Orcinol

Orcinol, systematically named 5-methylbenzene-1,3-diol or 3,5-dihydroxytoluene, is a naturally occurring phenolic compound with the molecular formula C₇H₈O₂ and a molecular weight of 124.14 g/mol.^[1]^[2] It is a 5-alkylresorcinol derivative where the alkyl group is a methyl substituent, and it appears as white to light beige crystals or powder with a CAS number of 504-15-4.^[2]^[3] This compound is primarily found in various lichen species, such as Roccella tinctoria and Lecanora, where it serves as a precursor to phenolic methyl ethers and contributes to the biosynthesis of lichen metabolites like depsides and depsidones.^[4]^[5]^[6] It also occurs as a metabolite in some fungi.^[2] Orcinol has been isolated from natural sources and can be synthesized chemically, with applications spanning analytical chemistry, dye production, and biological research.^[7] In analytical chemistry, orcinol is a key reagent in Bial's test, where it reacts with furfural derived from pentoses under acidic conditions to produce a characteristic green-colored complex, enabling the detection of carbohydrates like ribose and deoxyribose.^[7]^[8] Industrially, it is utilized in the synthesis of the violet dye orcein, traditionally extracted from lichens for staining elastic fibers, cells, and chromosomes in histological preparations.^[9] Beyond these, orcinol derivatives show promise in proteomics research, as inhibitors of melanogenesis in biological models, in bone disorder treatments such as osteoclastogenesis inhibition, and in the development of pharmaceuticals with antioxidant, antimicrobial, and anxiolytic properties.^[10]^[11]^[3]^[12]

Chemical identity and properties

Structure and nomenclature

Orcinol is an organic compound with the molecular formula C₇H₈O₂ and CAS number 504-15-4.^[2] Its systematic structure consists of a benzene ring bearing two hydroxyl groups in a meta configuration and a methyl substituent, specifically as 5-methylbenzene-1,3-diol.^[13] This arrangement positions the hydroxyl moieties at carbon atoms 1 and 3 of the ring, with the methyl group attached to carbon 5, conferring the compound its characteristic phenolic properties.^[14] The preferred IUPAC name for orcinol is 5-methylbenzene-1,3-diol, reflecting its status as a dihydroxy derivative of toluene.^[13] Alternative systematic nomenclature includes 3,5-dihydroxytoluene, emphasizing the positions of the functional groups relative to the methyl substituent.^[15] Common trivial names encompass orcin and orcinol itself, the latter derived from its historical isolation from lichen extracts.^[15] Orcinol is structurally related to resorcinol (benzene-1,3-diol) as its 5-methylated analog, differing by the addition of a single methyl group that modifies its substitution pattern without altering the core meta-diol framework.^[13] As a symmetric, planar molecule lacking any chiral centers or axes of asymmetry, orcinol is achiral and exhibits no optical isomers.^[14] This absence of stereoisomerism simplifies its chemical behavior and spectroscopic characterization, aligning with its representation in standard databases via non-stereospecific identifiers such as the InChI string InChI=1S/C7H8O2/c1-5-2-6(8)4-7(9)3-5/h2-4,8-9H,1H3.^[14]

Physical and chemical properties

Orcinol is a colorless crystalline solid with a molar mass of 124.139 g/mol. It exhibits a melting point of 109 °C and a boiling point of 291 °C at standard pressure.^[16] The density of orcinol is 1.29 g/cm³.^[17] It is soluble in water (80 g/L), alcohols and ethers.^[17]

Property	Value
Molar mass	124.139 g/mol
Melting point	109 °C
Boiling point	291 °C
Density	1.29 g/cm³
Solubility in water	80 g/L

Orcinol appears as a crystalline solid and remains stable under normal laboratory conditions, though it is sensitive to oxidation upon exposure to air, potentially leading to discoloration.^[18] Chemically, orcinol behaves as a weak acid due to its two phenolic hydroxyl groups, with pKa ≈ 9.56.^[17] It functions as a reducing agent, capable of donating electrons in oxidative environments, akin to other polyphenols.^[3] Orcinol also possesses potential for keto-enol tautomerism, predominantly existing in the enol form under standard conditions. Spectroscopic characterization of orcinol includes UV absorption maxima near 274 nm in aqueous solution, attributable to π-π* transitions in the aromatic ring.^[1] In ¹H NMR spectra (in DMSO-d₆), the aromatic protons resonate at approximately δ 6.0 ppm, while the methyl group appears at δ 2.1 ppm.^[19]

