Abstract

Tirpate, systematically named N-[(2,4-dimethyl-1,3-dithiolan-2-yl)methylidene]-N'-methylhydroxylamine-O-carboxamide, is an organosulfur compound with molecular formula C₈H₁₄N₂O₂S₂ and molecular weight of 234.34 g/mol. This crystalline solid compound belongs to the oxime carbamate chemical class and functions as a potent nematicide. Tirpate exhibits a complex molecular architecture featuring a 1,3-dithiolane ring system conjugated with carbamate and oxime functional groups. The compound demonstrates limited aqueous solubility but high lipophilicity, contributing to its biological activity. First developed as an agricultural pesticide, tirpate has been classified as an extremely hazardous substance under U.S. regulations due to its high toxicity profile. Its chemical properties include thermal stability up to 150°C and susceptibility to hydrolysis under alkaline conditions.

Introduction

Tirpate represents a structurally complex organosulfur compound that combines elements of carbamate chemistry with a dithiolane ring system. The compound, identified by CAS Registry Number 26419-73-8, belongs to the class of oxime carbamates characterized by the presence of both carbamate (RNHC(O)OR') and oxime (C=NOH) functional groups. Tirpate was developed during the mid-20th century as part of broader research into sulfur-containing pesticides, with particular application as a soil nematicide for agricultural use. The molecular architecture incorporates a 1,3-dithiolane heterocycle, which contributes significantly to both the compound's chemical reactivity and biological activity. The presence of two sulfur atoms within a five-membered ring system creates unique electronic properties that distinguish tirpate from simpler carbamate pesticides. Although commercial production has been discontinued in many regions, tirpate remains chemically significant as a model compound for studying structure-activity relationships in pesticidal chemicals.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of tirpate features a central 1,3-dithiolane ring with methyl substituents at positions 2 and 4, creating a chiral center at carbon 2. The ring system adopts an envelope conformation with sulfur-sulfur distance of approximately 2.08 Å and carbon-sulfur bond lengths averaging 1.81 Å. The dithiolane ring is connected through a methylidene bridge to an oxime functional group, which in turn is conjugated with a carbamate moiety. The C=N bond of the oxime group measures 1.29 Å with characteristic sp² hybridization. The carbamate portion displays partial double bond character between carbon and oxygen (1.23 Å) and between carbon and nitrogen (1.35 Å), resulting in delocalization across the O-C-N system. Molecular orbital analysis reveals highest occupied molecular orbitals localized on sulfur atoms and the oxime nitrogen, while the lowest unoccupied molecular orbitals are predominantly π* orbitals associated with the carbonyl and imine groups.

Chemical Bonding and Intermolecular Forces

Tirpate exhibits diverse bonding patterns with bond dissociation energies ranging from 65 kcal/mol for C-S bonds to 88 kcal/mol for C=O bonds. The molecule possesses a calculated dipole moment of 4.2 Debye with vector orientation toward the carbamate oxygen. Intermolecular forces include strong dipole-dipole interactions between carbonyl groups, with estimated interaction energies of 3.5 kcal/mol. The compound also demonstrates capacity for weak hydrogen bonding through both oxygen and nitrogen atoms, with hydrogen bond energies measuring 2.1-3.8 kcal/mol. Van der Waals interactions contribute significantly to crystal packing, particularly through the hydrophobic methyl groups and dithiolane ring. The compound's partition coefficient (log P) of 2.3 indicates moderate lipophilicity, while polar surface area calculations yield 78.4 Ų. These bonding characteristics collectively influence tirpate's physical properties and reactivity patterns.

Physical Properties

Phase Behavior and Thermodynamic Properties

Tirpate exists as a white to pale yellow crystalline solid at standard temperature and pressure. The compound melts at 89-91°C with enthalpy of fusion measuring 28.6 kJ/mol. Crystalline forms exhibit orthorhombic symmetry with space group P2₁2₁2₁ and unit cell parameters a = 8.42 Å, b = 12.37 Å, c = 14.85 Å. Density measurements yield 1.32 g/cm³ at 20°C. The compound sublimes at reduced pressure with sublimation point of 65°C at 0.1 mmHg. Heat capacity measurements indicate C_p = 312 J/mol·K at 25°C. The refractive index of crystalline tirpate is 1.582 at sodium D-line wavelength. Solubility characteristics include limited aqueous solubility (120 mg/L at 25°C) but high solubility in organic solvents such as dichloromethane (45 g/100 mL), acetone (38 g/100 mL), and ethyl acetate (22 g/100 mL). Vapor pressure is measured at 2.3 × 10^-4 mmHg at 25°C.

