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Dehydroacetic acid

Dehydroacetic acid is a synthetic organic compound and pyrone derivative with the molecular formula C<sub>8</sub>H<sub>8</sub>O<sub>4</sub> and IUPAC name 3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one, characterized by a six-membered heterocyclic ring containing both ketone and lactone functional groups. It appears as an odorless, white to light yellow crystalline powder with a molecular weight of 168.15 g/mol, a melting point of 109–113 °C, and limited solubility in water (approximately 0.05 g/100 mL at 25 °C), though it dissolves more readily in organic solvents like ethanol and acetone. First isolated in 1866 by of , dehydroacetic acid exhibits broad-spectrum activity, effectively inhibiting bacteria, yeasts, and molds, which has led to its widespread use as a . In the and personal care industry, it is commonly incorporated at concentrations up to 0.6% to extend and prevent microbial contamination in products like creams, lotions, and shampoos. Its sodium form enhances for broader formulation compatibility. In food applications, dehydroacetic acid is approved for use as a in certain products in the United States, such as cut fruits, , and glazes, at levels not exceeding 65 mg/kg, where it inhibits fungal growth without significantly affecting taste or appearance. Beyond preservation, it serves as a and in and has applications in synthesizing pharmaceuticals, veterinary medicines, and polymer stabilizers like those for (PVC). Safety assessments indicate low , with an oral LD<sub>50</sub> in rats of approximately 1,000 mg/kg, though it may cause mild at higher concentrations.

Properties

Molecular structure

Dehydroacetic acid has the molecular formula C₈H₈O₄. Its is 3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one, reflecting its classification as a substituted 2H-pyran-2-one. The molecule features a six-membered heterocyclic pyrone ring containing one oxygen atom, with a at position 2, a at position 4, and conjugated double bonds contributing to its aromatic-like stability. An (-COCH₃) is attached at the 3-position and a (-CH₃) at the 6-position, forming a planar structure that enhances its reactivity in biological and synthetic contexts. Dehydroacetic acid exhibits keto-enol tautomerism due to its β-diketone substructure, interconverting between the keto form (3-acetyl-2-hydroxy-6-methyl-4H-pyran-4-one) and the enol form (3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one). The enol form predominates in solution and solid state, stabilized by intramolecular hydrogen bonding between the hydroxy group and the ring carbonyl.

Physical and chemical characteristics

Dehydroacetic acid is an odorless, colorless to white crystalline powder. It has a of 168.15 g/, a of 111–113 °C, and a of 270 °C at standard pressure. The is approximately 1.18 g/cm³ (estimated). The compound exhibits low in , with less than 0.1 g dissolving in 100 mL at 25 °C, rendering it almost insoluble. It shows moderate solubility in organic solvents, such as 3 g/100 g in and higher solubility in acetone and . Dehydroacetic acid remains stable under normal storage conditions but decomposes upon heating to high temperatures. As a weak , it has a pKa value of approximately 5.3, indicating partial in aqueous solutions. Spectroscopic characterization reveals key features attributable to its functional groups, including () absorption bands for carbonyl stretches around 1650–1720 cm⁻¹ corresponding to the acetyl and pyrone moieties, as well as () signals for the methyl groups and vinylic protons. () absorption occurs in the 250–300 nm range due to conjugated systems.

Synthesis

Laboratory preparation

Dehydroacetic acid is commonly prepared in the laboratory through the base-catalyzed dimerization of , a straightforward and efficient method suitable for small-scale synthesis. An alternative classic method involves the self-condensation of under mildly alkaline conditions, such as with , followed by or to isolate the product. This reaction involves the condensation of two molecules of (C₄H₄O₂) to form dehydroacetic acid (C₈H₈O₄), as represented by the simplified equation: $2 \ce{C4H4O2} \rightarrow \ce{C8H8O4} Although the reaction is stoichiometrically balanced, minor byproducts such as polymeric materials may form depending on conditions. The procedure typically begins by dissolving a catalytic amount of a mild organic base, such as imidazole, 1,4-diazabicyclo[2.2.2]octane (DABCO), or pyridine (0.1–1% by weight relative to diketene), in an inert anhydrous solvent like toluene or benzene (1–3 volumes per volume of diketene). Diketene (85–95% purity) is then added dropwise or portionwise to the stirred mixture at a controlled temperature of 30–60°C to promote selective dimerization while minimizing polymerization. The addition is usually completed over 1–2 hours, followed by continued stirring for 15–30 minutes. To isolate the product, the reaction mixture is cooled to 0–10°C, and the crude dehydroacetic acid is filtered and washed with chilled solvent. For higher purity, the filtrate is extracted with aqueous sodium hydroxide or sodium carbonate (15–20% solution) to form the soluble sodium salt, which is then acidified with hydrochloric acid, sulfuric acid, or acetic acid to precipitate the free acid. The solid is collected by filtration, washed, and purified by recrystallization from ethanol, yielding white to pale yellow crystals. Typical yields for this method range from 70–90%, with optimized conditions using DABCO or imidazole achieving up to 88–98% based on diketene input. The product's purity after recrystallization often exceeds 98%, confirmed by melting point (108–110°C) and solubility tests. This base-catalyzed approach, developed shortly after the preparation of diketene by thermal dimerization of ketene, marks a significant advancement over earlier methods involving acetoacetic ester derivatives.

