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m -Cresol

m-Cresol, also known as 3-methylphenol, is an organic compound with the molecular formula C₇H₈O, serving as one of the three isomeric forms of cresol, where the methyl group is positioned meta to the hydroxyl group on the benzene ring.^[1] It appears as a colorless to pale yellow liquid with a characteristic tarry, phenolic odor and is moderately soluble in water.^[2] m-Cresol exhibits key physical properties including a melting point of 11–12 °C, a boiling point of 202 °C, and a density of 1.03 g/cm³ at 20 °C, making it a viscous liquid at room temperature that solidifies upon cooling.^[1] Chemically, it is stable under normal conditions but sensitive to light, air, and heat, potentially darkening over time; it reacts vigorously with strong oxidizers, bases, and certain acids like nitric acid.^[1] Its pKa value of approximately 10.01 indicates weak acidity, typical of phenols.^[2] Industrial production of m-cresol primarily involves fractional distillation from coal tar or petroleum fractions, where it occurs naturally alongside ortho- and para-isomers, followed by separation via distillation or sulfonation methods.^[2] Synthetic routes include chlorination or sulfonation of toluene, followed by hydrolysis to yield the meta isomer selectively.^[1] Global production volumes are significant, with annual outputs in the European Economic Area exceeding 10,000 tonnes, mainly for use in polymers and chemical manufacturing.^[3] As a versatile chemical intermediate, m-cresol is widely employed in the synthesis of pesticides such as fenthion and fenitrothion, antioxidants, synthetic vitamin E (via methylation to 2,3,6-trimethylphenol), herbicides, fumigants, and resins.^[4] It also functions as a disinfectant and preservative in products like Lysol and certain insulin formulations, and finds applications in photographic developers, explosives, and fragrances due to its solvent properties.^[2] Additionally, trace amounts occur naturally in some foods, tobacco smoke, and as a toluene metabolite in biological systems.^[1] m-Cresol poses significant health and environmental hazards; it is toxic if swallowed or absorbed through the skin, causing severe burns, eye damage, and potential central nervous system depression.^[3] Acute oral toxicity in rats shows an LD50 of 242–2,020 mg/kg, and it is classified as a possible human carcinogen (Group C) with experimental evidence of neoplastic effects.^[1]^[2] Environmentally, it is harmful to aquatic life with long-lasting effects and exhibits moderate mobility in soil.^[3] Occupational exposure limits include a TLV of 20 mg/m³, emphasizing the need for protective measures in handling.^[1]

Properties

Physical properties

m-Cresol, systematically named 3-methylphenol, is an organic compound with the molecular formula C₇H₈O and a molar mass of 108.14 g/mol.^[1] Its structure consists of a phenol ring substituted with a methyl group at the meta position. The compound appears as a colorless to yellowish viscous liquid at room temperature.^[1] Key physical properties of m-cresol are summarized in the following table:

Property	Value	Conditions
Density	1.034 g/cm³	20 °C
Melting point	11.8 °C	-
Boiling point	202.2 °C	1013 hPa
Flash point	86 °C	Closed cup
Vapor pressure	0.11 mmHg	25 °C
Viscosity	12.9 mPa·s	25 °C

These values are derived from experimental measurements and indicate m-cresol's liquid state under ambient conditions, with a relatively low melting point allowing it to remain fluid near room temperature.^[1]^[5]^[6] m-Cresol exhibits moderate solubility in water, approximately 2.3 g/100 mL at 20 °C, and is fully miscible with ethanol and diethyl ether.^[5]^[1] Compared to its isomers, m-cresol has a boiling point intermediate between o-cresol (191 °C) and p-cresol (202 °C).^[1]

