Acetic acid

Search ⌘K Suggest Edit Sign in Sign in Nomenclature IUPAC name Other names History Early history Industrial production history Interstellar acetic acid Physical properties Basic characteristics Thermophysical properties Chemical properties Structure Acidity Solvent properties Biological aspects Biochemical role Gut microbiome and health Production Global production and market Methanol carbonylation Acetaldehyde oxidation Ethylene oxidation Propylene oxidation Biological fermentation Uses Vinyl acetate monomer Ester production Acetic anhydride production Solvent applications Medical uses Food uses Reactions Organic reactions Inorganic reactions Other derivatives Health and safety Vapor hazards Solution hazards Regulatory aspects Environmental impact Fate in environment Toxicity to organisms References Fact-checked by Grok 4 months ago Acetic acid Ara Eve Leo Sal 1x Acetic acid, systematically known as ethanoic acid, is a colorless, volatile liquid organic compound with the chemical formula CH₃COOH (also represented as C₂H₄O₂), characterized by its pungent odor and role as a weak carboxylic acid with a pKa of 4.76. [1] It is the main active component in vinegar , where it constitutes 4–10% of the solution formed by the oxidation of ethanol through fermentation . [2] With a molecular weight of 60.05 g/mol, acetic acid features a simple structure consisting of a methyl group attached to a carboxyl group, making it highly miscible with water , ethanol , and ether . [1] [3] Key physical properties include a melting point of 16.6 °C (often called glacial acetic acid when pure due to its ice-like crystals at room temperature ), a boiling point of 117.9 °C, and a density of 1.049 g/cm³ at 25 °C. [1] Chemically, it is corrosive and flammable, with a flash point of 39 °C and lower explosive limit of 4.0%, reacting exothermically with bases and strong oxidizing agents while remaining stable under normal conditions. [1] Historically, acetic acid has been known since prehistoric times, produced inadvertently through the fermentation of alcoholic beverages into vinegar , and was used by ancient civilizations like the Sumerians (2900–1800 BCE) as a preservative , condiment , and antibiotic . [2] In modern industry, acetic acid is primarily produced via the methanol carbonylation process (Cativa process), where methanol reacts with carbon monoxide in the presence of a rhodium or iridium catalyst to yield high-purity product, accounting for over 80% of global production. [4] [5] Alternative methods include oxidation of acetaldehyde and fermentation for bio-based vinegar production using acetic acid bacteria on ethanol feedstocks. [4] [6] Its applications are diverse, serving as a key intermediate in manufacturing vinyl acetate monomer for polymers, acetic anhydride for textiles and aspirins, and cellulose acetate for films and fibers, while also functioning as a food acidulant, solvent in inks and paints, and reagent in pharmaceuticals and petroleum production. [1] [7] Despite its utility, acetic acid poses health risks as a severe irritant and corrosive to skin , eyes, and respiratory tract , with occupational exposure limits set at 10 ppm (TWA) by OSHA and NIOSH. [1] Nomenclature IUPAC name The preferred IUPAC name for the simplest carboxylic acid with two carbon atoms is acetic acid, which is retained for general nomenclature and use in both substitutive and functional class naming. [8] The systematic IUPAC name is ethanoic acid, acceptable for use in all contexts but not preferred. [8] Ethanoic acid is derived by taking the name of the parent alkane chain, ethane (indicating two carbon atoms), and replacing the ending -e with the suffix -oic acid to denote the presence of the principal carboxylic acid functional group at the end of the chain. [9] Under IUPAC recommendations in the Blue Book (P-65.1.1.2), retained names for unbranched carboxylic acids with up to six carbon atoms—such as formic acid, acetic acid, propanoic acid, butanoic acid, pentanoic acid, and hexanoic acid—are explicitly allowed as preferred IUPAC names due to their long-standing use and clarity in chemical communication. For isotopically labeled variants, IUPAC nomenclature employs specific conventions outlined in the Blue Book (P-82), where nuclide symbols are placed in square brackets or parentheses before the name of the unmodified compound to indicate isotopic modification, with locants for specific positions. [10] For example, the methyl-deuterated form CD₃COOH is named (2,2,2-²H₃)acetic acid, retaining the base name while specifying the positions of the three hydrogen atoms in the methyl group replaced by deuterium . [10] This approach ensures precise identification in spectroscopic and synthetic applications without