Natural occurrence and biosynthesis

Occurrence in lichens

Orcinol is primarily sourced from various lichen species, where it serves as a foundational compound in secondary metabolism. Notable examples include Roccella tinctoria, which contains precursors like lecanoric acid, Lecanora parella and other Lecanora spp., and Usnea spp. such as Usnea longissima. These lichens produce orcinol as part of their phenolic metabolites, often in symbiotic associations between fungi and algae.^[7]^[20] In lichen extracts, orcinol and its derivatives can reach up to 20% of the total dry weight in some species, contributing to ecological adaptations such as UV protection by absorbing harmful radiation and antimicrobial activity against bacteria and fungi. For instance, orcinol exhibits antibacterial effects, with related compounds showing minimum inhibitory concentrations as low as 7.5 µg/mL. These roles enhance lichen survival in extreme environments.^[21]^[22]^[23] Historically, orcinol has been isolated from lichens through traditional methods involving ammonia fermentation of species like Roccella tinctoria, which converts precursors into orcinol-based dyes. A key biochemical process is the decarboxylation of orsellinic acid, a lichen acid, to yield orcinol, as demonstrated in enzymatic studies from lichens such as Lasallia pustulata. This decarboxylation is catalyzed by orsellinate decarboxylase.^[24] Biosynthetically, orcinol in lichens is derived from the polyketide pathway, where polyketide synthases (PKSs) assemble units from acetyl-CoA and malonyl-CoA in the fungal partner of the lichen symbiosis. This process forms orcinol rings that serve as building blocks for depsides and depsidones, with gene clusters like those encoding PKS16 identified in species such as Cladonia grayi. Recent studies (as of 2023) have identified additional polyketide synthase genes in lichens and microbes, enhancing understanding of orcinol's evolutionary conservation.^[21]^[25]^[26]

Occurrence in other organisms

Orcinol, primarily known from lichen sources, has also been identified in various non-lichen organisms, where it contributes to defensive and ecological functions. In certain ant species, such as Colobopsis saundersi (synonym Camponotus saundersi), orcinol forms a component of the toxic adhesive secretion, or "exploding glue," produced by minor workers during defensive rupture of their abdominal wall. This secretion immobilizes predators and pathogens through its sticky, polymerizing properties, enhancing colony protection in Southeast Asian rainforests. In plants, orcinol occurs in trace amounts in some mosses and higher plants, potentially aiding in chemical defense and allelopathic interactions. For instance, the moss Physcomitrella patens expresses 2'-oxoalkylresorcinol synthase (PpORS), a type III polyketide synthase that generates alkylresorcinol derivatives from acetyl-CoA, marking an early evolutionary step in plant phenolic metabolism. In higher plants like Rhododendron dauricum, orsellinic acid is biosynthesized by a specialized type III polyketide synthase, serving as a precursor to meroterpenoids such as daurichromenic acid, which exhibits anti-HIV activity. Orcinol can be derived from decarboxylation of orsellinic acid. These occurrences suggest orcinol's role in inhibiting competitor growth via allelopathy, though concentrations are typically low compared to lichen production.^[27]^[28]^[29] Microbial synthesis of orcinol extends beyond lichen symbioses, with production documented in free-living fungi and bacteria. Fungi such as Fusarium graminearum generate orsellinic acid via polyketide synthase PKS14, which decarboxylates to orcinol, contributing to fungal virulence. Similarly, Aspergillus fumigatus converts radiolabeled orsellinic acid to orcinol derivatives like fumigatol, highlighting polyketide assembly in non-symbiotic contexts.^[30]^[31] In bacteria, iterative type I polyketide synthases in species like Streptomyces facilitate orsellinic acid biosynthesis, a direct precursor to orcinol, underscoring microbial contributions to phenolic diversity.^[32] Evolutionarily, orcinol represents a foundational polyketide, derived from simple acetate units via type III synthases in plants and type I in microbes, serving as a precursor to complex phenolics like depsides and meroterpenoids across kingdoms. Its conservation highlights an ancient pathway for aromatic metabolite evolution, predating lichen associations and enabling adaptations in defense and signaling.^[26]