Chemical properties

m-Cresol, as a phenolic compound, displays moderate acidity attributable to the phenolic hydroxyl group, which can dissociate to form a resonance-stabilized phenoxide ion. The pKa of this group is approximately 10.1 at 25°C.^[1] This value is slightly higher than that of unsubstituted phenol (pKa ≈ 10.0), rendering m-cresol a marginally weaker acid due to the electron-donating inductive effect of the meta-methyl substituent, which reduces the stability of the conjugate base without significant resonance involvement.^[1] In electrophilic aromatic substitution reactions, the hydroxyl group strongly activates the ring and directs incoming electrophiles to the ortho and para positions relative to itself (positions 2, 4, and 6). The meta-methyl group provides additional activation through hyperconjugation and inductive effects, preferentially directing to its own ortho and para positions (2, 4, and 6), thereby enhancing overall reactivity at these shared sites compared to phenol alone.^[7] This cooperative directing effect makes m-cresol highly susceptible to substitutions such as halogenation, nitration, and sulfonation under mild conditions.^[7] m-Cresol exhibits notable sensitivity to oxidation, especially when exposed to air or oxidizing agents, resulting in the formation of colored impurities such as quinone derivatives. This reactivity stems from the ease with which the phenolic ring undergoes oxidative coupling or dehydrogenation to yield para-quinoid structures, a common behavior among phenols that can lead to discoloration in stored samples.^[1] The structural features of m-cresol enable significant hydrogen bonding interactions, primarily through the hydroxyl group acting as both a donor and acceptor, which promotes self-association in neat liquid or solution and enhances solubility in protic solvents. The methyl group contributes modestly via weak C-H···O interactions, influencing intermolecular packing and viscosity in binary mixtures.^[1]^[8] Spectroscopically, m-cresol is characterized by a broad O-H stretching band in the infrared spectrum at approximately 3300–3350 cm⁻¹, indicative of hydrogen-bonded phenolic hydroxyl groups. In ¹H NMR spectroscopy (in CDCl₃ or similar solvents), the methyl protons appear as a singlet at δ ≈ 2.3 ppm, the aromatic protons resonate as a complex multiplet between δ 6.7 and 7.3 ppm (with distinct signals for H-2, H-4, H-5, and H-6), and the OH proton signal varies widely (δ 4–12 ppm) depending on concentration, solvent, and hydrogen bonding.^[9]^[10]

Production

Industrial production

m-Cresol is primarily produced industrially through extraction from coal tar and various synthetic routes, with the former serving as a traditional source and the latter enabling larger-scale, controlled production. In coal tar extraction, high-temperature coke oven tar from coking processes contains a mixture of phenols, including cresols where m-cresol constitutes approximately 40-45% of the total cresols (typically a few percent of the overall tar). The tar is first fractionally distilled to isolate the phenolic fraction (boiling range ~180-220°C), followed by alkaline extraction with sodium hydroxide to form sodium phenates, acidification to liberate the crude cresol mixture, and further purification via fractional distillation or sulfonation to separate isomers based on differing reactivity—m-cresol and p-cresol form monosulfonic acids more readily than o-cresol, allowing selective isolation.^[11]^[12] Synthetic production of m-cresol often involves the alkaline hydrolysis of chlorotoluene mixtures, a variant of the Raschig-Hooker process adapted from phenol synthesis; chlorotoluenes (prepared by chlorination of toluene) are hydrolyzed under high temperature and pressure with caustic soda, yielding a cresol mixture enriched in m-cresol (up to 60-70% selectivity). Alternatively, methylation of phenol with methanol over acidic catalysts (e.g., alumina or zeolites) at 300-450°C produces a mixture of cresol isomers (o:m:p ratios ~40:30:30), which is then separated. The cymene process, analogous to the cumene hydroperoxide route for phenol, involves alkylation of toluene with propylene to form m-cymene, followed by air oxidation to the hydroperoxide and acid-catalyzed cleavage to m-cresol and acetone, though this is less common for the meta isomer compared to the para variant.^[13]^[14]^[15] Global production capacity for m-cresol exceeds 60,000 metric tons per year as of 2024, representing a significant portion of total cresol output (estimated at around 150,000-200,000 tons annually across all isomers), with major producers including Sasol (South Africa and United States), Lanxess (Germany), and SI Group (United States). Isomer separation from mixed streams is challenging due to close boiling points (m-cresol 202.9°C, p-cresol 201.9°C, o-cresol 191.0°C), making fractional distillation energy-intensive (often requiring multi-stage columns with high reflux ratios and energy inputs of 5-10 GJ/ton) and yields typically 80-95% after purification via adsorption, crystallization, or sulfonation-desulfonation sequences to achieve >99% purity.^[16]^[17]^[18]