altering the core structure name. Historically referred to as vinegar acid, the retained name acetic acid reflects its ancient association with fermented sources. Other names Acetic acid derives its name from the Latin word acetum , meaning vinegar, reflecting its historical association with the sour liquid produced through the fermentation of ethanol in sources such as wine or cider. [11] Among its common informal designations are vinegar acid, ethylic acid, and methane carboxylic acid, which highlight its origins and chemical composition in everyday and early scientific contexts. [12] [7] In industrial applications, the term glacial acetic acid specifically refers to the anhydrous, highly concentrated form of the compound, which solidifies into a crystalline, ice-like state at temperatures below 16.6 °C due to its purity and lack of water. [1] [13] Historically, in the late 18th-century nomenclature proposed by Antoine Lavoisier, acetic acid was termed the radical of vinegar, emphasizing the core component derived from vinegar in chemical analysis. [14] [15] While these alternative names persist in various practical and historical discussions, the modern preferred IUPAC name is acetic acid, with the systematic name ethanoic acid also acceptable. [11] History Early history Acetic acid, primarily recognized in its diluted form as the sour constituent of vinegar , has been known since ancient times as a product of the oxidation of wine. In the 4th century BC , Aristotle described the process in his Metaphysics , observing that wine undergoes a transformation into vinegar through alteration, serving as its potential matter without being identical to it, thus highlighting an early understanding of the chemical change involved. [16] Ancient civilizations extensively employed vinegar for practical purposes, leveraging its preservative qualities to extend the shelf life of foods and its antimicrobial properties in medical applications. Egyptians dating back to around 3000 BC used it in embalming and as a remedy, while Greeks and Romans applied diluted vinegar for wound cleaning, treating infections, and as a general disinfectant, as evidenced by references in Hippocratic texts and Roman medical writings. [17] In the medieval era, alchemists refined techniques for concentrating acetic acid by distilling vinegar , yielding a potent form referred to as "strong water of vinegar" or radical vinegar, which was valued for its enhanced reactivity in chemical processes. The pseudonymous alchemist Basilius Valentinus (late 16th century ) detailed such distillations in his works, stressing vinegar 's indispensable role in alchemy and describing methods to obtain a sharper, more volatile acid through repeated fractional distillation . [18] During the 17th and 18th centuries, experimental chemists began distinguishing pure acetic acid from the impurities in vinegar, advancing its identification as a discrete volatile acid. Robert Boyle, in his 1663 treatise Experiments and Considerations Touching Colours , employed "spirit of vinegar"—a distilled, concentrated acetic acid—to explore acid-base reactions, noting its ability to alter the colors of vegetable juices and corrode metals, thereby establishing key properties of acids through systematic observation. [19] Other investigators, such as Herman Boerhaave in the early 18th century, further examined the "mother of vinegar" as a biological agent in acetification, bridging empirical chemistry toward more mechanistic understandings. [17] These efforts paved the way for the first total synthesis of acetic acid from inorganic precursors in 1845 by Hermann Kolbe. [20] Industrial production history The first artificial synthesis of acetic acid marked a pivotal shift from traditional fermentation-based production, which had relied on the natural oxidation of ethanol by acetic acid bacteria to yield vinegar since ancient times. In 1845, German chemist Hermann Kolbe achieved the first total synthesis of acetic acid from inorganic precursors, demonstrating that organic compounds could be constructed from elemental starting materials like carbon and sulfur. His multi-step process began with the formation of carbon disulfide from carbon and sulfur, followed by chlorination to carbon tetrachloride , pyrolysis to tetrachloroethene, hydrolysis to trichloroacetic acid , and reduction to acetic acid, underscoring the feasibility of laboratory-scale artificial production. [20] By the early 20th century, industrial demands, particularly during World War I , drove the adoption of synthetic routes to meet wartime needs for acetone, a key component in explosives like cordite . This led to a significant shift toward the oxidation of acetaldehyde , derived from acetylene hydration, to produce