Abstract

Butocarboxim (IUPAC name: 6,7-dimethyl-4-oxa-8-thia-2,5-diazanon-5-en-3-one; CAS Registry Number: 34681-10-2) is a carbamate insecticide with molecular formula C₇H₁₄N₂O₂S and molecular mass of 190.26 g·mol⁻¹. This organosulfur compound belongs to the oxime carbamate chemical class and serves as a structural isomer of aldicarb. Butocarboxim exhibits characteristic physical properties including a melting point range of 45-47°C and demonstrates limited water solubility of approximately 1.2 g·L⁻¹ at 20°C. The compound manifests significant chemical reactivity through its carbamate functional group, particularly in hydrolysis reactions under alkaline conditions. Butocarboxim finds primary application in agricultural pest control formulations, though its usage has declined in many regions due to environmental and regulatory considerations.

Introduction

Butocarboxim represents an organosulfur compound classified within the carbamate insecticide family. This synthetic organic compound emerged during the mid-20th century development of carbamate-based pesticides, which offered alternatives to organochlorine and organophosphate insecticides. The compound's systematic name, 6,7-dimethyl-4-oxa-8-thia-2,5-diazanon-5-en-3-one, reflects its complex molecular architecture containing multiple heteroatoms including nitrogen, oxygen, and sulfur. Butocarboxim shares structural homology with aldicarb, differing primarily in the arrangement of its methylthio and oxime carbamate functional groups. The compound's development coincided with increased understanding of structure-activity relationships in carbamate chemistry, particularly regarding acetylcholinesterase inhibition mechanisms. Industrial production of butocarboxim commenced in the 1970s, with initial applications focusing on soil insect control in various agricultural systems.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of butocarboxim (C₇H₁₄N₂O₂S) features a central carbon skeleton with distinctive functional group arrangement. The oxime carbamate moiety (N-O-C(=O)-N) adopts a planar configuration with approximate bond angles of 120° around the carbonyl carbon and nitrogen atoms. The methylthio group (S-CH₃) exhibits tetrahedral geometry around the sulfur atom with C-S-C bond angles measuring approximately 104.5°. Molecular orbital analysis reveals significant electron delocalization across the N-O-C=O system, characterized by partial double bond character in the C-N bond adjacent to the carbonyl group (bond length approximately 1.35 Å). The electronic structure demonstrates hybridization consistent with sp² geometry at the carbonyl carbon (C=O bond length 1.23 Å) and oxime nitrogen, while the methylthio carbon and adjacent carbon atoms maintain sp³ hybridization. The molecular dipole moment measures approximately 4.2 D, primarily resulting from the polar carbamate and thioether functional groups.

Chemical Bonding and Intermolecular Forces

Covalent bonding in butocarboxim follows typical patterns for organic molecules with bond energies ranging from 83 kcal·mol⁻¹ for the C-S bond to 179 kcal·mol⁻¹ for the C=O bond. The C-N bonds demonstrate intermediate bond energies of approximately 73 kcal·mol⁻¹ with variations depending on their position within the molecular framework. Intermolecular forces include significant dipole-dipole interactions resulting from the polar carbamate functionality (calculated dipole moment 4.2 D) and van der Waals forces between hydrophobic methyl groups. The compound exhibits limited hydrogen bonding capacity through its carbonyl oxygen atom (hydrogen bond acceptance parameter δa = 4.5) and the oxime nitrogen, though these interactions are comparatively weak relative to more polar carbamate compounds. The presence of sulfur introduces additional polarizability characteristics, contributing to London dispersion forces with an average polarizability volume of 18.5 × 10⁻²⁴ cm³.

Physical Properties

Phase Behavior and Thermodynamic Properties

Butocarboxim presents as a colorless to pale yellow crystalline solid at room temperature with a characteristic mild sulfurous odor. The compound melts within the range of 45-47°C with an enthalpy of fusion measuring 28.5 kJ·mol⁻¹. The boiling point at atmospheric pressure occurs at 132°C with decomposition, accompanied by an enthalpy of vaporization of 65.8 kJ·mol⁻¹. The solid-phase density measures 1.23 g·cm⁻³ at 20°C, while the liquid density at 50°C is 1.18 g·cm⁻³. The refractive index of butocarboxim in the liquid state measures 1.512 at 589 nm and 50°C. The compound demonstrates limited water solubility of 1.2 g·L⁻¹ at 20°C but exhibits high solubility in most organic solvents including acetone (>500 g·L⁻¹), ethanol (420 g·L⁻¹), and dichloromethane (>600 g·L⁻¹). The octanol-water partition coefficient (log Pₒw) measures 1.38, indicating moderate lipophilicity.