Laboratory synthesis

One established laboratory method for synthesizing m-cresol involves the carbonylation of methallyl chloride with acetylene and carbon monoxide, catalyzed by nickel carbonyl. This reaction is carried out under mild pressure and temperature conditions in a suitable autoclave, yielding m-cresol in approximately 80% efficiency.^[19] Selective methylation of phenol with methanol over acidic zeolite catalysts, such as MCM-22 with a SiO₂/Al₂O₃ ratio of 25–30, provides another route to m-cresol. The gas-phase reaction occurs at around 300°C in a fixed-bed reactor, resulting in ~40% phenol conversion and >95% selectivity to cresols, with m-cresol comprising up to 50% of the cresol products due to the influence of strong Brønsted acid sites. Isomer-specific isolation of m-cresol from the mixture is achieved through fractional distillation or preparative chromatography.^[20] m-Cresol can be obtained by reducing m-hydroxybenzaldehyde (3-hydroxybenzaldehyde) via the Wolff–Kishner reaction, which employs hydrazine hydrate and potassium hydroxide under reflux in a high-boiling solvent like diethylene glycol, converting the aldehyde to the methyl group with good yields typical for aromatic aldehydes.^[21] A classical laboratory sequence for m-cresol begins with commercially available m-nitrotoluene or m-toluidine (noting that direct nitration of toluene yields only ~3-4% m-nitrotoluene), followed by reduction to m-toluidine using iron or catalytic hydrogenation if starting from the nitro compound, and then diazotization with sodium nitrite in acidic medium at low temperature (~0°C) followed by hydrolytic decomposition in refluxing benzene-water mixture. This final step from m-toluidine affords m-cresol in 87% yield with >99.5% purity after neutralization and rectification.^[22] Regardless of the synthetic route, purification to laboratory-grade standards (>99% purity) is routinely performed using vacuum distillation under reduced pressure (e.g., 10–20 mmHg at 80–100°C) to minimize decomposition, or silica gel column chromatography with hexane-ethyl acetate eluents for analytical samples. Standard laboratory equipment, including round-bottom flasks, condensers, and rotary evaporators, suffices for these scales, though pressurized setups are required for carbonylation.

Applications

Synthetic applications

m-Cresol serves as a key starting material in organic synthesis, particularly for producing pharmaceuticals and fine chemicals through electrophilic aromatic substitutions that leverage its phenolic reactivity.^[23] The meta-positioned methyl group influences regioselectivity, often favoring ortho-functionalization relative to the hydroxyl group due to steric hindrance at the adjacent position.^[24] One prominent application is the synthesis of thymol, a monoterpenoid phenol used as an antiseptic and in pharmaceuticals. Thymol is produced via the acid-catalyzed alkylation of m-cresol with propylene in the gas phase, typically over zeolite or alumina catalysts at 200–300°C and 1–5 bar pressure, following the reaction C₇H₈O + C₃H₆ → C₁₀H₁₄O.^[25] This process achieves m-cresol conversions of up to 80% with thymol selectivities around 75–87%, depending on catalyst and conditions.^[26] The regioselectivity favors the 6-position (ortho to OH, meta to CH₃) due to the steric bulk of the meta-methyl group, which hinders substitution at the 2-position between the OH and CH₃ substituents, directing the isopropyl group to form 2-isopropyl-5-methylphenol (thymol).^[27] This method originated in the early 20th century, with foundational work by Niederl and Natelson in 1936 demonstrating the intramolecular rearrangement of m-cresyl ethers or direct alkylation under acidic conditions to yield thymol and its isomers.^[27] m-Cresol is also employed in the synthesis of antioxidants and insecticides through nitration of the aromatic ring. Nitration introduces nitro groups, primarily at the 4- or 6-position, yielding intermediates like 4-nitro-m-cresol, which serve as precursors for phenolic antioxidants and certain contact insecticides. For instance, mixtures of m- and p-cresol undergo nitration to produce antioxidants such as alkylated nitro-phenols, while pure m-cresol nitration supports insecticide formulations, with 4-nitro-m-cresol used in the synthesis of the insecticide fenitrothion.^[28] Sulfonation, another electrophilic process, functionalizes m-cresol at ortho or para positions to form sulfonic acids.^[29] In the production of vitamin E (α-tocopherol), m-cresol is transformed into trimethylhydroquinone (TMHQ) intermediates via sequential methylation and oxidation, followed by coupling with isophytol. The process begins with ortho/para-directed methylation of m-cresol to 2,3,6-trimethylphenol, exploiting the meta-methyl steric effects to enhance selectivity at unhindered ortho sites, yielding TMHQ after oxidation; this chromanol precursor then undergoes acid-catalyzed condensation with isophytol to form tocopherol.^[23] This route, widely adopted industrially, highlights m-cresol's utility in stereoselective coupling reactions for nutraceutical synthesis.^[30]