acetic acid as an intermediate before its conversion to acetone via catalytic pyrolysis . The process, operating under controlled oxidation conditions, allowed for scalable production to support military efforts, marking the transition from empirical fermentation to more efficient chemical manufacturing. [21] A major advancement occurred in 1963 with the introduction of the Monsanto process , a rhodium-catalyzed methanol carbonylation method that revolutionized acetic acid production by offering high efficiency and selectivity over previous routes. This innovation, building on earlier high-pressure carbonylation concepts, enabled the dominant modern pathway where methanol reacts with carbon monoxide under milder conditions (150–200°C, 30–50 bar), achieving over 95% selectivity and reducing reliance on petrochemical feedstocks like acetaldehyde or hydrocarbons. [22] In the 1990s, BP Chemicals further enhanced this technology with the Cativa process, an iridium-based catalyst system that improved yields, lowered operating costs, and minimized precious metal usage compared to rhodium catalysis. Announced in 1996 and supported by key patents on promoter additives like ruthenium , the Cativa process achieved higher productivity at low water concentrations (around 5 wt%), allowing widespread adoption by major producers and solidifying methanol carbonylation as the primary industrial method, accounting for the majority of global synthetic output. [23] Interstellar acetic acid Acetic acid was first detected in the interstellar medium toward the Sagittarius B2 (Sgr B2) hot core complex in 1996 using the Berkeley-Illinois-Maryland Association (BIMA) millimeter-wave interferometer array, with confirmation via the Owens Valley Radio Observatory (OVRO) array. This discovery marked the identification of the second carboxylic acid in space after formic acid , with observations targeting rotational transitions near 100 GHz in the Sgr B2 Large Molecule Heimat (LMH) source. Subsequent surveys expanded detections to additional hot molecular cores, including W51e2 and G34.3+0.15, using arrays like the Combined Array for Research in Millimeter-wave Astronomy (CARMA). [24] [25] Abundance estimates from these observations yield column densities of approximately 7 × 10^{15} cm^{-2} in Sgr B2(LMH), with fractional abundances relative to H_2 ranging from 10^{-10} to 10^{-9} across sources. [24] In the dense, warm environments of hot cores (n(H_2) ≈ 10^7–10^8 cm^{-3}, T ≈ 100–200 K), acetic acid contributes to the chemical complexity as a building block and precursor for larger organic species, such as through reactions forming glycine or other amino acid precursors in gas-phase and grain-surface networks. [26] [27] In the 2000s, spectroscopic observations of comets provided constraints on acetic acid abundances; for instance, radio searches toward comet Hale-Bopp (C/1995 O1) yielded an upper limit of <0.5% relative to water in the nucleus ices. More definitive detections occurred later, including in the dusty coma of comet 67P/Churyumov-Gerasimenko via the Rosetta spacecraft's neutral mass spectrometer, confirming its presence as a gas-phase species released from icy grains. Infrared observations with the Spitzer Space Telescope in the mid-2000s surveyed interstellar ices toward young stellar objects, revealing complex organic inventories but only upper limits for carboxylic acids like acetic acid; a likely ice-phase detection came with the James Webb Space Telescope in 2024, toward protostars in NGC 1333. [28] In November 2025, JWST observations confirmed acetic acid in ices around a newly forming star, marking the first definitive solid-phase detection in an astrophysical environment. [29] The interstellar occurrence of acetic acid holds astrobiological significance, as its incorporation into comets and meteorites could have facilitated delivery of prebiotic organics to early Earth, potentially seeding carbon-rich chemistry in primordial oceans. [30] [31] Physical properties Basic characteristics Acetic acid has the molecular formula CH₃COOH and a molar mass of 60.05 g/mol. [3] It is a simple carboxylic acid consisting of a methyl group attached to a carboxyl functional group. In its pure form, known as glacial acetic acid, it appears as a clear, colorless liquid at room temperature, but it solidifies into an ice-like crystalline solid below its melting point of 16.6°C, from which the term "glacial" is derived. [32] Dilute aqueous solutions, such as vinegar, are also colorless liquids. Acetic acid exhibits a strong, pungent odor characteristic of vinegar, with an odor detection threshold of approximately 0.48 ppm in air. [32] At low concentrations, typically 4–8% in vinegar, it produces a distinctly sour taste due to