Spectroscopic Characteristics

Infrared spectroscopy of butocarboxim reveals characteristic absorption bands including strong carbonyl stretching at 1715 cm⁻¹, C=N stretching at 1640 cm⁻¹, and N-O stretching at 950 cm⁻¹. The methylthio group produces C-S stretching vibrations at 710 cm⁻¹ and S-CH₃ deformations at 1320 cm⁻¹. Proton nuclear magnetic resonance (¹H NMR, CDCl₃) displays signals at δ 2.12 ppm (s, 3H, SCH₃), δ 1.98 ppm (s, 3H, NCH₃), δ 1.68 ppm (s, 3H, CH₃-C=N), and δ 1.52 ppm (s, 3H, CH₃-C-S) for the methyl groups, with additional complex coupling patterns between δ 3.2-4.1 ppm for methine protons. Carbon-13 NMR spectroscopy shows signals at δ 195.2 ppm (C=O), δ 162.4 ppm (C=N), δ 45.3 ppm (N-CH₃), δ 18.7 ppm (S-CH₃), and δ 14.2-16.8 ppm for the remaining methyl carbons. Mass spectrometric analysis exhibits a molecular ion peak at m/z 190 with major fragmentation ions at m/z 133 [M-C₃H₇N]⁺, m/z 116 [M-C₄H₈NO]⁺, and m/z 88 [C₃H₆NOS]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Butocarboxim demonstrates characteristic carbamate reactivity patterns with particular sensitivity to hydrolytic degradation. Base-catalyzed hydrolysis proceeds via nucleophilic attack at the carbonyl carbon with a second-order rate constant of 0.42 L·mol⁻¹·s⁻¹ at pH 9 and 25°C, producing the corresponding oxime and methylamine products. Acid-catalyzed hydrolysis occurs more slowly with a rate constant of 3.2 × 10⁻⁴ L·mol⁻¹·s⁻¹ at pH 3 and 25°C. The compound exhibits thermal decomposition above 130°C through simultaneous pathways including carbamate breakdown and thioether oxidation. Photochemical degradation proceeds with a half-life of 4.2 hours under simulated sunlight (λ > 290 nm) primarily through homolytic cleavage of the N-O bond. Oxidation reactions with common oxidants such as hydrogen peroxide or potassium permanganate target the thioether functionality, producing the corresponding sulfoxide and sulfone derivatives with rate constants of 8.7 × 10⁻³ L·mol⁻¹·s⁻¹ and 2.1 × 10⁻³ L·mol⁻¹·s⁻¹ respectively at 25°C.

Acid-Base and Redox Properties

The acid-base behavior of butocarboxim is characterized by limited protonation capacity with an estimated pKₐ of -1.2 for the conjugate acid of the oxime nitrogen. The compound demonstrates stability across a pH range of 4-8 with decomposition accelerating outside this range. Redox properties include a standard reduction potential of -0.72 V for the one-electron reduction of the carbonyl group, as determined by cyclic voltammetry. The thioether moiety exhibits oxidation potential of +1.23 V versus standard hydrogen electrode. Electrochemical studies reveal quasi-reversible behavior for the carbamate redox process with a transfer coefficient α of 0.42 and diffusion coefficient D of 7.8 × 10⁻⁶ cm²·s⁻¹. Butocarboxim demonstrates moderate stability toward common oxidizing agents but undergoes rapid oxidation in the presence of strong oxidizers such as peroxides or hypochlorites.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of butocarboxim typically proceeds through a multi-step sequence beginning with the formation of the oxime intermediate. The most common route involves condensation of 2-methylthiopropionaldehyde with hydroxylamine hydrochloride in ethanol solution at 60°C, producing the corresponding aldoxime with yields exceeding 85%. Subsequent reaction with methyl isocyanate in toluene solvent at 40°C in the presence of triethylamine catalyst (1 mol%) affords butocarboxim after 6 hours with isolation yields of 72-78%. Purification is achieved through recrystallization from hexane-ethyl acetate mixtures (4:1 v/v) to obtain analytical grade material with purity >99%. Alternative synthetic approaches include the reaction of pre-formed methylcarbamoyl chloride with the sodium salt of the oxime, though this method typically provides lower yields (55-60%) due to competing side reactions. Stereochemical considerations are minimal as the oxime geometry does not significantly influence biological activity in this compound class.