Spectroscopic Characteristics

Infrared spectroscopy of tirpate reveals characteristic vibrations including strong carbonyl stretch at 1715 cm^-1, C=N stretch at 1640 cm^-1, and N-H stretch at 3320 cm^-1. The dithiolane ring shows C-S stretching vibrations at 690 cm^-1 and 720 cm^-1. Proton NMR spectroscopy (CDCl₃, 300 MHz) displays methyl singlets at δ 1.42 ppm (3H, CH₃), δ 1.65 ppm (3H, CH₃), and δ 3.12 ppm (3H, N-CH₃), with methylene protons appearing as AB quartet at δ 3.28 ppm (J = 14.2 Hz) and δ 3.45 ppm (J = 14.2 Hz). The methine proton of the dithiolane ring resonates at δ 4.12 ppm. Carbon-13 NMR shows signals at δ 22.4 ppm and δ 24.8 ppm (ring CH₃), δ 36.2 ppm (N-CH₃), δ 48.7 ppm (CH₂S), δ 62.4 ppm (CH), δ 158.2 ppm (C=N), and δ 161.5 ppm (C=O). Mass spectral analysis exhibits molecular ion peak at m/z 234 with major fragments at m/z 177 [M-C₃H₅NS]⁺ and m/z 132 [C₅H₈S₂]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Tirpate undergoes hydrolysis as its primary degradation pathway, with pseudo-first order rate constants of 2.3 × 10^-3 h^-1 at pH 7 and 25°C. Alkaline hydrolysis proceeds 28 times faster than acid-catalyzed hydrolysis, with Arrhenius activation energy of 64.8 kJ/mol. The hydrolysis mechanism involves nucleophilic attack at the carbonyl carbon, leading to cleavage of the carbamate bond and release of methylamine and carbon dioxide. The oxime functionality demonstrates thermal lability with decomposition onset at 150°C and activation energy of 118 kJ/mol. Photochemical degradation occurs under UV irradiation (λ > 290 nm) with quantum yield of 0.13 in aqueous solution. Oxidation reactions preferentially target sulfur atoms, forming sulfoxide and sulfone derivatives with second-order rate constant of 4.7 M^-1s^-1 for reaction with hydrogen peroxide. Reduction of the oxime group occurs with sodium cyanoborohydride at pH 4.0, yielding the corresponding amine derivative.

Acid-Base and Redox Properties

The oxime functionality in tirpate exhibits weak acidity with pK_a of 9.8 for oxime proton dissociation. The carbamate nitrogen shows basic character with protonation pK_a of 3.2. The compound demonstrates stability between pH 5-7, with degradation half-lives of 312 days at pH 5, 42 days at pH 7, and 6 hours at pH 9. Redox properties include reduction potential of -0.87 V vs. SCE for the oxime group and oxidation potential of +1.23 V vs. SCE for sulfur atoms. Cyclic voltammetry reveals irreversible oxidation waves at +1.05 V and +1.38 V corresponding to successive oxidation of sulfur centers. The compound functions as a weak antioxidant with radical scavenging capacity of 0.42 mmol Trolox equivalents/g compound. Electrochemical impedance spectroscopy indicates charge transfer resistance of 4.7 kΩ·cm² in neutral aqueous solution.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The synthesis of tirpate proceeds through a multistep sequence beginning with 2,4-dimethyl-1,3-dithiolane. This intermediate is prepared by reaction of 2,3-butanedione with 1,2-ethanedithiol in benzene with p-toluenesulfonic acid catalysis, yielding 85-90%. The dithiolane undergoes formylation using dichloromethyl methyl ether and titanium tetrachloride to give the 2-formyl derivative in 75% yield. Condensation with N-methylhydroxylamine hydrochloride in ethanol with sodium acetate buffer at pH 5.0 produces the oxime intermediate. Final carbamoylation employs methyl isocyanate in dichloromethane with triethylamine catalyst, providing tirpate in 68% yield after recrystallization from hexane-ethyl acetate. The overall yield from starting materials is 43-47%. Alternative routes have been developed using O-(N-methylcarbamoyl)hydroxylamine for direct reaction with the formyl-dithiolane derivative, achieving slightly higher yields of 72% but requiring more elaborate purification.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides effective separation of tirpate using DB-5 capillary column (30 m × 0.32 mm × 0.25 μm) with temperature programming from 100°C to 280°C at 15°C/min. Retention time is 9.8 minutes under these conditions. Mass spectrometric detection in selected ion monitoring mode uses m/z 234, 177, and 132 for confirmation, with detection limit of 0.1 ng/mL. High-performance liquid chromatography employs C₁₈ reverse-phase column with acetonitrile-water (65:35) mobile phase at 1.0 mL/min, with UV detection at 230 nm. Method validation shows linearity range of 0.5-100 μg/mL, correlation coefficient of 0.9998, and quantitation limit of 0.2 μg/mL. Thin-layer chromatography on silica gel with toluene-ethyl acetate (7:3) development gives R_f value of 0.45, visualized with vanillin-sulfuric acid reagent. Fourier-transform infrared spectroscopy provides characteristic fingerprint region between 600-800 cm^-1 for confirmation of dithiolane ring structure.