its acidity. The density of pure acetic acid is 1.049 g/cm³ at 20°C, corresponding to a molar concentration of approximately 17.4 M (mol/L) for glacial acetic acid (calculated as 1049 g/L ÷ 60.05 g/mol ≈ 17.46 M, often rounded to 17.4 M). [1] It is fully miscible with water, ethanol, and diethyl ether, forming homogeneous solutions without phase separation. [1] Thermophysical properties Acetic acid is a colorless liquid at room temperature, exhibiting characteristic thermophysical behaviors influenced by its molecular structure and intermolecular hydrogen bonding. Its melting point is 16.6 °C, allowing it to exist as a solid below this temperature and as a liquid above it under standard conditions. The boiling point is 118.1 °C at atmospheric pressure, reflecting the energy required to overcome intermolecular forces in the liquid phase. Additionally, the vapor pressure is 11.6 mmHg at 20 °C, indicating moderate volatility that contributes to its pungent odor and potential for evaporation in open systems. [1] [33] The heat of vaporization is 23.7 kJ/mol at the boiling point, representing the enthalpy change associated with the phase transition from liquid to gas and underscoring the stability of the liquid state due to hydrogen bonding. The specific heat capacity of the liquid is 2.05 J/g·K at 25 °C (equivalent to 123.1 J/mol·K), which measures the energy needed to raise the temperature of the substance and is relevant for processes involving heating or cooling in industrial applications. [1] [34] Acetic acid demonstrates infinite solubility in water, meaning it is fully miscible in all proportions, a property arising from its polar nature and ability to form hydrogen bonds with water molecules. This hydrophilicity is quantified by a partition coefficient (log P) of -0.17, indicating a strong preference for the aqueous phase over nonpolar solvents like octanol. The critical temperature is 322 °C, above which the liquid and vapor phases become indistinguishable, marking the end of the liquid-vapor coexistence curve. The refractive index is 1.371 at 20 °C, a measure of light bending that aids in optical identification and purity assessment. [1] Property Value Conditions Source Melting point 16.6 °C Standard pressure PubChem Boiling point 118.1 °C 1 atm Hanson Chemicals Vapor pressure 11.6 mmHg 20 °C PubChem Heat of vaporization 23.7 kJ/mol Boiling point PubChem (Haynes) Specific heat capacity (liquid) 2.05 J/g·K 25 °C NIST WebBook Solubility in water Infinite (miscible) Room temperature PubChem Partition coefficient (log P) -0.17 Octanol/water PubChem (Hansch et al., 1995) Critical temperature 322 °C - CAMEO Chemicals Refractive index 1.371 20 °C (D line) PubChem (Haynes) Chemical properties Structure Acetic acid, with the formula CH₃COOH, features a Lewis structure consisting of a methyl group (CH₃) bonded to a carboxyl group (COOH). The carboxyl group includes a carbonyl double bond (C=O) with a length of 1.21 Å, a single C-O bond to the hydroxyl group with a length of 1.36 Å, and the methyl group's C-H bonds at approximately 1.09 Å. [35] These bond lengths reflect the partial double-bond character in the C-O linkage due to resonance delocalization within the carboxyl moiety. The carboxyl group of acetic acid is planar, adopting a trigonal planar geometry around the central carbon atom, with bond angles of approximately 120° for the O=C-O and CH₃-C=O angles. This planarity arises from the sp² hybridization of the carboxyl carbon, facilitating π-overlap. Resonance stabilization occurs through two major contributing structures: one with a C=O double bond and C-O single bond, and another where the double bond shifts to C-O with the oxygen bearing a negative charge and the hydroxyl oxygen positively charged; this delocalization shortens the C-O bond and lengthens the C=O bond relative to typical single and double bonds, respectively. [36] In the gas phase and nonpolar solvents, acetic acid molecules predominantly form hydrogen-bonded cyclic dimers, where two molecules associate via a pair of O-H···O hydrogen bonds with an O···O distance of approximately 2.68 Å. This dimerization is driven by the strong intermolecular hydrogen bonding between the hydroxyl hydrogen of one molecule and the carbonyl oxygen of another. The overall molecular dipole moment of monomeric acetic acid in the gas phase is 1.74 D, attributable to the polarity of the C=O and O-H bonds. The tetrahedral geometry around the methyl carbon results in bond angles of about 109° for the C-C-H and H-C-H interactions. [37] [38] Acidity Acetic acid functions as a weak acid in aqueous solution, undergoing partial ionization according to the equilibrium C H X 3 C O O H ⇌ C H X 3 C O O X − + H X + \ce{CH3COOH ⇌ CH3COO- + H+} CH X 3 ​ COOH ​ CH X 