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Cresol

Cresol, also known as hydroxytoluene or methylphenol, refers to a group of three isomeric compounds with the molecular C₇H₈O, consisting of ortho-cresol (2-methylphenol), meta-cresol (3-methylphenol), and para-cresol (4-methylphenol). These aromatic are typically colorless to pale yellow liquids with a sharp, odor, moderate in (around 20-30 g/L at 25°C depending on the ), and high in solvents. Cresols occur naturally in , , and biological processes such as the of aromatic , but are also produced synthetically on an industrial scale. Commercially, they are obtained as mixtures (cresylic acids) from or , where cresols form a significant portion of the phenolic compounds, or via synthetic routes like the of phenol. Global production is approximately 200,000 metric tons annually as of 2024. Cresols serve as versatile intermediates in the production of resins, antioxidants, herbicides (e.g., ), and pharmaceuticals (e.g., from ), and are used as disinfectants and antiseptics. They exhibit as irritants and have regulated occupational exposure limits (e.g., OSHA PEL of 5 TWA) due to health risks, with moderate environmental persistence.

Chemical Properties

Structure and Isomers

Cresols are a group of aromatic organic compounds classified as monomethylphenols or hydroxytoluenes, featuring a single (-CH₃) substituted on the ring of phenol (C₆H₅OH). All isomers share the molecular formula C₇H₈O and a of 108.14 g/. The term "cresylic acid" is an alternative designation, often applied to commercial mixtures of these compounds rather than the pure isomers. The three distinct isomers differ based on the position of the relative to the hydroxyl (-OH) group: ortho-cresol (, also 2-methylphenol), meta-cresol (, 3-methylphenol), and para-cresol (p-cresol, 4-methylphenol). A common commercial form is mixed tricresol, which contains varying proportions of these isomers. The structural variation influences their physical properties, such as boiling points and , though all exhibit moderate in due to the polar hydroxyl group.
IsomerSystematic NameCAS NumberBoiling Point (°C)Water Solubility (g/L at 25°C)
o-Cresol2-Methylphenol~26
m-Cresol3-Methylphenol~23
p-Cresol4-Methylphenol~22
These values highlight subtle differences arising from steric and electronic effects of the methyl group's position. The naming convention for cresols traces its origins to the , when they were first isolated from fractions obtained during the of , a byproduct of . The term derives from "creosote," the oily distillate containing these phenolic compounds, reflecting their historical association with coal processing.

Physical and Chemical Properties

Cresols are colorless to pale yellow liquids or low-melting solids with a characteristic , though they darken upon exposure to air and light. The liquid isomers exhibit densities around 1.03–1.05 g/cm³ at 20–25°C, with at 1.047 g/cm³, at 1.034 g/cm³, and p-cresol at 1.034 g/cm³. Melting points vary by isomer due to packing efficiency in the solid state: melts at 31.0°C, at 12°C, and p-cresol at 35°C, rendering liquid at while the others are near-solid. points range from 191–203°C at standard pressure, reflecting strong intramolecular hydrogen bonding. Vapor pressures are low, typically 0.1–0.3 mmHg at 25°C; for instance, p-cresol has a of 0.11 mmHg.
Propertyo-Cresolm-Cresolp-Cresol
Density (g/cm³ at 20–25°C)1.0471.0341.034
Melting Point (°C)31.01235
Vapor Pressure (mmHg at 25°C)0.300.140.11
Water Solubility (g/100 mL at 25°C)2.592.272.15
Solubility profiles show cresols are sparingly soluble in , with values decreasing from o- to p-isomer (2.59, 2.27, and 2.15 g/100 mL at 25°C, respectively), attributed to the hydrophobic influencing hydration of the hydroxyl. They are fully miscible with organic solvents such as alcohols, ethers, and , facilitating their use in solvent-based applications. Chemically, cresols behave as weak acids with pKa values of 10.29 for , 10.09 for , and 10.26 for p-cresol, closely mirroring phenol's pKa of 10.0 due to delocalization of the phenolate anion. This acidity enables hydrogen bonding, both intra- and intermolecular, which contributes to their elevated boiling points and compared to non-hydrogen-bonding analogs. Cresols undergo oxidation in the presence of air or oxidants, forming quinone methides or colored polymeric impurities, a reactivity stemming from the activated aromatic ring. They are stable under neutral conditions up to their boiling points but prone to auto-oxidation, leading to discoloration over time. Spectroscopically, cresols exhibit characteristic absorption for the O-H stretch at approximately 3300 cm⁻¹, broadened by , with C-O stretching around 1200–1250 cm⁻¹. In ¹H NMR, the methyl protons appear at 2.2–2.3 as a , while aromatic protons resonate between 6.7–7.3 , and the OH signal varies widely (4–12 ) depending on concentration and due to exchange. ¹³C NMR shows the methyl carbon at about 20–21 and ipso carbon (attached to OH) shifted downfield to 150–155 .