Material science applications

m-Cresol undergoes copolymerization with formaldehyde to produce m-cresol-formaldehyde resins, which serve as key components in adhesives and coatings for composite materials. These resins are particularly employed in surface modification of polyester fibers to enhance interfacial adhesion in rubber composites, such as those used in tires, conveyor belts, and V-belts. Compared to traditional resorcinol-formaldehyde-latex systems, m-cresol-formaldehyde latex offers a less toxic alternative while maintaining strong bonding performance. Optimal formulations, such as a cresol-to-formaldehyde molar ratio of 1:2 and a resin-to-latex weight ratio of 0.23, yield peeling forces up to 7.3 N per sample and H-pull-out forces of 56.8 N, with fiber breaking strength reduced by less than 5%. Curing typically occurs at 180–200°C, enabling efficient processing for industrial applications. In conductive polymers, m-cresol functions as a secondary doping agent for polyaniline, particularly in its emeraldine salt form when primarily doped with camphorsulfonic acid. This secondary doping induces a conformational shift from a compact coil to an extended coil structure, significantly enhancing electron mobility, solubility in organic solvents, and film-forming capabilities. As a result, polyaniline films cast from m-cresol solutions exhibit conductivities exceeding 10³ S/cm, far surpassing those obtained from solvents like chloroform or N-methyl-2-pyrrolidinone, where residual solvent content remains around 15 wt% post-evaporation. This improvement supports applications in flexible electronics and sensors, with crystallinity reaching approximately 50% and domain sizes of ~50 Å. m-Cresol-based terpolymers, such as those formed with formaldehyde and urea or salicylic acid, are utilized in chelating ion-exchange resins for selective sorption of heavy metal ions from aqueous solutions. These resins demonstrate high affinity for ions like Cu²⁺ and Pb²⁺, facilitating water purification processes. Additionally, derivatives like tricresyl phosphate, synthesized from m-cresol and phosphorus oxychloride, act as flame-retardant plasticizers and stabilizers in polyvinyl chloride (PVC) formulations, improving thermal stability and flexibility in vinyl plastics without compromising mechanical integrity. Recent developments since 2010 have explored m-cresol derivatives in enzyme-mediated polymerization to create bio-inspired phenolic polymers with potential for sustainable materials, though biodegradability remains limited compared to aliphatic polyesters.

Occurrence

Biological sources

m-Cresol is secreted by male African elephants (Loxodonta africana) from their temporal glands during musth, a periodic state of heightened aggression and reproductive activity, where it serves as a component of pheromonal signals for communication and dominance display.^[31]^[32] Studies of temporal gland secretions have identified m-cresol alongside p-cresol and phenol, with its presence varying by sample but contributing to the overall volatile profile that conveys musth status to other elephants.^[33] In certain ant species, notably Colobopsis saundersi (synonym Camponotus saundersi), m-cresol is a key constituent of mandibular gland secretions expelled during defensive autothysis, a suicidal behavior where workers rupture their bodies to release toxic fluids that deter predators and protect the colony.^[34] This compound acts as a corrosive toxin within a mixture that includes resorcinol, 6-methylsalicylic acid, and 2,4-dihydroxyacetophenone, with m-cresol comprising a major portion—up to significant levels approaching 10% in related exploding ant taxa—enhancing the secretion's antimicrobial and repellent properties.^[35]^[36] m-Cresol also occurs naturally in tobacco smoke, arising from the pyrolysis of plant lignins and other biomass components during combustion, where it forms alongside other phenols like o-cresol and p-cresol through thermal breakdown of phenolic precursors in tobacco leaves.^[37] This process mimics natural degradation but is biologically rooted in the lignin structure of Nicotiana plants. Trace amounts of m-cresol are found naturally in various foods, including asparagus shoots, coffee beans, and spices, derived from phenolic compounds in plant materials.^[38]^[39] It also appears as a minor metabolite of toluene in biological systems, such as in human urine following exposure, where toluene is hydroxylated to form cresol isomers including m-cresol.^[40]^[41] Biosynthesis routes for m-cresol in natural organisms remain understudied, unlike the better-characterized p-cresol pathway from tyrosine.