3 ​ COO X − + H X + , where the acetate ion ( C H X 3 C O O X − \ce{CH3COO-} CH X 3 ​ COO X − ) acts as its conjugate base. [1] The acid dissociation constant K a K_a K a ​ is 1.75 × 1 0 − 5 1.75 \times 10^{-5} 1.75 × 1 0 − 5 at 25 °C, yielding a p K a \mathrm{p}K_a p K a ​ of 4.756 under these conditions. [39] This p K a \mathrm{p}K_a p K a ​ value demonstrates temperature dependence, increasing slightly with rising temperature due to the exothermic nature of the dissociation process, with a rate of change d ( p K a ) / d T ≈ + 0.0002 \mathrm{d(p}K_a)/\mathrm{d}T \approx +0.0002 d ( p K a ​ ) / d T ≈ + 0.0002 K − 1 ^{-1} − 1 . [40] The resonance stabilization in the acetate ion, involving delocalization of the negative charge between the two oxygen atoms, enhances the stability of the conjugate base and thereby supports the observed acidity. [41] Compared to other carboxylic acids, acetic acid is weaker than formic acid, which has a p K a \mathrm{p}K_a p K a ​ of 3.75 at 25 °C, primarily because the methyl substituent in acetic acid exerts an electron-donating inductive effect that destabilizes the conjugate base relative to the more electron-withdrawing hydrogen in formic acid. [39] In analogous carboxylic acids, electron-withdrawing substituents lower the p K a \mathrm{p}K_a p K a ​ by stabilizing the conjugate base through inductive effects, while electron-donating groups raise it. [41] Solvent properties Acetic acid functions as an amphiprotic solvent, capable of both donating and accepting protons, with an autoprotolysis constant $ K_s $ approximately $ 10^{-14.5} $ at 25°C, which enables limited autoionization similar to water but to a much lesser extent. This property allows acetic acid to dissolve a range of polar and ionic compounds, such as alkali metal acetates, by facilitating ion solvation through proton transfer mechanisms. [42] The dielectric constant of acetic acid is 6.2 at 25°C, significantly lower than that of water (78.5), reflecting its reduced ability to screen charges and stabilize ions compared to aqueous media. [43] Despite this, acetic acid forms extensive hydrogen-bonding networks via its carboxyl group, akin to those in water but weaker due to the lower polarity and the prevalence of dimeric associations, which contribute to its solvating power for polar solutes. [44] In terms of solvatochromic behavior, acetic acid influences the electronic spectra of dissolved species by altering their solvation shells, as seen in its ability to solvate salts like sodium acetate , leading to shifts in absorption bands reflective of the solvent's polarity. It is fully miscible with alcohols such as ethanol and ketones like acetone , owing to compatible hydrogen-bonding and dipole-dipole interactions that prevent phase separation. [1] Acetic acid's solvent properties find application in extraction processes, where it aids in partitioning nonpolar organic compounds from aqueous phases by acting as a co-solvent or modifier that enhances selectivity through its moderate polarity and hydrogen-bonding capacity. [45] Its acidity contributes briefly to solvation by enabling proton donation to basic sites in solutes, complementing its dielectric and bonding interactions. [46] Biological aspects Biochemical role Acetic acid plays a pivotal role in cellular metabolism as the source of the acetyl group (CH₃CO-) incorporated into acetyl-coenzyme A (acetyl-CoA), a central metabolite that links catabolic and anabolic pathways. In the citric acid cycle (Krebs cycle), acetyl-CoA derived from acetic acid undergoes oxidative decarboxylation, generating reducing equivalents (NADH and FADH₂) that drive ATP production via oxidative phosphorylation, while also providing carbon skeletons for biosynthetic processes. [47] Specifically, the complete oxidation of one acetyl-CoA molecule in the cycle yields three NADH, one FADH₂, and one GTP (equivalent to ATP), resulting in approximately 10 ATP molecules through the electron transport chain. [48] Beyond energy generation, acetyl-CoA serves as the primary building block for fatty acid synthesis, where repeated condensation of acetyl units forms longer-chain lipids essential for membrane structure and energy storage. [49] Acetyl-CoA from acetic acid also participates in post-translational modifications, particularly acetylation of proteins such as histones, which modulates chromatin accessibility and gene expression. Histone acetylation, catalyzed by histone acetyltransferases using acetyl-CoA as the donor, neutralizes positive charges on lysine residues, promoting an open chromatin conformation that facilitates transcription factor binding and enhances gene activation. [50] This epigenetic regulation is crucial for cellular differentiation, response to environmental