Production

Natural Occurrence and Extraction

Cresols occur naturally as components of , a generated during the high-temperature of at approximately 1000°C in coke production processes. This tar constitutes the primary natural source of cresols. Within the fraction of , cresols comprise 20-30% by mass, representing a significant portion of the extractable . Other natural sources include byproducts from petroleum refining and wood obtained through the of or similar hardwoods. The extraction of cresols from coal tar begins with to isolate the light oil , which boils in the range of 170-230°C and is enriched with including cresols. This is then subjected to alkali extraction using aqueous (NaOH), where the cresols react to form water-soluble sodium cresolates. Subsequent acidification of the aqueous phase with a strong acid, such as , liberates the free cresols, which are then separated by further or . These processes yield a mixture of cresol isomers, typically comprising about 40% , 40% p-cresol, and 20% . Global production of cresols from has historically been substantial, with estimates indicating approximately 7,000-10,000 tons annually in recent years, though total cresylic acid output (including cresols) exceeded 84,000 metric tons by 2023, with about 39% derived from processing. The first isolation of cresols from occurred in the , credited to chemists Auguste Laurent and Gerhardt during their pioneering work on aromatic compounds from coal derivatives.

Industrial Synthesis

The primary industrial synthesis of cresol involves the vapor-phase of phenol with at temperatures exceeding 300°C, typically using solid catalysts such as (MgO) or alumina to promote C-alkylation over O-alkylation. This process operates under with a high methanol-to-phenol ratio, yielding a of o-, m-, and p-cresol s alongside byproducts like and xylenols; selectivity for the combined m- and p-s is approximately 60%, depending on catalyst acidity and reaction conditions. MgO-based catalysts favor ortho-substitution due to their basic nature, while acidic alumina promotes meta- and para-s through intermediates. The reaction proceeds via initial O-methylation to , followed by rearrangement or direct C-methylation, enabling scalable production but requiring separation of the via . Alternative industrial routes start from toluene derivatives. Chlorotoluenes, obtained by side-chain or ring chlorination of , undergo hydrolysis with aqueous NaOH at high pressure and temperatures around 350°C, often yielding cresol mixtures enriched in due to isomerization of the ortho and para precursors. Similarly, toluene sulfonates—produced by sulfonation of with —are fused with NaOH at elevated temperatures (300–400°C) under pressure, providing a corrosive but established method for cresol, particularly p-cresol, with the sulfate byproduct recoverable as . Isomer-specific syntheses are employed for high-purity requirements. p-Cresol is synthesized via diazotization of p-toluidine with in acidic media to form the diazonium , followed by thermal or acid-catalyzed to displace nitrogen and yield the phenol; this route achieves high selectivity but involves hazardous intermediates, limiting it to smaller scales. is predominantly accessed through the MgO-catalyzed of phenol, though of o-chlorotoluene under alkaline conditions offers an alternative, albeit with potential migration. Recent advancements focus on sustainable bio-based routes, such as microbial of using engineered strains expressing tyrosine decarboxylase and subsequent pathways, enabling selective p-cresol production at laboratory titers up to 1.3 g/L in the . These methods leverage renewable feedstocks and genetic modifications for improved flux through the , representing emerging alternatives to processes despite current scale-up challenges. Global cresol production is estimated at approximately 280,000 tons per year as of 2023, with dominating as the leading manufacturer due to its integrated infrastructure and capacity expansions.