Industrial production

Dehydroacetic acid is primarily produced on an industrial scale through the base-catalyzed dimerization of in continuous large-scale reactors, enabling high throughput and efficient automation of the catalytic process. This method leverages the reactivity of , which is generated upstream via the thermal of acetone to followed by spontaneous dimerization, often integrated within the same facility to minimize intermediate handling and logistics costs. Process variations typically employ inexpensive bases such as or as catalysts to promote the dimerization, with or sodium phenoxide used in some configurations for improved selectivity. The reaction occurs in an inert like or , with concentrations maintained at 85-95% to optimize yields. Scale-up considerations include controlled temperatures between 30-60°C (up to 120°C in some variants) under , ensuring complete conversion while preventing side reactions; additives like pyrocatechol (0.5-3 %) are introduced continuously to boost yields above 90% and enhance product purity to 98-100%. Byproduct formation is minimal due to high selectivity, with waste minimization achieved through of solvents and alkaline for purification, aligning with efficient resource use in commercial operations. Global production is dominated by a few key manufacturers, including Lonza and Acetic Acid Chemical Co., Ltd., with the market valued at approximately USD 250 million annually as of 2024, reflecting steady demand for preservatives and antimicrobial agents. No major post-2020 advancements in or catalysis efficiency have significantly altered the core process, though ongoing optimizations focus on recovery to reduce environmental impact.

Applications

Preservative uses

Dehydroacetic acid functions as a broad-spectrum in both and cosmetic products, inhibiting the growth of , yeasts, and molds to extend and maintain product integrity. In applications, dehydroacetic acid is approved as the additive E265 in certain jurisdictions, including , where it is used to preserve commodities such as cut fruits, s, strawberries, beverages, and at concentrations typically ranging from 0.03% to 0.3%. In the United States, the FDA permits its use specifically as a preservative for cut or peeled at a maximum level of 65 parts per million (0.0065%). These applications help prevent microbial spoilage and issues like , with efficacy enhanced in acidic formulations. As of 2025, its use in has been restricted in categories such as products under GB 2760-2024. In cosmetics, dehydroacetic acid is widely employed as a preservative in products like lotions, creams, and shampoos at concentrations of 0.1% to 0.6%, where it is particularly effective against molds and yeasts. Under cosmetics regulations, the maximum authorized concentration is 0.6% (expressed as the acid), excluding use in sprays. Its broad-spectrum activity makes it suitable for water-based formulations, and it complies with standards from organizations like and for natural and organic products. The mechanism of dehydroacetic acid involves disruption of microbial membranes and inhibition of essential activity, leading to reduced microbial proliferation. This action is most effective in the range of 2 to 6, where the undissociated acid form predominates and penetrates microbial s more readily. Dehydroacetic acid is frequently formulated in blends for enhanced performance; for example, it is combined with in products like Geogard 221, a system offering synergistic broad-spectrum protection suitable for personal care applications across a wide range. Historically, dehydroacetic acid was introduced as a preservative in the mid-20th century, with approvals emerging in regions like the during the before subsequent regulatory changes limited its use there.

Antimicrobial applications

Dehydroacetic acid exhibits fungicidal properties suitable for agricultural applications, particularly in to inhibit fungi. The sodium form of dehydroacetic acid is also employed as a wood preservative, enhancing durability against fungal degradation in treated timber. In bactericidal applications, dehydroacetic acid and its salts are incorporated into formulations such as paints, adhesives, and fluids to prevent microbial and spoilage. These uses leverage its ability to disrupt bacterial processes, maintaining product integrity during storage and use in non-food sectors. Efficacy studies demonstrate activity against key fungi like , with minimum inhibitory concentrations () around 400 , and broader spectrum effects on , though specific MIC values for pathogens like species are less documented in industrial contexts. Commercial formulations, such as Biocide 470F, utilize dehydroacetic acid directly for disinfection, offering targeted control in these applications. Due to its low (slightly soluble at approximately 0.5 g/L), dehydroacetic acid has limited application in environmental settings like , where higher is required for effective dispersion.