Synthesis

Laboratory synthesis

One classical laboratory method for synthesizing orcinol involves the treatment of dehydroacetic acid with barium hydroxide, leading to ring-opening of the pyrone structure to form a linear triketone intermediate, followed by cyclization and reduction under basic conditions to yield orcinol. This historical approach, first reported by Collie and Myers in 1893, proceeds by heating dehydroacetic acid in a concentrated solution of barium hydroxide, precipitating a barium salt of the opened intermediate, which upon acidification and extraction with solvents like chloroform and alcohol affords orcinol after recrystallization. The reaction occurs under basic conditions at elevated temperatures (boiling), with early experiments yielding low amounts, such as approximately 3% based on starting material.^[33] A common route to orcinol utilizes the selective replacement of amino groups with hydroxyl functionalities through diazotization of 3,5-diaminotoluene, derived from nitration and reduction of toluene or related precursors. The process begins with reduction of 3,5-dinitrotoluene using hydrogen and a palladium on carbon catalyst to obtain 3,5-diaminotoluene, which is then diazotized with sodium nitrite in acidic medium and hydrolyzed in the presence of copper sulfate by boiling in water or alcohol to introduce the hydroxyl groups at positions 3 and 5, directly yielding orcinol. This method operates under acidic conditions for diazotization at low temperatures (0-5°C) followed by heating to 100°C for hydrolysis, achieving overall yields around 55% with high purity (>99%).^[34] Modern laboratory syntheses often employ decarboxylation of orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), a straightforward thermal process that removes the carboxylic acid group to form orcinol. Orsellinic acid is heated in a high-boiling solvent such as glycerol or quinoline at 180-200°C, or under basic conditions with alkali, leading to clean decarboxylation and isolation of orcinol by extraction and distillation. This method provides high yields, typically 80-90%, and is favored for its simplicity in small-scale preparations.^[35]

Industrial production

Orcinol, also known as 5-methylresorcinol, is primarily produced on an industrial scale from Kukersite oil shale in Estonia, where it serves as the main water-soluble phenol in the derived shale oil. The primary industrial production of orcinol occurs through extraction from shale oil obtained from Kukersite oil shale in Estonia by Viru Keemia Grupp. The process begins with the pyrolysis of oil shale in retorts, such as the Kiviter or Petroter systems, yielding crude shale oil containing approximately 30% phenolic compounds.^[36]^[37] The phenolic fraction is then separated via liquid-liquid extraction using aqueous sodium hydroxide or other solvents to isolate the water-soluble phenols, followed by acidification to recover the crude phenols. Orcinol is subsequently purified from this fraction through fractional distillation, exploiting its boiling point of around 240–250°C, and crystallization to achieve a high degree of purity.^[38] Commercial products include high-purity anhydrous and monohydrate forms of 5-methylresorcinol, suitable for use as analytical reagents and raw materials in pharmaceuticals and dyes.^[38] Alternative industrial routes to orcinol, such as isolation from coal tar phenols or synthesis from petroleum-derived cresols, have been explored but are less common in modern production due to the efficiency and availability of the shale oil process.^[39] The shale-based method is economically advantageous, leveraging the abundant reserves of Kukersite shale, which supports cost-effective large-scale operations with commercial-grade orcinol exceeding 95% purity.^[38]

Reactions

Key chemical reactions

Orcinol, as a 1,3-dihydroxybenzene derivative, exhibits high reactivity toward electrophilic aromatic substitution due to the activating and ortho/para-directing effects of its two hydroxyl groups. The positions 2, 4, and 6 (numbered relative to the methyl group at position 5) are particularly activated, facilitating substitutions such as halogenation and nitration. For instance, bromination in aqueous solution yields mono-, di-, and tribrominated products primarily at these positions, with the reaction proceeding via electrophilic attack on the electron-rich aromatic ring.^[40] Similarly, chlorination of orcinol follows a comparable mechanism, leading to chlorinated derivatives at the activated sites, as observed in kinetic studies of phenolic chlorination.^[41] Nitration also occurs readily, producing polynitro derivatives like trinitro-orcinate under controlled conditions, highlighting the compound's susceptibility to multiple electrophilic attacks.^[42] The phenolic hydroxyl groups of orcinol enable classic reactions of alcohols and phenols, including esterification and etherification. Esterification involves reaction with carboxylic acids or their derivatives under acidic conditions to form esters, protecting the hydroxyl functionalities while altering solubility and reactivity. Etherification, particularly alkylation, proceeds via treatment with alkyl halides in the presence of a base, yielding alkyl ethers. A representative example is the O-methylation of orcinol to form 3,5-dimethoxytoluene (also known as orcinol dimethyl ether), achieved using methyl iodide and a base such as potassium carbonate in acetone:

\ce{C6H3(CH3)(OH)2 + 2 CH3I + 2 K2CO3 -> C6H3(CH3)(OCH3)2 + 2 KI + 2 CO2 + 2 H2O}

This Williamson ether synthesis variant is efficient for complete dimethylation, though partial monomethylation can be controlled by stoichiometry.^[43] Intramolecular hydrogen bonding in orcinol, involving the meta-oriented hydroxyl groups, influences its conformational preferences, reactivity, and solubility properties. This bonding stabilizes certain tautomers and reduces the availability of hydroxyl protons for intermolecular interactions, thereby modulating electrophilic substitution rates and enhancing crystallinity in solid states. Such effects are evident in spectroscopic studies and contribute to orcinol's distinct behavior compared to ortho- or para-dihydroxybenzenes.^[44] Orcinol's reactivity patterns also extend briefly to oxidation, such as oxidative condensation to orcein in the presence of ammonia and air or other oxidants.