Applications

Disinfectants and Antiseptics

Cresols function as effective disinfectants and antiseptics primarily through their structure, which enables them to disrupt bacterial membranes. The hydrophobic nature of the compounds allows penetration into bilayers, while their acidity facilitates and subsequent damage to the cytoplasmic membrane, leading to leakage of intracellular constituents such as ions and disruption of the proton motive force. This uncouples and denatures proteins, resulting in cell death; at higher concentrations, cresols coagulate cytoplasmic components for irreversible bactericidal effects. Historically, cresols gained prominence in applications following Joseph Lister's pioneering work in 1867, where he employed carbolic acid (phenol) mixtures—closely related phenolic compounds including cresols—for wound dressings and surgical sterilization to prevent infection. This approach dramatically reduced postoperative rates and laid the foundation for modern antisepsis. Key commercial products emerged soon after, including , introduced in 1889 as a 50% cresol solution in saponified (), which served as a household and medical . Other formulations like , containing 40–50% carbolic oil rich in cresols and related , were developed for environmental in settings such as barns and kennels. Carbolic , incorporating 3–5% cresol for mild action, became widely used for personal hygiene and surface cleaning. In terms of efficacy, cresols demonstrate broad-spectrum antimicrobial activity, particularly against such as Staphylococcus aureus, at concentrations of 0.1–1%, where they exhibit bacteriostatic to bactericidal effects within minutes by inducing membrane lysis. They are less potent against due to outer membrane barriers but remain valuable in veterinary disinfectants for stables and household cleaners targeting environmental pathogens. Modern applications include diluted aqueous solutions, such as 2–3% cresol, for wound irrigation to cleanse debris and inhibit without excessive tissue irritation. Additionally, cresols serve as preservatives in paints at low concentrations to prevent microbial spoilage during storage and application.

Industrial and Chemical Uses

Cresols serve as key intermediates in the production of alkyl resins through condensation reactions with , yielding materials essential for adhesives, laminates, and coatings. These cresol- resins, often variants of traditional formulations, exhibit enhanced thermal stability and adhesion properties compared to pure counterparts, making them suitable for applications in electrical laminates and composite materials. For instance, cresylic novolac resins derived from cresol are widely employed in formulations for manufacturing, where their solubility and curing characteristics support precise patterning in electronics production. In applications, cresol mixtures function effectively in flotation processes, where they act as frothers to enhance separation by stabilizing air bubbles that carry hydrophobic particles to the surface. Additionally, cresols are utilized in as solvents for resin-based formulations, aiding in the dissolution of components for high-quality applications. As dispersants in production, cresols contribute to the uniform distribution of pigments, preventing aggregation and improving color consistency in and industrial operations. Other industrial uses include cresols' role as additives in fuels and lubricants, where low concentrations (typically 0.01-0.1%) provide oxidation inhibition to extend and maintain performance under high-temperature conditions. Research continues into bio-based cresol from for sustainable in adhesives and composites. The global cresol market was valued at USD 731.6 million in , with chemical intermediates accounting for approximately 52% of applications as of 2025, driven by demand in synthesis. The sector further amplifies this, with growing reliance on cresol-based for boards and insulators.