Environmental presence

m-Cresol occurs naturally as a fossil-derived compound in coal tar and petroleum deposits, where it is obtained through processes such as fractional distillation of crude oil and destructive distillation of coal. Trace levels appear in coal-derived materials as natural analogs from ancient plant lignins.^[42] It is also present in crude oil and shale oil as a component of these natural hydrocarbon mixtures.^[29]^[43] In industrial settings, m-cresol is detected in wastewater from coking operations and phenolic resin production, with concentrations ranging from approximately 2.7 mg/L to 950 mg/L in coal gasification effluents and up to 1,230 mg/L in coal liquefaction processes.^[44] For example, raw coking wastewater has been reported to contain m-cresol at levels around 183 mg/L.^[45] m-Cresol undergoes rapid biodegradation in soil under aerobic conditions, primarily by microorganisms such as Pseudomonas putida, with a reported half-life of about 0.6 days in uncultivated grassland surface soil.^[46] Its environmental distribution is influenced by a log K_ow of 1.94, indicating moderate hydrophobicity, and a soil adsorption coefficient (K_oc) ranging from 22 to 3,420, which promotes partitioning into soil organic matter over water but allows some mobility depending on soil pH and type.^[47] Volatilization from soil and water is limited due to a low Henry's law constant (approximately 1.2 × 10⁻⁶ atm-m³/mol).^[29] Monitoring data from U.S. Environmental Protection Agency (EPA) and related assessments indicate low ambient levels of cresols, including m-cresol, in air near industrial sites, with concentrations of 0.3–3.3 μg/m³ reported near wood treatment facilities and a national median of 1.59 μg/m³ in ambient air samples.^[44] In sediments and soils, m-cresol is only occasionally detected, mainly at hazardous waste sites contaminated by petroleum spills or coal tar, at levels such as 1,400 μg/L in associated groundwater, owing to its rapid aerobic degradation.^[29] European Environment Agency reports similarly note infrequent detections in sediments, emphasizing its persistence under anaerobic conditions but overall transience in oxic environments.

Safety and regulation

Health effects

m-Cresol poses significant health risks through various exposure routes, primarily dermal absorption and inhalation, where it is harmful upon skin contact and toxic if breathed in.^[48] It is readily absorbed through the skin due to its moderate water solubility, allowing penetration and systemic distribution.^[49] Under the Globally Harmonized System of Classification and Labelling of Chemicals (GHS), m-cresol is designated as toxic if swallowed (H301), toxic in contact with skin (H311), and causing severe skin burns and eye damage (H314).^[50] Acute exposure to m-cresol is highly corrosive to the skin, eyes, and mucous membranes, resulting in chemical burns, severe irritation, and potential tissue damage. Inhalation leads to respiratory tract irritation, including symptoms such as coughing, shortness of breath, and in severe cases, pulmonary edema.^[46] Oral ingestion exhibits acute toxicity with an LD₅₀ of 242 mg/kg in rats, manifesting in symptoms like nausea, vomiting, abdominal pain, convulsions, and organ damage, particularly to the gastrointestinal tract, liver, and kidneys. Chronic exposure to m-cresol may cause liver and kidney damage, as evidenced by increased organ weights and histopathological changes in animal studies.^[46] Cresols, including m-cresol, have not been classified by the International Agency for Research on Cancer (IARC) with respect to their carcinogenicity to humans due to inadequate evidence; however, the U.S. Environmental Protection Agency (EPA) designates them as possible human carcinogens (Group C).

