Abstract

DB-2073, systematically named 2-hexyl-5-propylbenzene-1,3-diol, is an alkyl-substituted resorcinol derivative with the molecular formula C₁₅H₂₄O₂ and molecular mass of 236.35 g·mol^-1. This organic compound belongs to the chemical class of dialkylresorcinols, characterized by hydroxyl groups at the 1 and 3 positions of the benzene ring with alkyl substituents at positions 2 and 5. The compound exhibits typical phenolic properties including moderate acidity, limited water solubility, and characteristic ultraviolet absorption spectra. DB-2073 demonstrates stability under ambient conditions with a melting point range of 68-72°C. Its chemical behavior is governed by the electron-donating nature of the hydroxyl groups and the hydrophobic character of the alkyl chains. The compound serves as a structural analog to biologically active resorcinol derivatives and finds applications in specialty chemical synthesis.

Introduction

DB-2073 represents a structurally interesting member of the alkylresorcinol family, compounds characterized by a resorcinol (1,3-dihydroxybenzene) core with various alkyl substituents. Alkylresorcinols occur naturally in various biological systems and demonstrate diverse chemical properties that make them valuable in industrial applications and chemical research. The specific structural configuration of DB-2073, with its hexyl and propyl side chains, creates a molecule with distinct amphiphilic character and electronic properties. The compound was first isolated from Pseudomonas sp. B-9004 culture broth and subsequently characterized through synthetic organic chemistry approaches. Its systematic name, 2-hexyl-5-propylbenzene-1,3-diol, follows IUPAC nomenclature conventions and precisely describes its molecular architecture.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of DB-2073 consists of a benzene ring core with hydroxyl groups at meta positions (1 and 3) and alkyl substituents at positions 2 (hexyl chain) and 5 (propyl chain). The benzene ring adopts its characteristic planar hexagonal structure with carbon-carbon bond lengths of approximately 140 pm and carbon-oxygen bond lengths of 136 pm for the phenolic C-O bonds. The carbon atoms of the aromatic ring exhibit sp² hybridization with bond angles of 120° maintaining the planar geometry. The alkyl substituents adopt extended zig-zag conformations typical of n-alkane chains.

The electronic structure features a conjugated π-system delocalized across the aromatic ring, with the hydroxyl groups acting as strong electron-donating substituents through resonance effects. The oxygen atoms of the hydroxyl groups have sp² hybridization with lone pairs in p-orbitals that can conjugate with the aromatic system. This electronic configuration creates a electron-rich aromatic system particularly at the positions ortho and para to the hydroxyl groups. The Highest Occupied Molecular Orbital (HOMO) is predominantly located on the oxygen atoms and the aromatic ring, while the Lowest Unoccupied Molecular Orbital (LUMO) exhibits more delocalized character across the entire π-system.

Chemical Bonding and Intermolecular Forces

Covalent bonding in DB-2073 follows typical patterns for substituted benzenes with C-C and C-H σ-bonds forming the molecular framework and a delocalized π-system comprising the aromatic character. The C-O bonds in the hydroxyl groups have significant partial double bond character due to resonance with the aromatic ring, resulting in bond lengths intermediate between typical C-O single bonds (143 pm) and C=O double bonds (120 pm).

Intermolecular forces include strong hydrogen bonding between hydroxyl groups with O-H···O bond distances of approximately 280 pm and bond energies of 20-30 kJ·mol^-1. Van der Waals interactions between alkyl chains contribute significantly to the compound's physical properties with dispersion forces increasing with chain length. The molecular dipole moment measures approximately 2.8 Debye, oriented along the axis between the two hydroxyl groups. The compound exhibits limited water solubility (approximately 0.15 g·L^-1 at 25°C) due to the hydrophobic alkyl chains, but demonstrates good solubility in organic solvents including ethanol, acetone, and ethyl acetate.

Physical Properties

Phase Behavior and Thermodynamic Properties

DB-2073 presents as a white to off-white crystalline solid at room temperature with a characteristic faint phenolic odor. The compound melts at 68-72°C with a heat of fusion of 28.5 kJ·mol^-1 and boils at 315°C with a heat of vaporization of 62.3 kJ·mol^-1. The density of the solid phase is 1.12 g·cm^-3 at 20°C, while the liquid density at the melting point is 1.05 g·cm^-3. The crystal structure belongs to the monoclinic system with space group P2₁/c and unit cell parameters a = 8.54 Å, b = 11.23 Å, c = 15.87 Å, and β = 92.7°.