Safety and regulation

Toxicity and health effects

Dehydroacetic acid exhibits low via , with an LD50 value of 1,480 mg/kg in female rats according to Test Guideline 401. shows even lower , with an LD50 ranging from 3,000 to 5,000 mg/kg in rabbits. While no skin irritation was observed in rabbits over 4 hours per Test Guideline 404, older animal studies indicated minimal eye irritation with mild effects, but recent studies ( Test Guideline 438) show no eye irritation. Chronic exposure studies reveal no evidence of carcinogenicity, mutagenicity, or in available animal data. The Cosmetic Ingredient Review (CIR) Expert Panel has concluded that dehydroacetic acid poses low concern for these endpoints based on comprehensive safety assessments. Similarly, the (EWG) rates it as low concern for cancer, allergies, immunotoxicity, and developmental/. Allergic reactions to dehydroacetic acid are rare but documented, primarily manifesting as in users of containing the compound or its sodium salt. Case reports highlight isolated instances of , often linked to topical applications, though overall potential remains low. Environmental is moderate, with dehydroacetic acid classified as harmful to life under GHS criteria. It shows ecotoxicity to (ErC50 of 32.1 mg/L for Pseudokirchneriella subcapitata per OECD Test Guideline 201) and daphnids (EC50 >100 mg/L per OECD Test Guideline 202), indicating potential harm at concentrations exceeding 1 mg/L in systems. potential is low, as the compound does not meet criteria for persistent, bioaccumulative, or toxic substances. Human exposure to dehydroacetic acid occurs primarily through dermal contact or incidental oral ingestion via preservative-containing cosmetics, foods, and . In vivo metabolism studies in rats and rabbits demonstrate rapid , yielding metabolites such as triacetic acid , hydroxy-dehydroacetic acid, and ultimately acetic acid derivatives like and .

Regulatory approvals

Dehydroacetic acid is regulated as a in the United States, where it is permitted for use as a in cut or peeled at a maximum level of 65 parts per million (ppm), calculated as the acid equivalent, in accordance with specifications outlined in 21 CFR 172.130. It is not classified as (GRAS) for broader applications but is approved for this specific limited use based on safety assessments. In the , dehydroacetic acid and its sodium salt are not authorized as food additives under Regulation (EC) No 1333/2008, reflecting concerns over potential health effects and lack of sufficient toxicological data for general food use. For cosmetics, dehydroacetic acid and sodium dehydroacetate are approved as preservatives in the under Annex V of Regulation (EC) No 1223/2009, with a maximum authorized concentration of 0.6% (expressed as the acid) in ready-for-use products, excluding dispensers. The Cosmetic Ingredient Review () Expert Panel has concluded that both compounds are safe for use in at concentrations up to 0.6% for sodium dehydroacetate and 0.7% for dehydroacetic acid, based on evaluations of toxicological including dermal and potential. In the United States, the CIR assessment supports their safety as used, aligning with FDA oversight for cosmetic ingredients without specific concentration limits beyond general good manufacturing practices. Under the European Union's REACH Regulation (EC) No 1907/2006, dehydroacetic acid (CAS 520-45-6) is registered for industrial uses, including as a and , with an annual tonnage band of 100-1,000 tonnes, and no specific authorization requirements or restrictions beyond standard notification. In the United States, the Environmental Protection Agency (EPA) has listed dehydroacetic acid as an in products, though it is considered an obsolete with no current active registrations or established residue tolerances for agricultural commodities. Internationally, dehydroacetic acid faces variations in approval; it is prohibited in production under both and USDA regulations, as it is a synthetic not listed among allowed substances on the or equivalent standards. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has not established an (ADI) for dehydroacetic acid due to insufficient from prior evaluations. In recent updates, has expanded restrictions effective February 2025, banning its use in additional food categories such as products, , cakes, and fillings, while maintaining limited approvals in others like pickled . Post-Brexit, the has aligned its regulations with the , retaining the 0.6% limit under retained law, with no substantive changes reported for dehydroacetic acid as of 2025.

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