Environmental and regulatory aspects

m-Cresol exhibits moderate acute toxicity to aquatic organisms, with a 96-hour LC50 of 13 mg/L reported for rainbow trout (Oncorhynchus mykiss).^[3] It is also harmful to invertebrates, showing a 48-hour EC50 of 22 mg/L for water flea (Daphnia magna), and to algae, with a 72-hour EC50 of 29 mg/L for Pseudokirchneriella subcapitata.^[3] These values indicate potential ecological risks in contaminated water bodies, though m-cresol demonstrates low bioaccumulation potential due to its log Kow of 1.96 and rapid metabolism in organisms.^[52]^[44] Under the EU REACH regulation, m-cresol is registered and classified as acutely toxic to aquatic life (Aquatic Acute 1) and causing long-term adverse effects (Aquatic Chronic 2), requiring risk assessments for environmental releases.^[3] In the United States, it is listed as a hazardous air pollutant by the EPA and designated a hazardous substance under CERCLA.^[53] In Canada, as of February 2025, m-cresol is listed as a restricted ingredient in cosmetics under Health Canada's Cosmetic Ingredient Hotlist.^[54] For occupational exposure, NIOSH recommends a REL of 2.3 ppm (10 mg/m³) as an 8-hour TWA, with a PEL of 5 ppm (22 mg/m³) TWA (skin notation) set by OSHA, and an IDLH of 250 ppm; these limits stem from observed respiratory and dermal health effects in workers.^[48] Waste containing m-cresol is managed as hazardous under RCRA, assigned code U052 for unused commercial products and D024 under the toxicity characteristic with a regulatory level of 200 mg/L. Recent evaluations in the 2020s, including under EU REACH criteria, have concluded that m-cresol does not possess endocrine-disrupting properties.^[55]

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Characterization of chars from pyrolysis of lignin - ScienceDirect.com
The amount of phenol was independent of the source of lignin but the amount of m- and p-cresol was much lower from tobacco lignin than that from the wood lignin ...Missing: smoke | Show results with:smoke
[39]
[PDF] OH CH OH CH m-Cresol p-Cresol
Cresols (mixed isomers) are widespread in nature, occurring, for instance, in many plants, petroleum, coal tar, crude oil and, volcanic actions. They are ...
[40]
[PDF] Toxicological Profile for Cresols
Estimated releases of 270 pounds (~0.1 metric tons) of o-cresol, 780 pounds (~0.4 metric tons) of m-cresol, 666 pounds (~0.3 metric tons) of p-cresol, and ...Missing: energy | Show results with:energy
[41]
The occurrence and fate of phenolic compounds in a coking ...
Aug 6, 2025 · It was found that 3-cresol was the dominant compound in the raw coking wastewater with a concentration of 183 mg L(-1), and that chlorophenols ...
[42]
[PDF] ATSDR Cresols Tox Profile
The ATSDR toxicological profile succinctly characterizes the toxicologic and adverse health effects information for the hazardous substance described ...<|separator|>
[43]
Cresol - PubChem - NIH
Cresols is a colorless to yellow liquids with a tarry odor. Flash point 178 °F. Insoluble in water. Density 8.7 lb / gal. Poisonous by ingestion and skin ...
[44]
NIOSH Pocket Guide to Chemical Hazards - m-Cresol - CDC
m-Cresol ; Lower Explosive Limit. (300°F): 1.1% ; Incompatibilities & Reactivities. Strong oxidizers, acids ; Exposure Routes. inhalation, skin absorption, ...Missing: compound properties structure production
[45]
https://www.researchgate.net/publication/249996856_The_occurrence_and_fate_of_phenolic_compounds_in_a_coking_wastewater_treatment_plant
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https://www.atsdr.cdc.gov/toxprofiles/tp34.pdf
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https://pubchem.ncbi.nlm.nih.gov/compound/Cresol
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https://www.cdc.gov/niosh/npg/npgd0155.html
[49]
Initial List of Hazardous Air Pollutants with Modifications | US EPA
Initial List of Hazardous Air Pollutants with Modifications ; 108394, m-Cresol ; 106445, p-Cresol ; 98828, Cumene ; 94757, 2,4-D, salts and esters.
[50]
[PDF] m-Cresol - SAFETY DATA SHEET
Nov 28, 2024 · The substance does not have endocrine disrupting properties in accordance with the criteria set out in Commission. Delegated Regulation (EU) ...