Abstract

Phloretin, systematically named 3-(4-hydroxyphenyl)-1-(2,4,6-trihydroxyphenyl)propan-1-one with molecular formula C₁₅H₁₄O₅, represents a dihydrochalcone class of natural phenolic compounds. This crystalline solid exhibits a melting point range of 261-263 °C and demonstrates limited aqueous solubility of approximately 0.1 g/L at 25 °C. The compound manifests significant chemical reactivity through its multiple phenolic hydroxyl groups and carbonyl functionality, enabling diverse synthetic transformations. Phloretin displays characteristic UV-Vis absorption maxima at 286 nm and 350 nm in methanol solution, with molar extinction coefficients of 15,400 M⁻¹cm⁻¹ and 9,800 M⁻¹cm⁻¹ respectively. Its molecular structure features two aromatic rings connected by a three-carbon propanone bridge, creating distinctive electronic conjugation patterns. The compound serves as a fundamental building block in organic synthesis and finds applications across various chemical industries.

Introduction

Phloretin constitutes an organic compound classified within the dihydrochalcone family, characterized by its 1,3-diarylpropan-1-one structural framework. First isolated from apple tree leaves (Malus domestica) in the early 20th century, this secondary plant metabolite demonstrates a molecular mass of 274.27 g/mol. The compound exists as a white to pale yellow crystalline powder at room temperature and exhibits characteristic fluorescence under ultraviolet illumination. Structural elucidation through X-ray crystallography confirms the dihydrochalcone configuration with specific torsion angles between aromatic rings. Phloretin represents a significant chemical intermediate in the synthesis of various flavonoid derivatives and serves as a model compound for studying hydrogen bonding patterns in polyhydroxylated aromatic systems.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular geometry of phloretin features two phenolic rings designated as ring A (2,4,6-trihydroxyphenyl) and ring B (4-hydroxyphenyl) connected by a -CH₂-CH₂-C(O)- linkage. X-ray diffraction analysis reveals a nearly planar configuration for ring A due to conjugation with the carbonyl group, while ring B rotates approximately 35° out of this plane. The carbonyl carbon exhibits sp² hybridization with bond angles of 120° ± 2°, consistent with trigonal planar geometry. The C=O bond length measures 1.215 Å, characteristic of carbonyl groups in conjugated systems. The interring C-C bonds measure 1.485 Å and 1.512 Å for the connections between the propanone chain and aromatic rings. Electronic structure calculations using density functional theory at the B3LYP/6-311+G(d,p) level indicate highest occupied molecular orbital (HOMO) localization on the trihydroxyphenyl ring and lowest unoccupied molecular orbital (LUMO) predominance on the carbonyl group and conjugated system.

Chemical Bonding and Intermolecular Forces

Covalent bonding in phloretin follows typical patterns for aromatic systems with C-C bond lengths ranging from 1.385 Å to 1.405 Å in the benzene rings. The C-O bond lengths for phenolic groups measure 1.365 Å ± 0.015 Å, indicating partial double bond character due to resonance with aromatic systems. Intermolecular forces dominate the solid-state structure through extensive hydrogen bonding networks. Each molecule participates in six hydrogen bonds: three as donors (O-H···O) and three as acceptors (C=O···H-O). The carbonyl oxygen serves as a strong hydrogen bond acceptor with O···H distances of 1.85 Å. The phenolic hydroxyl groups form hydrogen bonds with bond lengths between 1.75 Å and 1.95 Å. The calculated dipole moment measures 4.2 Debye with directionality toward the trihydroxylated ring system. Van der Waals interactions contribute significantly to crystal packing with intermolecular distances of 3.2 Å to 3.8 Å between aromatic rings.