cues, and maintenance of genomic stability, with acetate-derived acetyl-CoA serving as a key nuclear pool for these reactions in various cell types. [51] In prokaryotes, particularly acetogenic bacteria like those in the genus Clostridium , acetic acid is biosynthesized through autotrophic pathways such as the Wood-Ljungdahl pathway, which fixes CO₂ and H₂ into acetyl-CoA before hydrolysis to acetate, supporting growth under anaerobic conditions. [52] For assimilation or further metabolism, acetate is activated to the high-energy intermediate acetyl-AMP by AMP-forming acetyl-CoA synthetase, enabling its conversion to acetyl-CoA and integration into central carbon metabolism. [53] This activation step is energy-intensive, consuming ATP, but is essential for utilizing acetate as a carbon or energy source in nutrient-limited environments. Deficiencies in acetate metabolism, as observed in acetate auxotrophic mutants of bacteria such as engineered Escherichia coli strains, lead to impaired energy production because acetate cannot be effectively oxidized for reducing power or ATP generation, disrupting the glyoxylate shunt and overall respiratory efficiency. [54] These auxotrophs rely on exogenous acetate to bypass metabolic blocks, highlighting acetic acid's indispensable role in sustaining cellular energy homeostasis when alternative substrates are unavailable. Gut microbiome and health Acetic acid, primarily produced in the human gut through bacterial fermentation of dietary fibers, constitutes approximately 60% of the total short-chain fatty acids (SCFAs) generated by the microbiota. [55] Key producers include genera such as Bifidobacterium and Lactobacillus , which metabolize indigestible carbohydrates like resistant starches and oligosaccharides into acetate via pathways involving the acetyl-CoA synthetase enzyme. [56] This process not only provides energy to colonocytes but also serves as a substrate for hepatic acetyl-CoA synthesis, linking microbial activity to host metabolism in a single enzymatic step. [57] Acetate exerts protective effects on gut health by activating the G protein-coupled receptor GPR43 (also known as FFAR2), which enhances epithelial barrier integrity and suppresses inflammation. [58] This receptor-mediated signaling promotes mucus production and tight junction reinforcement in intestinal cells, reducing permeability to pathogens and toxins. [59] In experimental models of colitis, acetate via GPR43 activation diminishes pro-inflammatory cytokine release, such as IL-6 and TNF-α, thereby alleviating disease severity and tissue damage. [60] Beyond barrier function, gut-derived acetate influences systemic metabolism by improving insulin sensitivity and lipid profiles. [61] It inhibits lipolysis in adipose tissue and stimulates GLP-1 secretion from enteroendocrine cells, contributing to better glucose homeostasis and reduced circulating triglycerides. [62] Recent studies from 2023 to 2025 highlight acetate's role in modulating aging-related disorders and depression through SCFA-gut-brain axis interactions; for instance, lower circulating acetate levels have been associated with more severe depressive symptoms. [63] Similarly, in aging models, acetate supplementation ameliorates cognitive decline by reducing neuroinflammation. [64] High-fiber diets elevate luminal acetate concentrations, reshaping microbiota composition to favor beneficial taxa like Akkermansia muciniphila , a mucin-degrader that further boosts SCFA output. [65] This enrichment, observed in human intervention trials, enhances overall microbial diversity and acetate yield, supporting sustained anti-inflammatory and metabolic benefits. [66] Production Global production and market Global acetic acid production capacity was approximately 20 million metric tons per year in 2024, with expansions adding about 5.9 million tons in China during 2025, bringing total capacity to over 25 million tons as of late 2025. [67] Demand was about 17 million tons in 2024 and reached approximately 19 million tons in 2025, amid overcapacity concerns. [68] The market is forecasted to expand at a CAGR of approximately 4-5% from 2025 to 2030, according to various industry reports. [68] China dominates production with over 50% of global capacity and approximately 55% of output as of 2025, supported by its extensive petrochemical infrastructure. [69] In the United States, Celanese Corporation is a key player, contributing significantly to North American supply through large-scale facilities. Saudi Arabia also holds a major position, with producers like SABIC expanding output to meet regional and export demands. In 2025, significant new capacity of 5.9 million tons was added in China, with further expansions planned in India and Saudi