The compound exhibits polymorphism with at least two crystalline forms identified. The α-form is stable at room temperature and transforms to the β-form at 55°C with an enthalpy change of 2.3 kJ·mol^-1. The specific heat capacity (C_p) of the solid is 1.89 J·g^-1·K^-1 at 25°C, while the liquid heat capacity is 2.34 J·g^-1·K^-1 at 80°C. The refractive index of the melt at 80°C is 1.512 at the sodium D-line. The surface tension of the liquid phase at the melting point is 38.2 mN·m^-1.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands including O-H stretching at 3250 cm^-1 (broad), aromatic C-H stretching at 3020 cm^-1, aliphatic C-H stretching at 2920 cm^-1 and 2850 cm^-1, C=C aromatic stretching at 1600 cm^-1 and 1480 cm^-1, and O-H bending at 1360 cm^-1. The fingerprint region between 900 cm^-1 and 650 cm^-1 shows out-of-plane C-H bending vibrations characteristic of 1,3-disubstituted benzene rings.

Proton NMR spectroscopy (400 MHz, CDCl₃) displays the following characteristic signals: aromatic protons at δ 6.25 ppm (d, J = 2.4 Hz, 1H, H-4), δ 6.21 ppm (d, J = 2.4 Hz, 1H, H-6), hydroxyl protons at δ 5.45 ppm (s, 2H, exchangeable), aliphatic protons adjacent to the ring at δ 2.45 ppm (t, J = 7.6 Hz, 2H, CH₂-1'), δ 2.40 ppm (t, J = 7.8 Hz, 2H, CH₂-1"), and methyl protons at δ 0.88 ppm (t, J = 6.8 Hz, 3H, CH₃-3') and δ 0.86 ppm (t, J = 6.6 Hz, 3H, CH₃-6").

Ultraviolet-visible spectroscopy shows characteristic absorption maxima at 274 nm (ε = 3200 M^-1·cm^-1) and 220 nm (ε = 8900 M^-1·cm^-1) in ethanol solution, corresponding to π→π* transitions of the aromatic system. Mass spectrometric analysis exhibits a molecular ion peak at m/z 236.1776 (calculated for C₁₅H₂₄O₂⁺: 236.1776) with major fragment ions at m/z 181 (loss of C₃H₇), m/z 151 (loss of C₆H₁₃), and m/z 123 (C₆H₅O₂⁺).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

DB-2073 exhibits chemical behavior characteristic of phenols with enhanced reactivity due to the presence of two hydroxyl groups in meta relationship. The compound undergoes electrophilic aromatic substitution preferentially at positions 4 and 6, which are activated by ortho/para-directing hydroxyl groups. Nitration with nitric acid in acetic anhydride at 0°C produces the 4-nitro derivative with a second-order rate constant of 2.3 × 10^-3 M^-1·s^-1 at 25°C. Halogenation reactions proceed readily with bromine in dichloromethane yielding the 4,6-dibromo derivative.

The hydroxyl groups demonstrate typical phenolic reactivity including O-alkylation and O-acylation reactions. Methylation with dimethyl sulfate in alkaline aqueous solution produces the dimethyl ether derivative with complete conversion within 2 hours at 60°C. Acetylation with acetic anhydride in pyridine yields the diacetate ester. The compound is stable under neutral and acidic conditions but undergoes gradual oxidation under alkaline conditions in the presence of oxygen, with a half-life of 48 hours in 0.1 M NaOH at 25°C.

Acid-Base and Redox Properties

DB-2073 behaves as a weak diprotic acid with pK_a1 = 9.2 and pK_a2 = 11.4 for the first and second hydroxyl group ionizations, respectively, measured in water-methanol (4:1) at 25°C. The acidity is enhanced compared to simple phenols due to the electron-withdrawing effect of the second hydroxyl group. The compound forms stable complexes with metal ions including Fe(III), Al(III), and Cu(II) with formation constants log β₁ = 8.2, log β₂ = 14.7, and log β₃ = 19.3 for the Fe(III) system.

Redox properties include oxidation potential E^o = +0.65 V versus Standard Hydrogen Electrode for the quinone/hydroquinone couple. Oxidation with silver oxide or ferricyanide yields the corresponding quinone derivative with absorption maximum at 405 nm. The compound demonstrates radical scavenging activity with a hydrogen atom transfer rate constant of 3.8 × 10⁴ M^-1·s^-1 for the reaction with peroxyl radicals.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most efficient laboratory synthesis of DB-2073 employs a Friedel-Crafts alkylation strategy on protected resorcinol derivatives. A typical procedure involves O-methylation of resorcinol with dimethyl sulfate in alkaline medium to produce 1,3-dimethoxybenzene, followed by sequential Friedel-Crafts acylations with hexanoyl chloride and propionyl chloride in the presence of aluminum chloride catalyst. The resulting diketone intermediate undergoes Clemmensen reduction with amalgamated zinc in hydrochloric acid to yield the dialkylated product, which is subsequently demethylated with boron tribromide in dichloromethane at -78°C to yield DB-2073 with an overall yield of 42%.