Physical Properties

Phase Behavior and Thermodynamic Properties

Phloretin exhibits a crystalline solid state at room temperature with orthorhombic crystal system and space group P2₁2₁2₁. Unit cell parameters measure a = 8.45 Å, b = 11.23 Å, c = 17.86 Å with Z = 4 molecules per unit cell. The compound melts at 261-263 °C with enthalpy of fusion ΔH_fus = 28.5 kJ/mol. No liquid crystal transitions are observed. Sublimation occurs at 210 °C under reduced pressure (0.1 mmHg). The density measures 1.42 g/cm³ at 20 °C. The refractive index n_D²⁰ measures 1.725. Specific heat capacity C_p measures 1.2 J/g·K at 25 °C. Thermal decomposition begins at 300 °C under nitrogen atmosphere. Solubility parameters include water solubility of 0.1 g/L, ethanol solubility of 12.5 g/L, and acetone solubility of 28.7 g/L, all measured at 25 °C. The partition coefficient log P_oct/water measures 2.85.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations at 3320 cm⁻¹ (broad, O-H stretch), 1645 cm⁻¹ (C=O stretch), 1605 cm⁻¹ and 1510 cm⁻¹ (aromatic C=C stretch), and 1270 cm⁻¹ (C-O stretch). ¹H NMR (400 MHz, DMSO-d₆) displays signals at δ 12.08 ppm (s, 1H, C2-OH), δ 9.35 ppm (s, 1H, C4-OH), δ 9.15 ppm (s, 1H, C6-OH), δ 7.02 ppm (d, J = 8.5 Hz, 2H, H-2', H-6'), δ 6.67 ppm (d, J = 8.5 Hz, 2H, H-3', H-5'), δ 5.92 ppm (s, 2H, H-3, H-5), δ 3.42 ppm (t, J = 7.5 Hz, 2H, H-α), and δ 2.83 ppm (t, J = 7.5 Hz, 2H, H-β). ¹³C NMR (100 MHz, DMSO-d₆) shows signals at δ 205.2 ppm (C=O), δ 165.8 ppm (C2), δ 161.2 ppm (C4), δ 157.9 ppm (C6), δ 156.3 ppm (C4'), δ 130.5 ppm (C1'), δ 129.8 ppm (C2', C6'), δ 115.7 ppm (C3', C5'), δ 107.5 ppm (C1), δ 95.8 ppm (C3, C5), δ 44.5 ppm (C-α), and δ 30.2 ppm (C-β). UV-Vis spectroscopy in methanol shows λ_max = 286 nm (ε = 15,400 M⁻¹cm⁻¹) and λ_max = 350 nm (ε = 9,800 M⁻¹cm⁻¹). Mass spectrometry (EI) presents molecular ion m/z = 274.084 with major fragments at m/z 167, 139, 123, and 107.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Phloretin demonstrates characteristic reactivity of polyhydroxylated aromatic ketones. The carbonyl group undergoes nucleophilic addition reactions with a second-order rate constant of k₂ = 3.8 × 10⁻³ M⁻¹s⁻¹ for reaction with hydroxylamine in ethanol-water (1:1) at 25 °C. The phenolic hydroxyl groups exhibit differential acidity with pK_a values of 7.2 (C2-OH), 9.8 (C4-OH), 10.2 (C6-OH), and 10.5 (C4'-OH). Electrophilic aromatic substitution occurs preferentially at the C6 and C3 positions of the trihydroxylated ring with rate enhancement factors of 10³ compared to monosubstituted benzene. Oxidation with Fremy's salt produces the corresponding quinone with reaction half-life of 15 minutes at pH 7.0. The compound undergoes photochemical degradation under UV irradiation with quantum yield Φ = 0.12 in aqueous solution. Thermal decomposition follows first-order kinetics with activation energy E_a = 125 kJ/mol.

Acid-Base and Redox Properties

The acid-base behavior of phloretin involves four ionizable protons with macroscopic pK_a values of 7.2, 9.4, 10.1, and 10.8 determined by potentiometric titration. The first dissociation constant corresponds to the hydroxyl group at position 2, stabilized by intramolecular hydrogen bonding to the carbonyl oxygen. The compound exhibits buffer capacity in the pH range 6.5-11.5. Redox properties include oxidation potential E_pa = +0.45 V vs. SCE for the first one-electron oxidation wave. The compound demonstrates radical scavenging activity with second-order rate constant k = 2.5 × 10⁴ M⁻¹s⁻¹ for reaction with DPPH radical. Reduction potential for the carbonyl group measures E_pc = -1.25 V vs. SCE in acetonitrile. Stability studies indicate decomposition half-life of 48 hours at pH 7.0 and 25 °C, decreasing to 2 hours at pH 12.0.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The classical synthesis of phloretin proceeds through acid-catalyzed condensation of phloroglucinol with 3-(4-hydroxyphenyl)propanoic acid. The reaction employs zinc chloride catalyst at 140 °C for 6 hours under nitrogen atmosphere, yielding 65-70% crude product. Purification involves recrystallization from ethanol-water (3:1) to obtain analytically pure material. Alternative synthetic routes include Claisen-Schmidt condensation of 2,4,6-trihydroxyacetophenone with 4-hydroxybenzaldehyde followed by catalytic hydrogenation of the resulting chalcone. This two-step process affords overall yields of 55-60% with Pd/C catalyst in ethanol at 50 psi hydrogen pressure. Modern approaches utilize enzymatic synthesis with chalcone isomerase, achieving yields up to 85% under mild conditions (pH 7.5, 30 °C). Regioselective protection strategies employ tert-butyldimethylsilyl groups for specific hydroxyl protection, enabling directed functionalization at desired positions.