Purity Assessment and Quality Control

Pharmaceutical-grade tirpate specifications require minimum purity of 98.5% by HPLC area normalization. Common impurities include the desmethyl analog (N-demethyltirpate) at 0.3-0.8%, hydrolysis products (2,4-dimethyl-1,3-dithiolane-2-carbaldehyde oxime) at 0.2-0.5%, and oxidation products (sulfoxides) at 0.1-0.4%. Residual solvents are limited to 500 ppm for dichloromethane and 3000 ppm for hexane. Heavy metal content must not exceed 10 ppm lead, 5 ppm cadmium, and 3 ppm mercury. Karl Fischer titration specifies water content below 0.5% w/w. Ash content determination yields less than 0.1% residue on ignition. Stability testing indicates shelf life of 24 months when stored in amber glass containers at 4°C under nitrogen atmosphere. Accelerated stability testing at 40°C and 75% relative humidity shows degradation of 1.2% per month.

Applications and Uses

Industrial and Commercial Applications

Tirpate found primary application as a soil nematicide in agricultural settings, particularly for control of root-knot nematodes (Meloidogyne species) in vegetable crops and fruit orchards. Application rates typically ranged from 5-10 kg active ingredient per hectare, incorporated into soil before planting. The compound demonstrated efficacy through inhibition of acetylcholinesterase in nematodes, with EC₅₀ values of 0.8-2.3 mg/L against various species. Secondary uses included incorporation into polymer matrices for controlled-release formulations and combination with other pesticides for broad-spectrum soil treatment. Market production reached maximum capacity in the 1980s with annual global production estimated at 500-700 metric tons. Economic factors including regulatory restrictions and development of resistance led to gradual phase-out of commercial production. Current uses are restricted to research applications and specialized agricultural situations where alternative nematicides are ineffective.

Historical Development and Discovery

Tirpate was first synthesized in 1968 during systematic investigation of sulfur-containing heterocycles as agricultural chemicals. Initial patent protection was granted to researchers at a major chemical company seeking alternatives to existing carbamate nematicides. The discovery emerged from structure-activity relationship studies demonstrating that incorporation of dithiolane rings enhanced nematicidal activity compared to simpler carbamates. Development throughout the 1970s focused on optimization of synthetic routes and formulation strategies. Environmental concerns regarding soil persistence and potential groundwater contamination led to increased regulatory scrutiny in the 1980s. The United States Environmental Protection Agency classified tirpate as an extremely hazardous substance in 1987 under the Emergency Planning and Community Right-to-Know Act. Manufacturing discontinuation occurred gradually through the 1990s as newer compounds with improved environmental profiles replaced tirpate in agricultural practice. The compound remains historically significant as an example of structure-based pesticide design incorporating heterocyclic sulfur systems.

Conclusion

Tirpate represents a chemically sophisticated organosulfur compound that combines dithiolane, oxime, and carbamate functionalities within a single molecular architecture. Its structural features confer unique physical properties including crystalline solid state behavior, limited aqueous solubility, and moderate lipophilicity. The compound's reactivity is dominated by hydrolysis pathways, particularly under alkaline conditions, and oxidation at sulfur centers. Although commercial applications have declined due to environmental and regulatory considerations, tirpate remains chemically significant as a model system for studying heterocyclic sulfur chemistry and structure-activity relationships in pesticidal compounds. The synthetic methodology developed for tirpate preparation continues to inform strategies for constructing complex sulfur-containing molecules. Future research directions may explore modified analogs with reduced environmental persistence while maintaining biological activity, as well as applications in materials science leveraging the compound's unique electronic properties derived from its sulfur heterocycle system.