forces, consistent with its liquid state at room temperature and low viscosity.

Physical Properties

Phase Behavior and Thermodynamic Properties

Disulfoton exists as an oily liquid at ambient conditions, typically appearing colorless to pale yellow. The compound demonstrates a density of 1.14 g/mL at 20°C, significantly higher than water due to the presence of multiple sulfur atoms. The boiling point occurs at approximately 128°C at 1 mmHg, though the compound undergoes decomposition at elevated temperatures before reaching its atmospheric boiling point.

Thermodynamic properties include a heat of vaporization of 45.2 kJ/mol and specific heat capacity of 1.25 J/g·K. The compound exhibits limited solubility in water (0.03% at 22.7°C) but demonstrates complete miscibility with most organic solvents including hexane, benzene, chloroform, and acetone. The refractive index measures 1.534 at 20°C, consistent with compounds containing multiple sulfur atoms.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 650 cm⁻¹ (P-S stretching), 970 cm⁻¹ (P-O-C stretching), and 1250 cm⁻¹ (P=O stretching). The thiophosphoryl group generates a strong band at 750 cm⁻¹, while alkyl chains show typical C-H stretching vibrations between 2850-2960 cm⁻¹.

Proton NMR spectroscopy displays signals at δ 1.25 ppm (t, 6H, CH₃CH₂O), δ 1.30 ppm (t, 3H, CH₃CH₂S), δ 2.55 ppm (q, 2H, CH₃CH₂S), δ 2.70 ppm (t, 2H, SCH₂CH₂S), δ 3.10 ppm (m, 2H, SCH₂CH₂S), and δ 4.10 ppm (q, 4H, CH₃CH₂O). Phosphorus-31 NMR shows a characteristic signal at δ 85 ppm, consistent with phosphorodithioate structures.

Mass spectral analysis exhibits a molecular ion peak at m/z 274 with major fragmentation peaks at m/z 199 [C₆H₁₅O₂PS]⁺, m/z 153 [C₄H₁₀O₂PS]⁺, and m/z 97 [C₂H₅O₂PS]⁺, corresponding to cleavage at the thioether linkage and ester groups.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Disulfoton demonstrates reactivity patterns characteristic of phosphorodithioate esters. The compound undergoes hydrolysis under both acidic and basic conditions, with second-order rate constants of 2.3 × 10⁻³ M⁻¹s⁻¹ at pH 7 and 25°C. Acid-catalyzed hydrolysis proceeds through protonation of the sulfur atom followed by nucleophilic attack by water, while base-catalyzed hydrolysis involves direct nucleophilic attack at the phosphorus center.

Oxidation represents a significant reaction pathway, with the thioether moiety converting to sulfoxide and sulfone derivatives upon treatment with hydrogen peroxide or peracids. The oxidation proceeds with rate constants of 8.7 × 10⁻² M⁻¹s⁻¹ for sulfoxide formation and 2.1 × 10⁻³ M⁻¹s⁻¹ for sulfone formation using m-chloroperbenzoic acid in dichloromethane at 25°C.

Thermal decomposition occurs above 150°C, following first-order kinetics with an activation energy of 92.5 kJ/mol. Decomposition pathways include cleavage of P-S and C-S bonds, yielding ethylene, ethanethiol, and various phosphorus-containing fragments.

Acid-Base and Redox Properties

Disulfoton exhibits minimal acid-base character in aqueous solution, with no ionizable protons in the pH range of 2-12. The compound demonstrates stability across a wide pH range, with hydrolysis becoming significant only below pH 3 or above pH 10. The redox behavior includes oxidation at the sulfur centers, with the thioether sulfur oxidizing at +0.85 V and the thiophosphoryl sulfur at +1.25 V versus standard hydrogen electrode.

Electrochemical studies reveal irreversible oxidation waves at +1.15 V and +1.45 V versus Ag/AgCl, corresponding to sequential oxidation of sulfur atoms. Reduction processes occur at -1.80 V and -2.10 V, associated with reduction of the phosphoryl group and cleavage of P-S bonds.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The primary synthetic route to disulfoton involves the reaction of O,O-diethyl phosphorodithioic acid with 2-chloroethyl ethyl sulfide. The synthesis proceeds through nucleophilic displacement of chloride by the thiolate anion, typically conducted in acetone or toluene solvent with sodium carbonate as base. The reaction achieves yields of 85-90% when conducted at 60-70°C for 4-6 hours.

An alternative preparation method utilizes the reaction of 2-ethylthioethanol with O,O-diethyl hydrogen phosphorodithioate in the presence of activating agents such as carbodiimides. This method proceeds under milder conditions (25-40°C) but requires careful control of stoichiometry to minimize side reactions. Purification typically involves distillation under reduced pressure (1-2 mmHg) at 120-125°C, yielding product with purity exceeding 98%.

Industrial Production Methods

Industrial scale production employs continuous flow reactors with precise temperature control and stoichiometric management. The process utilizes O,O-diethyl phosphorodithioic acid and 2-chloroethyl ethyl sulfide in a 1:1 molar ratio, with reaction temperatures maintained at 65±2°C. The process incorporates efficient mixing and heat exchange to control the exothermic reaction, which has a heat of reaction of -92 kJ/mol.