Derivatives

Derivatives of o-Cresol

o-Cresol serves as a key starting material for various derivatives due to its reactivity at the phenolic hydroxyl group and the ortho-methyl substitution, which influences in subsequent transformations. One prominent derivative is mephenesin, a centrally used to alleviate muscle spasms and tension. is synthesized through the etherification of with 3-chloro-1,2-propanediol, a chlorinated of obtained via chlorination of followed by selective steps to form the . This process yields 3-(2-methylphenoxy)propane-1,2-diol, which exhibits transient muscle relaxation by depressing the without significant sedation at therapeutic doses. Another important class of o-cresol derivatives includes , particularly those designed for stabilization. For instance, 4,6-bis(octylthiomethyl)-o-cresol is a multifunctional hindered synthesized by alkylating o-cresol with octylthiomethyl groups, providing synergistic protection against oxidative degradation through both and thioether mechanisms. This compound is widely applied in polyolefins, elastomers, and adhesives to enhance thermal and oxidative stability during processing and long-term use. Similarly, 4,6-di-tert-butyl-o-cresol is prepared by tert-butylation of o-cresol at the available ortho and para positions relative to the hydroxyl group, yielding a liquid with high and efficacy in preventing formation in lubricants and synthetic rubbers. These derivatives leverage the ortho-methyl group of o-cresol to modulate steric hindrance, improving their performance in high-temperature environments compared to non-substituted . In the dye industry, o-cresol undergoes sulfonation to form o-cresol-4-sulfonic acid, which acts as an intermediate for synthesizing sulfur-containing dyes and pigments. The sulfonation typically involves heating o-cresol with concentrated , directing the sulfonic group to the para position due to the ortho-directing effects of the hydroxyl and methyl substituents. This sulfonic acid derivative is then further modified, often through or coupling reactions, to produce water-soluble azo dyes and couplers used in coloration and dye formulations. For example, reduction of nitro-substituted analogs derived from sulfonated intermediates yields amino-o-cresol variants like 5-amino-o-cresol, employed as oxidative dye couplers in permanent hair colorants for shades ranging from brown to black. These transformations highlight o-cresol's role in enabling the introduction of polar groups for improved dye and substantivity. o-Cresol accounts for approximately 25% of the overall cresol market (as of 2024), driven by demand in pharmaceuticals, polymers, and specialty chemicals. This reflects o-cresol's selective use in high-value applications where its isomer-specific reactivity provides advantages over mixed cresol feedstocks.

Derivatives of m- and p-Cresol

Derivatives of p-cresol are primarily synthesized through and oxidation reactions, leveraging the unhindered position for efficient transformations into s, pharmaceutical intermediates, and fragrance components. A key example is the production of (BHT), achieved by acid-catalyzed of p-cresol with , resulting in a hindered phenol widely employed in stabilizing polymers, foodstuffs, and pharmaceuticals against oxidative degradation. This process typically yields high selectivity for the 2,6-di-tert-butyl substitution, enhancing BHT's thermal stability and efficacy in industrial applications. Another significant transformation involves the selective aerobic oxidation of p-cresol to p-hydroxybenzaldehyde using heterogeneous catalysts such as cobalt oxide under moderate pressure, achieving up to 95% selectivity at high conversion rates. p-Hydroxybenzaldehyde serves as a versatile intermediate in pharmaceutical synthesis, including the production of antibiotics like amoxicillin via 4-hydroxyphenylglycine and agents targeting conditions such as . Additionally, it acts as a building block for fragrance compounds, contributing to notes in perfumes and flavors. Recent advancements in sustainable synthesis include electrochemical methods for functionalizing p-cresol derivatives, enabling greener routes to value-added products like substituted for agrochemicals and materials. In contrast to o-cresol's role in phenoxy herbicides, m- and p-cresol derivatives emphasize linear reactivity suited for pesticides and nutraceuticals. For m-cresol, yields 3-methyl-4-nitrophenol, which undergoes phosphorothioation with dimethyl phosphorothiochloridate to form fenitrothion, an effective against agricultural pests like stem borers. also functions as a precursor to synthetic pyrethroids; it is first converted to 3-phenoxytoluene via with phenoxide, followed by side-chain oxidation to 3-phenoxybenzaldehyde, a critical intermediate in insecticides such as . Further derivatization of involves isopropylation with isopropanol or over catalysts like , producing with selectivities exceeding 80% under optimized conditions, which is subsequently hydrogenated to for use in pharmaceuticals and antiseptics. Sequential of also generates 2,3,6-trimethylphenol, a key intermediate in the industrial synthesis of (), valued for its antioxidant properties in nutritional supplements and . Emerging sustainable approaches, such as microbial in , enable of -derived compounds like , reducing reliance on feedstocks.

