Abstract

Edifenphos (C₁₄H₁₅O₂PS₂), systematically named O-ethyl S,S-diphenyl phosphorodithioate, represents an organophosphorus compound of significant agricultural importance. This phosphorodithioate ester exhibits a molecular mass of 310.37 g/mol and manifests as a pale yellow to brown liquid at ambient temperature with a characteristic aromatic odor. The compound demonstrates limited aqueous solubility of 56 mg/L at 20°C but shows good solubility in most organic solvents. Edifenphos functions as a systemic fungicide through inhibition of phosphatidylcholine biosynthesis in fungal pathogens. First synthesized in 1966, this compound maintains relevance in specific agricultural applications despite regulatory restrictions in certain jurisdictions due to its toxicological profile.

Introduction

Edifenphos belongs to the organophosphorus class of compounds, specifically categorized as a phosphorodithioate ester. The compound emerged from agricultural chemical research in the mid-1960s as part of efforts to develop systemic fungicides for rice cultivation. Bayer AG introduced edifenphos commercially in 1966 under the trade name Hinosan® for control of Pyricularia oryzae (rice blast fungus) and Pellicularia sasakii in rice crops. The molecular structure features a central phosphorus atom bonded to two sulfur atoms, one oxygen atom, and an ethyl group, creating an asymmetric tetrahedral coordination geometry. This structural arrangement confers specific reactivity patterns that underlie both its biological activity and chemical behavior.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of edifenphos centers around a phosphorus atom in tetrahedral coordination. Bond angles approximate the ideal tetrahedral angle of 109.5°, though slight distortions occur due to differences in ligand electronegativity. The phosphorus atom exhibits sp³ hybridization, with bond lengths measuring approximately 1.98 Å for P-S bonds, 1.77 Å for P=O bond, and 1.81 Å for P-O bond. The molecular point group symmetry is C₁, indicating no symmetry elements beyond identity. Electron distribution shows significant polarization of the P=O bond with a dipole moment of approximately 2.5 D, while the P-S bonds demonstrate partial double bond character due to dπ-pπ backbonding from sulfur to phosphorus.

Chemical Bonding and Intermolecular Forces

Covalent bonding in edifenphos follows typical patterns for organophosphorus compounds with tetracoordinate phosphorus. The P=O bond demonstrates substantial ionic character estimated at 40%, while P-S bonds show approximately 15% ionic character. Intermolecular forces include London dispersion forces between aromatic rings, with estimated interaction energies of 5-8 kJ/mol. Dipole-dipole interactions between polarized P=O groups contribute additional stabilization of approximately 10-15 kJ/mol. The compound lacks significant hydrogen bonding capacity due to absence of hydrogen bond donors, though weak C-H···O interactions may occur with bond energies below 4 kJ/mol.

Physical Properties

Phase Behavior and Thermodynamic Properties

Edifenphos presents as a viscous liquid at standard temperature and pressure, with a pale yellow to brown appearance depending on purity. The compound exhibits a melting point of -25°C and boiling point of approximately 180°C at 1 mmHg. Density measures 1.23 g/cm³ at 20°C, with a refractive index of 1.61. Vapor pressure remains low at 1.2 × 10⁻⁴ mmHg at 25°C. Thermodynamic parameters include heat of vaporization of 65 kJ/mol and heat capacity of 350 J/mol·K. The compound demonstrates limited thermal stability, beginning decomposition at temperatures above 150°C.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 1265 cm⁻¹ (P=O stretch), 970 cm⁻¹ (P-O-C stretch), and 650 cm⁻¹ (P-S stretch). Proton NMR spectroscopy shows signals at δ 1.35 ppm (triplet, 3H, CH₃), δ 4.20 ppm (quartet, 2H, OCH₂), and δ 7.30-7.50 ppm (multiplet, 10H, aromatic). Phosphorus-31 NMR displays a singlet at δ 58 ppm relative to 85% H₃PO₄. Carbon-13 NMR exhibits signals at δ 16.1 ppm (CH₃), δ 64.3 ppm (OCH₂), and aromatic carbons between δ 127-133 ppm. Mass spectrometry demonstrates molecular ion peak at m/z 310 with characteristic fragmentation patterns including loss of C₂H₄ (m/z 282), C₆H₅S (m/z 217), and C₆H₅ (m/z 233).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Edifenphos demonstrates reactivity characteristic of phosphorodithioate esters. Hydrolysis represents the primary degradation pathway, proceeding through both acid-catalyzed and base-catalyzed mechanisms. Alkaline hydrolysis occurs with second-order rate constants of 0.15 M⁻¹s⁻¹ at pH 9 and 25°C, following SN₂(P) mechanism with hydroxide attack at phosphorus. Acid-catalyzed hydrolysis proceeds more slowly with rate constants of 0.003 M⁻¹s⁻¹ at pH 5. Thermal decomposition follows first-order kinetics with activation energy of 85 kJ/mol, producing ethyl mercaptan, diphenyl disulfide, and phosphoric acid derivatives. Oxidation reactions occur readily with common oxidants, converting P-S bonds to P-O bonds.