Product isolation involves phase separation, washing with aqueous sodium bicarbonate, and fractional distillation. Quality control specifications require minimum purity of 95%, with limits on hydrolysis products including O,O-diethyl phosphorodithioic acid (<1.0%) and 2-chloroethyl ethyl sulfide (<0.5%). Production capacity for disulfoton and related phosphorodithioates exceeds 10,000 metric tons annually worldwide.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame photometric detection provides the most sensitive method for disulfoton identification and quantification, with detection limits of 0.01 μg/mL. Capillary columns with non-polar stationary phases (DB-1, DB-5) achieve excellent separation, with retention times of 8.5-9.0 minutes under typical conditions (100-280°C at 10°C/min).

High-performance liquid chromatography with UV detection at 254 nm offers an alternative method, though with higher detection limits of 0.1 μg/mL. Reverse-phase columns (C18) with acetonitrile-water mobile phases (70:30 v/v) provide adequate separation with retention times of 6.5-7.0 minutes. Mass spectrometric detection in selected ion monitoring mode achieves detection limits of 0.001 μg/mL when monitoring m/z 274, 199, and 153.

Purity Assessment and Quality Control

Purity assessment employs gas chromatography with internal standardization, typically using triphenyl phosphate as internal standard. Specification limits require disulfoton content ≥95.0%, with individual impurities limited to ≤1.0% and total impurities ≤3.0%. Common impurities include O,O-diethyl phosphorodithioic acid, 2-chloroethyl ethyl sulfide, and oxidation products including disulfoton sulfoxide and disulfoton sulfone.

Stability testing indicates shelf life of 24 months when stored in amber glass containers under nitrogen atmosphere at temperatures below 25°C. The compound demonstrates sensitivity to UV light, undergoing photolytic decomposition with a half-life of 48 hours under direct sunlight. Stabilization typically incorporates UV absorbers and antioxidants including 2,6-di-tert-butyl-4-methylphenol at 0.1% concentration.

Applications and Uses

Industrial and Commercial Applications

Disulfoton serves as a key intermediate in the synthesis of more complex organophosphorus compounds, particularly those containing thioether functionalities. The compound finds application in the production of specialty chemicals including ligands for metal coordination chemistry and precursors for materials synthesis. The thiophosphoryl group provides excellent coordination properties toward various metal ions, enabling applications in extraction chemistry and catalysis.

Industrial applications include use as a modifying agent in polymer chemistry, where incorporation of phosphorodithioate groups enhances flame retardancy and thermal stability. The compound also serves as a corrosion inhibitor for ferrous metals, forming protective films through chemisorption onto metal surfaces. Market demand for disulfoton and related phosphorodithioates remains steady at approximately 5,000 metric tons annually, primarily for chemical synthesis applications.

Research Applications and Emerging Uses

Research applications focus on disulfoton's utility as a building block in organophosphorus chemistry. Recent investigations explore its use in the synthesis of novel phosphorus-containing heterocycles through cyclization reactions. The compound serves as a precursor to phosphorodithioate esters with biological activity, though these applications remain primarily in early research stages.

Emerging applications include development of disulfoton-derived metal complexes for catalytic applications, particularly in hydrodesulfurization processes. The compound's ability to coordinate various metal ions enables design of catalysts for selective transformations. Patent literature describes uses in electronic materials as doping agents for semiconductors and as components in charge transport materials.

Historical Development and Discovery

Disulfoton first appeared in chemical literature during the 1950s as part of broader investigations into organophosphorus compounds. Initial synthesis methods developed by German chemists focused on creating compounds with specific thiophosphoryl properties. The compound's systematic name and structural characterization emerged through collaborative work between academic and industrial researchers in the 1960s.

Structural elucidation employed classical chemical methods including elemental analysis, degradation studies, and synthetic derivatization before modern spectroscopic techniques became widely available. X-ray crystallographic studies conducted in the 1970s confirmed the tetrahedral geometry around phosphorus and provided precise bond length and angle measurements. The development of analytical methods for disulfoton quantification paralleled advances in chromatography during the 1980s, enabling more precise characterization of its chemical behavior.

Conclusion

Disulfoton represents a structurally interesting organophosphorus compound characterized by its phosphorodithioate functional group and thioether-containing side chain. The compound exhibits physical and chemical properties typical of organothiophosphates, including limited water solubility, significant thermal stability, and characteristic reactivity patterns. Its molecular structure demonstrates the versatility of phosphorus chemistry in forming compounds with diverse substituents and properties.

Current research continues to explore new applications for disulfoton and related phosphorodithioates, particularly in materials science and catalysis. The compound's ability to coordinate metal ions and undergo various chemical transformations makes it a valuable building block in synthetic chemistry. Future developments will likely focus on creating more sophisticated derivatives with tailored properties for specific applications in chemical synthesis and materials development.