Oxidation and derivatization

Orcinol undergoes oxidative condensation in the presence of ammonia and atmospheric oxygen or hydrogen peroxide to form orcein, a reddish-brown dye with the molecular formula C₂₈H₂₄N₂O₇. This reaction involves the coupling of multiple orcinol units through electrophilic aromatic substitution and subsequent cyclization, resulting in a mixture of phenoxazone derivatives where orcein itself is the primary component.^[45]^[46] The process, first described in the late 19th century, typically proceeds under ammoniacal conditions, where orcinol is oxidized to reactive intermediates that condense with ammonia to yield the colored product.^[46] A simplified representation of the reaction stoichiometry is given by the equation:

$4 \text{ orcinol} + 2 \text{NH}_3 + \text{O}_2 \rightarrow \text{orcein} + \text{byproducts}

This equation captures the overall transformation but omits detailed mechanistic steps and side products, as the actual process generates a complex mixture of isomers.^[45] Such quinones serve as intermediates in further synthetic derivatizations or biochemical pathways involving resorcinolic compounds.^[47] Orcinol glucoside derivatives are synthesized via glycosylation, either enzymatically using UDP-glycosyltransferases (UGTs) or chemically through coupling with activated glucose donors. Enzymatic approaches, such as those employing orcinol-specific UGTs from plants like Curculigo orchioides, yield β-D-glucopyranosides with high regioselectivity at the 1-position of orcinol.^[48] These derivatives enhance the solubility and bioavailability of orcinol, mimicking natural glucosides found in certain orchids. Chemical glycosylation typically involves protecting groups and Lewis acid catalysis to attach glucose, though enzymatic methods are preferred for efficiency and stereocontrol in modern synthesis.^[48]

Applications and uses

In dyes and staining

Orcinol has played a central role in the production of natural purple dyes since ancient times, particularly in the Mediterranean region where it was extracted from lichens of the genus Roccella, such as Roccella tinctoria, to create orchil, a vibrant purple pigment used as a substitute for the rarer Tyrian purple derived from mollusks.^[49] This lichen-based industry flourished in areas like the Canary Islands and North Africa, supporting textile dyeing for clothing, parchments, and luxury goods by Greek, Roman, and later European cultures.^[50] In traditional dye production, orcinol serves as the key precursor for orcein, the primary coloring component of orchil, through ammoniacal oxidation where orcinol is exposed to ammonia and atmospheric oxygen or hydrogen peroxide, yielding a mixture of phenoxazone derivatives.^[7] The resulting orcein dye, with the empirical formula C₂₈H₂₄N₂O₇ and molecular weight of 500.51 g/mol, produces reddish-brown to purple hues depending on pH and mordants, and was applied to wool and silk fibers for durable coloration.^[51] Orcein has also found extensive use in histological staining, particularly for visualizing elastic fibers in tissue sections, where it binds selectively to these structures, imparting a dark brown color under light microscopy; for example, in La Cour's acetic-orcein method introduced in 1941, which enhances contrast for fibrous components in fixed specimens.^[52] This application remains standard in pathology for identifying elastin in organs like the aorta and skin, often combined with counterstains like Van Gieson's for broader tissue differentiation.^[45] In modern applications, synthetic analogs derived from orcinol, such as monoazo compounds formed by coupling diazonium salts with orcinol as the phenolic component, have been developed for textile dyeing, offering improved fastness to light and washing on synthetic fabrics like polyester compared to natural extracts.^[53] These orcinol-based azo dyes provide a range of orange to red shades and are valued for their straightforward synthesis and environmental compatibility relative to earlier heavy metal mordants.^[54]