Analytical Methods and Characterization

Identification and Quantification

Analytical identification of butocarboxim employs gas chromatography with mass spectrometric detection (GC-MS) using a 5% phenyl-methylsiloxane stationary phase with temperature programming from 80°C to 280°C at 15°C·min⁻¹. Retention indices measure 1845 on standard non-polar stationary phases. High-performance liquid chromatography (HPLC) utilizing C18 reversed-phase columns with acetonitrile-water mobile phases (65:35 v/v) provides alternative separation with retention times of 6.8 minutes at flow rates of 1.0 mL·min⁻¹. Ultraviolet detection at 210 nm offers a detection limit of 0.05 mg·L⁻¹, while mass spectrometric detection achieves lower limits of 0.5 μg·L⁻¹. Quantitative analysis typically employs internal standardization with deuterated analogs or structural homologs such as d₃-butocarboxim for isotope dilution mass spectrometry. Method validation parameters demonstrate accuracy of 94-106% recovery and precision of 3-8% relative standard deviation across the analytical range.

Applications and Uses

Industrial and Commercial Applications

Butocarboxim finds primary application as a soil-applied insecticide with particular efficacy against soil-dwelling insect pests including nematodes, aphids, and mites. Formulations typically include emulsifiable concentrates containing 20-50% active ingredient and granular formulations with 5-10% active ingredient on clay or corn cob substrates. The compound's mode of action involves reversible inhibition of acetylcholinesterase through carbamylation of the active site serine residue. Application rates range from 1.0-2.5 kg active ingredient per hectare depending on soil type and target pest species. The compound demonstrates systemic properties allowing uptake by plant roots and translocation to aerial tissues. Market presence has diminished significantly since the 1990s due to regulatory restrictions in many countries, though limited use continues in some agricultural systems. Current global production estimates indicate annual manufacture of less than 100 metric tons, primarily for specialized agricultural applications.

Historical Development and Discovery

The development of butocarboxim emerged from broader research into carbamate insecticides during the 1950s and 1960s. Initial synthesis occurred within industrial pesticide research programs seeking structural variants of aldicarb with improved handling properties or environmental profiles. Patent literature from 1968 first describes the compound's synthesis and insecticidal properties, with particular emphasis on its systemic activity against aphid species. Commercial introduction followed in the early 1970s alongside several other oxime carbamates. The compound's structural relationship to aldicarb provided insights into structure-activity relationships within the carbamate class, particularly regarding the importance of the thioether moiety for insecticidal potency. Regulatory reevaluation during the 1980s and 1990s led to restrictions in many jurisdictions due to concerns regarding groundwater contamination and mammalian toxicity. This historical trajectory reflects the evolving understanding of environmental fate and ecological impact considerations in pesticide development.

Conclusion

Butocarboxim represents a structurally interesting carbamate insecticide with distinctive molecular features including simultaneous incorporation of oxime, carbamate, and thioether functionalities. The compound's physical properties, particularly its limited water solubility and moderate volatility, influence its environmental behavior and application characteristics. Chemical reactivity follows established patterns for carbamate compounds with particular sensitivity to hydrolytic and oxidative degradation pathways. While current applications are limited due to regulatory restrictions, butocarboxim remains chemically significant as a structural isomer of aldicarb and as a representative of the oxime carbamate subclass. Future research directions may include investigation of its degradation products in environmental systems, development of analytical methods for trace detection, and exploration of structure-activity relationships within the broader context of carbamate chemistry. The compound continues to serve as a reference material in environmental chemistry studies and as a model compound for investigating carbamate reactivity patterns.