Health Effects

Acute Toxicity and Exposure Routes

Cresols, a group of methylphenols, pose significant acute toxicity risks primarily through three main exposure routes: , dermal contact, and . of cresol vapors acts as a respiratory irritant, causing symptoms such as dryness of the mucous membranes, nasal constriction, , and at concentrations as low as 6 mg/m³ (approximately 1.3 ppm) for in humans. Higher acute exposures can lead to and respiratory distress. Dermal exposure results in corrosive burns, with cresols rapidly absorbed through the skin, leading to severe , blistering, and even at low concentrations; systemic effects may follow due to . causes immediate gastrointestinal damage, including burns to the and , , , and potentially hemorrhage or of the digestive tract. Acute systemic effects from high-dose exposures, typically exceeding 100 mg/kg body weight via oral or dermal routes, include , muscular weakness, tremors, confusion, , and in severe cases, death from respiratory or cardiac failure. demonstrate the potency of these effects, with oral LD50 values for mixed cresols in rats ranging from 121 to 207 mg/kg depending on the (o-cresol: 121 mg/kg; : 242 mg/kg; p-cresol: 207 mg/kg), and dermal LD50 in rabbits at 300 mg/kg for p-cresol and up to 2,830 mg/kg for . To mitigate occupational risks, regulatory limits include an OSHA (PEL) of 5 (22 mg/m³) as an 8-hour time-weighted average and a NIOSH (REL) of 2.3 (10 mg/m³) as a 10-hour , both with notation due to concerns. Historical case studies illustrate the dangers of acute cresol poisoning, particularly from ingestion of Lysol, a disinfectant containing approximately 50% cresol. In a 1922 report by Isaacs, 52 cases of intentional Lysol ingestion were documented, primarily suicide attempts, with common symptoms including severe burns to the oral and gastrointestinal mucosa, vomiting, and collapse; two cases were fatal, and survivors often experienced renal failure characterized by oliguria and anuria. Another incident involved a fatal ingestion of 250 mL of cresol-based disinfectant, equivalent to about 2 g/kg, resulting in rapid onset of coma and multi-organ failure. These cases underscore the corrosive and systemic nature of cresol toxicity, with prompt medical intervention critical for survival.

Chronic Effects and Recent Studies

Chronic exposure to cresols has been associated with liver and kidney damage, as well as respiratory in occupational settings. Prolonged or dermal can lead to hepatic toxicity, including elevated liver enzymes and histopathological changes, while renal effects manifest as tubular necrosis and impaired filtration function. Respiratory may develop from repeated low-level exposures, resulting in chronic inflammation of the upper airways and potential reactions. Regarding carcinogenicity, the International Agency for Research on Cancer (IARC) has classified cresols as Group 3, not classifiable as to their carcinogenicity to humans, due to insufficient evidence from human and animal studies. p-Cresol, a prominent isomer, specifically induces mitochondrial dysfunction and DNA damage in colon and kidney cells. In human colonic epithelial cells, exposure to p-cresol disrupts mitochondrial respiration, reduces ATP production, and triggers oxidative stress, leading to impaired cell proliferation. Similarly, in kidney cells, p-cresol promotes mitochondrial inhibition and genotoxic effects, including DNA strand breaks, which contribute to cellular apoptosis and long-term organ impairment. These mechanisms underscore p-cresol's role as a uremic toxin in chronic kidney disease, where accumulation exacerbates renal pathology. Recent studies from 2025 have linked elevated urinary p-cresol levels to symptoms in animal models. Perinatal exposure to p-cresol in mice induced behavioral alterations reminiscent of core traits, such as social deficits and repetitive behaviors, without broadly impairing neurodevelopment. A confirmed higher urinary p-cresol in patients, suggesting it as a potential of dysbiosis influencing neurodevelopment. Additionally, research in Frontiers in (2025) detailed p-cresol's contribution to kidney impairment as a uremic , promoting oxidative damage, immune suppression, and mitochondrial dysfunction in renal tissues. Epidemiological data indicate that occupational exposure to cresols correlates with . Workers in industries involving cresol handling, such as chemical , report persistent skin irritation and . Mitigation strategies include to counteract cresol-induced , as demonstrated in studies from 2016. Proanthocyanidin-rich polyphenols prevented p-cresol's deleterious effects on colonic cells by scavenging and restoring mitochondrial function. These findings suggest potential therapeutic applications for supplementation in reducing from cresol exposure.