In analytical chemistry

Orcinol serves as a key reagent in Bial's test, a colorimetric assay primarily used for the detection and quantification of pentoses and related carbohydrates. In this procedure, the sample is mixed with Bial's reagent—consisting of orcinol dissolved in concentrated hydrochloric acid with a trace of ferric chloride—and heated at 100°C for several minutes. Under these acidic conditions, pentoses undergo dehydration to form furfural, which then condenses with orcinol in the presence of Fe³⁺ ions to produce a green-colored complex.^[55]^[56] The reaction's specificity arises from the differential dehydration rates of pentoses versus hexoses, with the latter forming hydroxymethylfurfural that yields a less intense or different color.^[55] The assay is quantified by measuring absorbance at approximately 630 nm, where the green complex exhibits maximum absorption. It demonstrates good sensitivity, detecting pentose concentrations in the range of 0.1–1 mg, making it suitable for both qualitative identification and quantitative analysis in biological samples such as nucleic acids or plant extracts. The mechanism involves acid-catalyzed dehydration followed by electrophilic aromatic substitution on orcinol, stabilized by the iron catalyst, providing a reliable indicator of pentose content without significant interference from common hexoses at standard conditions.^[55]^[57] Beyond pentoses, orcinol in conjunction with HCl and ferric chloride is employed in colorimetric assays for uronic acids, where these compounds are dehydrated to furfural derivatives that react similarly to form a green product. This adaptation allows for the estimation of uronic acid content in polysaccharides like pectin, with comparable sensitivity to the pentose assay. For sialic acids, the same Bial's reagent produces a blue-purple chromophore upon heating, enabling detection in glycoproteins and other sialoglycoconjugates; the method is accurate down to about 5 nmol (1.5 µg) and is particularly useful for bound sialic acids after hydrolysis. These applications highlight orcinol's versatility in carbohydrate analysis through condensation reactions under acidic conditions, though specificity can require prior separation to minimize interferences from neutral sugars.^[57]^[55]

Other applications

Orcinol has shown potential as a tyrosinase inhibitor in melanogenesis pathways, making it a candidate for skin-lightening agents in cosmetics. In B16F10 melanoma cells, orcinol at concentrations of 0.5 mM reduced mushroom tyrosinase activity by approximately 49%, while 0.2 mM inhibited cellular tyrosinase by about 93% after 72 hours. Additionally, at 200 μM, orcinol monohydrate suppressed melanin production by 76.6 ± 2.9% in B16 cells over 72 hours, with no significant cytotoxicity observed up to 1 mM. This inhibition occurs via upregulation of the MAPK/ERK signaling pathway, which downregulates microphthalmia-associated transcription factor (MITF) expression, thereby reducing tyrosinase and other melanogenic enzymes.^[58]^[59] Orcinol exhibits antioxidant properties through free radical scavenging, with applications in cosmetics and pharmaceuticals to mitigate oxidative stress. In DPPH assays, orcinol demonstrates notable scavenging activity, achieving up to 80% inhibition at 20 mg/mL, and its potency is comparable to resorcinol, outperforming some orsellinate derivatives in kinetic studies of radical reactivity. These properties stem from its phenolic structure, enabling hydrogen atom transfer to stabilize radicals, as evidenced in evaluations of lichen-derived compounds where orcinol contributed to overall antioxidant capacity.^[60]^[61]^[62] Orcinol and its derivatives have demonstrated antimicrobial activity against various bacteria and fungi. For instance, orcinol exhibits antibacterial effects against Gram-positive and Gram-negative strains, such as Staphylococcus aureus and Escherichia coli, with minimum inhibitory concentrations (MICs) in the range of 0.5–2 mg/mL in some studies. This activity is attributed to disruption of microbial cell membranes due to its phenolic nature.^[63]^[64] Orcinol monohydrate has shown anxiolytic properties in animal models without sedative effects. Oral administration at doses of 2.5 and 5 mg/kg in mice increased the time spent in open arms of the elevated plus-maze, indicating reduced anxiety-like behavior, potentially through modulation of the GABAergic system or serotonin pathways.^[65]^[66] As a precursor in organic synthesis, orcinol is used to prepare functionalized azacryptands for advanced materials in photonics. In a one-pot reaction, orcinol monohydrate reacts with tris(2-chloroethyl)amine hydrochloride under basic conditions in acetone/water, yielding an 18% of the azacryptand product after 48 hours of reflux. These azacryptands form luminescent lanthanide complexes (e.g., with Eu(III) emitting at 593-696 nm and Er(III) at 1542 nm), suitable for optical amplifiers and light-emitting diodes due to their thermal stability and efficient energy transfer.^[67] Regarding toxicity and safety, orcinol is considered a mild irritant to skin and eyes, potentially causing respiratory irritation upon inhalation, and it is harmful if swallowed. Acute oral LD50 values are 844 mg/kg in rats and 770 mg/kg in mice, indicating moderate toxicity. Handling requires protective equipment to avoid contact, and no specific regulatory status as GRAS was identified for food or cosmetic use.^[68]^[69]