Acid-Base and Redox Properties

The compound exhibits minimal acid-base character in aqueous solution, with no ionizable protons in the pH range 2-12. Redox behavior demonstrates reduction potential of -0.75 V versus standard hydrogen electrode for the P(V)/P(III) couple. Electrochemical reduction proceeds via two-electron transfer mechanism with cleavage of P-S bonds. Oxidation potential measures +1.2 V versus SHE for formation of sulfoxide derivatives. The compound remains stable under reducing conditions but undergoes rapid oxidation in the presence of atmospheric oxygen over extended periods.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of edifenphos typically proceeds via reaction of O,O-diethyl phosphorodithioate with thiophenol. The method involves preparation of sodium salt of thiophenol (10.0 g, 0.09 mol) in ethanol (100 mL), followed by addition of O,O-diethyl phosphorodithioate (16.5 g, 0.09 mol) with stirring at 0°C. Reaction proceeds for 4 hours with gradual warming to room temperature. After removal of ethanol under reduced pressure, the residue undergoes extraction with dichloromethane and washing with water. Purification by vacuum distillation yields edifenphos as a pale yellow liquid with typical yields of 75-80%. Alternative routes employ phosphorus oxychloride as starting material, proceeding through sequential reactions with ethanol and thiophenol.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame photometric detection provides sensitive determination of edifenphos with detection limits of 0.01 mg/L. Capillary columns with non-polar stationary phases (5% phenyl methylpolysiloxane) achieve separation with retention times of 12.3 minutes under temperature programming from 150°C to 280°C at 10°C/min. High-performance liquid chromatography with UV detection at 254 nm offers alternative quantification with limits of detection at 0.05 mg/L using C18 reverse-phase columns and acetonitrile-water mobile phases. Confirmatory analysis employs GC-MS with electron impact ionization, monitoring characteristic ions at m/z 310, 282, 233, and 217.

Purity Assessment and Quality Control

Purity assessment typically employs gas chromatography with internal standardization, requiring minimum purity of 95% for technical grade material. Common impurities include O,O-diethyl S-phenyl phosphorothioate (3-5%), diphenyl disulfide (1-2%), and diethyl phosphate (0.5-1%). Quality control specifications limit water content to 0.5% maximum and acidity (as H₂SO₄) to 0.3% maximum. Accelerated stability testing at 54°C for 14 days establishes shelf-life under normal storage conditions.

Applications and Uses

Industrial and Commercial Applications

Edifenphos serves primarily as a systemic fungicide in agricultural applications, specifically for control of rice blast disease caused by Pyricularia oryzae. Application rates typically range from 500-1000 g active ingredient per hectare, applied as foliar spray or granular formulation. The compound demonstrates translaminar movement within plant tissues, providing protective and curative action against fungal pathogens. Secondary applications include control of Rhizoctonia solani in various crops. Commercial formulations contain 30-50% active ingredient in emulsifiable concentrate or wettable powder forms.

Historical Development and Discovery

Development of edifenphos originated at Bayer AG's agricultural research facilities in Germany during the early 1960s. Researchers sought systemic fungicides with improved activity against rice pathogens, building upon earlier work with phosphorothioate compounds. Initial synthesis occurred in 1965, with patent protection granted in 1966 (German Patent 1,543,257). Commercial introduction followed in 1968 under the trade name Hinosan®. The compound represented one of the first phosphorodithioate fungicides with systemic activity, establishing a structural template for subsequent developments in agricultural chemistry. Despite its early promise, regulatory concerns regarding mammalian toxicity limited widespread adoption in many markets.

Conclusion

Edifenphos occupies a significant position in the history of organophosphorus fungicides, demonstrating the application of phosphorodithioate chemistry to agricultural problems. The compound's molecular architecture, featuring tetracoordinate phosphorus with mixed oxygen-sulfur ligation, confers specific physicochemical properties that underlie both its biological activity and environmental behavior. While current usage remains limited due to toxicological concerns, edifenphos continues to serve as a reference compound for studies of phosphorodithioate reactivity and degradation pathways. Further research may explore structural analogs with improved selectivity and reduced environmental impact, potentially building upon the established structure-activity relationships of this chemical class.