Environmental Impact

Fate in the Environment

Cresols enter the environment primarily through industrial effluents from processes such as and , where concentrations in wastewater can reach 586 mg/L for , 950 mg/L for , and 880 mg/L for p-cresol, as well as from sites associated with and production. Additional sources include atmospheric deposition following volatilization from contaminated soils or waters and emissions from of , , and fossil fuels. Cresols have been detected in raw at low concentrations, such as 15.3–216.5 µg/L for . In environmental compartments, cresols exhibit moderate persistence, primarily limited by . In , half-lives range from 0.6 to 1.6 days under aerobic conditions due to microbial , though persistence can extend to weeks under conditions. In , aerobic biodegradation results in half-lives of less than 24 hours to 7 days, while conditions slow this process to weeks or months; in oligotrophic or low-oxygen waters, cresols may persist longer. Atmospheric half-lives are short, on the order of 7 to 10 hours during the day due to reactions with hydroxyl radicals. Cresols demonstrate high mobility in the owing to their , which ranges from 21.5 g/L for p-cresol to 25.9 g/L for at 25°C, facilitating into from contaminated soils. Their octanol- partition coefficients (log K_ow) of 1.94 for p-cresol, 1.95 for , and 1.96 for indicate moderate potential for in aquatic organisms, though rapid typically limits this risk. organic carbon- partition coefficients (K_oc) vary from 17.5 to 3,420, suggesting high to very high mobility depending on and content. Degradation pathways for cresols involve both biological and abiotic processes. Bacterial oxidation under aerobic conditions oxidizes the , for example, converting p-cresol to p-hydroxybenzoic acid via intermediates like p-hydroxybenzyl alcohol and p-hydroxybenzaldehyde, followed by ring cleavage. Recent research indicates p-cresol can influence production in biological waste gas treatment systems. In surface waters, photolysis induced by UV light contributes to breakdown, with estimated half-lives of 4.4 days for p-cresol and 11 days for under exposure, though direct photolysis alone is slower at 300–400 days. These pathways underscore the role of microbial communities in mitigating cresol persistence in aerobic environments.

Ecological and Regulatory Concerns

Cresols demonstrate significant ecotoxicity to aquatic life, posing risks to fish and invertebrates. The 96-hour LC50 for o-cresol in rainbow trout (Oncorhynchus mykiss) is 7 mg/L, indicating moderate to high acute toxicity in salmonid species. Similarly, p-cresol exhibits high toxicity to aquatic invertebrates, with a 48-hour EC50 of 7.7 mg/L in Daphnia magna. These values highlight cresols' potential to cause lethal effects at low concentrations in freshwater ecosystems. Bioaccumulation of cresols in is low, with factors (BCF) typically below 100, limiting long-term trophic transfer. Cresols are rapidly biodegraded in by microbial communities. Under the European Union's REACH regulation, cresols are classified as skin irritants (Skin Irrit. 2, H315) and very toxic to aquatic life (Aquatic Acute 1, H400), requiring measures to prevent environmental release. In the United States, cresols are listed on the Toxic Substances Control Act (TSCA) inventory, subjecting them to reporting and control requirements; under the Clean Water Act (CWA), cresols are designated as toxic pollutants subject to guidelines for various industrial categories to protect . The global cresols market is projected to grow at a (CAGR) of 3.6% through 2030, reaching $476.7 million, which may elevate release risks into ecosystems without enhanced controls. offers a promising strategy for cresol mitigation, with species demonstrating high efficiency in degrading contaminants. Lab trials using Pseudomonas monteilii achieved over 90% removal of cresol in contaminated media under optimized conditions, supporting its application in soil and .

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