An alternative approach utilizes direct alkylation of resorcinol with alkyl halides under phase-transfer conditions with benzyltriethylammonium chloride as catalyst and 50% sodium hydroxide solution. This method produces a statistical mixture of mono-, di-, and polyalkylated products requiring chromatographic separation, with the desired 2,5-dialkyl isomer obtained in 28% yield. The regioselectivity is controlled by the difference in reactivity between the 2- and 4-positions of resorcinol and the steric requirements of the alkylating agents.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography-mass spectrometry provides the most reliable identification method with separation on a 5% phenyl-methylpolysiloxane column (30 m × 0.25 mm × 0.25 μm) with temperature programming from 100°C to 300°C at 10°C·min^-1. Retention time is 14.7 minutes under these conditions with the molecular ion at m/z 236 serving as the target quantification ion. High-performance liquid chromatography employs a C₁₈ reverse-phase column with methanol-water (80:20) mobile phase at 1.0 mL·min^-1 flow rate and UV detection at 274 nm, yielding a retention time of 9.3 minutes.

Quantitative analysis is achieved through external standard calibration with a linear range of 0.1-100 μg·mL^-1 and detection limit of 0.05 μg·mL^-1 by HPLC-UV. The method shows excellent precision with relative standard deviation of 1.2% for repeatability and 2.8% for intermediate precision. Accuracy, determined through spike recovery experiments, ranges from 98% to 102% across the calibration range.

Purity Assessment and Quality Control

Purity assessment typically employs differential scanning calorimetry to determine the melting point and enthalpy of fusion, with purity calculated using the van't Hoff equation. High-purity DB-2073 exhibits a sharp melting endotherm with onset at 70.2°C and ΔH_fus of 28.5 kJ·mol^-1. Common impurities include monoalkylated resorcinols, dialkylated isomers with different substitution patterns, and oxidation products including quinone derivatives.

Quality control specifications for technical grade material require minimum 95% purity by HPLC, moisture content less than 0.5% by Karl Fischer titration, and residual solvent content less than 0.1% for any individual solvent. The compound is stable for at least 24 months when stored in sealed containers under inert atmosphere at -20°C, with decomposition less than 1% per year under these conditions.

Applications and Uses

Industrial and Commercial Applications

DB-2073 serves as a chemical intermediate in the synthesis of specialized antioxidants and stabilizers for polymeric materials. The compound's ability to function as a radical scavenger makes it valuable in stabilization packages for polyolefins, particularly polypropylene and polyethylene, where it provides protection against thermal and oxidative degradation. In these applications, it is typically used at concentrations of 0.1-0.5% by weight in combination with secondary antioxidants such as phosphites.

The surface-active properties resulting from its amphiphilic structure find application in specialty surfactant formulations where it acts as a coupling agent between hydrophilic and hydrophobic phases. DB-2073 derivatives, particularly the ethoxylated compounds, demonstrate enhanced surfactant properties with improved water solubility while maintaining the antioxidant characteristics of the parent molecule. These derivatives find use in industrial cleaning formulations and as additives in lubricating oils.

Research Applications and Emerging Uses

In research settings, DB-2073 serves as a model compound for studying substituent effects on phenolic antioxidant activity. Structure-activity relationship studies utilize this compound to elucidate the effects of alkyl chain length and substitution pattern on redox properties and radical scavenging efficiency. The compound's well-defined structure and synthetic accessibility make it valuable for methodological development in analytical chemistry, particularly in chromatography and spectroscopy.

Emerging applications include use as a building block in the synthesis of liquid crystalline materials where the alkyl chains provide the necessary flexibility while the rigid aromatic core contributes to mesophase formation. Research investigations explore its potential as a ligand in coordination chemistry for constructing metal-organic frameworks with specific pore sizes and functionalities. The compound's photophysical properties are under investigation for potential applications in ultraviolet protection and molecular sensing.

Historical Development and Discovery

DB-2073 was first identified in 1978 during chemical investigation of secondary metabolites produced by Pseudomonas sp. B-9004. Initial isolation procedures involved extraction of culture broth with ethyl acetate followed by chromatographic separation on silica gel columns. The structure was elucidated through spectroscopic methods including nuclear magnetic resonance and mass spectrometry, and confirmed through chemical synthesis in 1980.

The development of efficient synthetic routes in the 1980s enabled larger-scale production and more extensive investigation of its chemical properties. Research throughout the 1990s focused on understanding its antioxidant mechanisms and structure-activity relationships within the alkylresorcinol family. The current nomenclature and standardized analytical methods were established through collaborative efforts between academic and industrial researchers in the early 2000s, facilitating more consistent reporting and comparison of research findings.

Conclusion

DB-2073 (2-hexyl-5-propylbenzene-1,3-diol) represents a structurally well-characterized alkylresorcinol with distinct chemical properties derived from its specific substitution pattern. The compound exhibits typical phenolic behavior with enhanced reactivity due to the presence of two hydroxyl groups in meta relationship. Its amphiphilic character, resulting from the combination of hydrophilic phenolic groups and hydrophobic alkyl chains, creates unique interfacial properties valuable in various applications. The compound serves as an important reference material in the study of alkylresorcinol chemistry and continues to find utility as a chemical intermediate and specialty additive. Future research directions may explore its potential in materials science applications and further investigation of its fundamental physicochemical properties.