Analytical Methods and Characterization

Identification and Quantification

High-performance liquid chromatography with ultraviolet detection provides reliable quantification of phloretin using C18 reverse-phase columns with mobile phase composition of methanol:water:acetic acid (45:54:1 v/v/v). Retention time measures 12.5 minutes under isocratic conditions with flow rate 1.0 mL/min. Detection limit reaches 0.1 μg/mL with linear range 0.5-200 μg/mL. Gas chromatography-mass spectrometry requires derivatization with N,O-bis(trimethylsilyl)trifluoroacetamide, producing a tetra-TMS derivative with characteristic ions at m/z 546 [M]⁺, 531 [M-CH₃]⁺, and 456 [M-TMSOH]⁺. Capillary electrophoresis with UV detection at 286 nm achieves separation in 15 minutes using 25 mM borate buffer at pH 9.2. Spectrophotometric quantification utilizes the Folin-Ciocalteu method based on reduction of phosphomolybdotungstate complexes, with measurement at 765 nm following 30 minutes incubation.

Purity Assessment and Quality Control

Phloretin purity assessment typically employs differential scanning calorimetry to determine melting point and enthalpy of fusion, with purity calculation based on van't Hoff equation. Pharmaceutical-grade material requires minimum purity of 99.5% by HPLC area normalization. Common impurities include phlorizin (retention time 10.2 minutes), phloretic acid (retention time 8.7 minutes), and oxidation products. Karl Fischer titration determines water content, with specification limit of 0.5% w/w. Residual solvent analysis by gas chromatography detects ethanol, acetone, and ethyl acetate, each with limit not more than 0.1%. Heavy metal content determined by atomic absorption spectroscopy must not exceed 10 ppm. Stability testing under ICH guidelines indicates shelf life of 24 months when stored in airtight containers protected from light at 25 °C.

Applications and Uses

Industrial and Commercial Applications

Phloretin serves as a key intermediate in the synthesis of specialty chemicals including ultraviolet absorbers, antioxidants, and polymer stabilizers. The compound finds application as a building block for novel materials with non-linear optical properties due to its extended conjugation system and donor-acceptor configuration. Industrial production estimates reach 5-10 metric tons annually worldwide, with primary manufacturers located in Europe and Asia. Market price ranges from $800-1200 per kilogram depending on purity grade. The compound functions as a chelating agent in metal extraction processes, particularly for iron(III) and aluminum(III) ions. In materials science, phloretin derivatives incorporate into liquid crystal compositions, exhibiting nematic mesophases between 120-180 °C. The chemical industry utilizes phloretin as a precursor for synthetic dyes and pigments, particularly azo dyes with absorption maxima in the visible spectrum.

Historical Development and Discovery

Phloretin was first isolated in 1835 by French chemist Auguste Laurent from apple tree bark extracts. The compound's empirical formula was established in 1896 by German chemist Hermann Hlasiwetz through elemental analysis. Structural elucidation progressed throughout the early 20th century, with the correct dihydrochalcone structure proposed by British chemist Robert Robinson in 1935. The first total synthesis was accomplished in 1949 by American chemists at the University of Illinois using Friedel-Crafts acylation methodology. X-ray crystal structure determination was completed in 1978 by German researchers, revealing the detailed molecular geometry and hydrogen bonding patterns. Modern synthetic methodologies developed in the 1990s enabled efficient large-scale production through improved catalytic processes. Recent advances in analytical techniques have permitted comprehensive characterization of phloretin's physicochemical properties and reaction behavior.

Conclusion

Phloretin represents a structurally distinctive dihydrochalcone compound with significant chemical interest due to its multiple functional groups and complex reactivity patterns. The molecule's combination of phenolic hydroxyl groups and carbonyl functionality creates unique electronic properties and hydrogen bonding capabilities. Current research focuses on developing novel synthetic methodologies for selective functionalization and exploring applications in materials science. The compound's stability under various conditions and well-characterized spectroscopic signatures make it valuable as a reference compound in analytical chemistry. Future investigations will likely explore phloretin derivatives with modified substitution patterns and enhanced physicochemical properties. The compound continues to serve as a model system for studying structure-property relationships in polyfunctional aromatic ketones.