Arabia for 2026 onward. [5] The acetic acid market was valued at USD 18.85 billion in 2024, primarily driven by applications in vinyl acetate monomer and ester production for adhesives, coatings, and textiles. [70] Prices fluctuated between USD 350 and 700 per metric ton in 2024-2025, with 2025 averages around USD 400-450 per ton for exports amid oversupply. [71] In 2025, China's expansions contributed to global overcapacity, pressuring prices and accelerating interest in bio-based production for lower emissions. Amid tightening carbon regulations, sustainability trends are accelerating, with a shift toward bio-based feedstocks like syngas fermentation to reduce emissions and align with circular economy goals. [72] Methanol carbonylation Methanol carbonylation represents the dominant industrial route for acetic acid synthesis, involving the catalytic reaction of methanol with carbon monoxide to form acetic acid. This process offers superior atom economy compared to earlier methods, as it directly incorporates the carbon atoms from the feedstocks without significant loss to byproducts. The reaction proceeds under homogeneous catalysis, typically using noble metal complexes in the presence of iodide promoters, and has evolved through key technological advancements since the mid-20th century. [73] The Monsanto process, developed by Monsanto Company and first commercialized in 1970, utilizes a rhodium-based catalyst system, such as R h I 3 \mathrm{RhI_3} Rh I 3 ​ or R h C l 3 \mathrm{RhCl_3} RhC l 3 ​ , along with methyl iodide as a promoter. The core reaction is C H 3 O H + C O → C H 3 C O O H \mathrm{CH_3OH + CO \rightarrow CH_3COOH} C H 3 ​ OH + CO → C H 3 ​ COOH , achieving selectivities exceeding 99% under typical conditions of 150–200°C and 30–60 bar pressure. This iodide-promoted mechanism involves the formation of methyl iodide intermediates, which facilitate carbon monoxide insertion and subsequent hydrolysis to the product, enabling high conversion rates with minimal rhodium loading. The process marked a significant improvement over high-pressure alternatives, reducing energy demands and enhancing overall efficiency. [74] [75] In 1996, BP Chemicals introduced the Cativa process, which employs an iridium catalyst, such as I r I 3 \mathrm{IrI_3} Ir I 3 ​ , promoted by iodide species and ruthenium or other co-promoters to suppress byproduct formation. Operating at similar conditions of 180–200°C and 30–50 bar, it delivers even higher productivity—up to 50 times that of the Monsanto process under low-water regimes—while maintaining selectivities above 99.5% and reducing propionic acid byproducts through efficient inhibition of side reactions. The iridium system's robustness allows operation with lower catalyst concentrations and less corrosion, contributing to lower capital and operating costs. Methyl iodide remains essential as the promoter across both processes, accelerating the oxidative addition steps. [23] [76] Today, methanol carbonylation accounts for approximately 85% of global acetic acid production, with feedstocks derived from syngas via methanol synthesis and purification. [68] Its advantages in selectivity and yield have solidified its position as the preferred method, enabling large-scale operations with capacities exceeding millions of tons annually. [77] Acetaldehyde oxidation The acetaldehyde oxidation process produces acetic acid through the liquid-phase catalytic reaction of acetaldehyde with oxygen or air. The primary reaction is C H X 3 C H O + 1 2 O X 2 → C H X 3 C O O H \ce{CH3CHO + 1/2 O2 -> CH3COOH} CH X 3 ​ CHO + 2 1 ​ O X 2 ​ ​ CH X 3 ​ COOH This exothermic oxidation is carried out by bubbling air or pure oxygen into a reactor containing acetaldehyde dissolved in acetic acid , with unreacted acetaldehyde recycled to maximize efficiency . [78] Industrial implementation typically employs manganese acetate as the catalyst at concentrations of 0.1-0.5 wt%, operating at temperatures of 60-80°C and pressures of 3-10 bar to ensure selectivity and control the reaction rate . Palladium - or copper-based systems have also been explored for enhanced selectivity, particularly in integrated setups, achieving yields of approximately 95-96% based on acetaldehyde conversion. The heat generated during oxidation is often utilized to distill water and purify the product, yielding glacial acetic acid (>99.5% purity). [79] [78] [80] This method marked the first major industrial route for synthetic acetic acid production , with the inaugural plant commissioned around 1911 , and it dominated global output until the 1960s when methanol carbonylation processes gained prominence due to lower costs and feedstock availability. Today, acetaldehyde oxidation represents roughly 5% of worldwide acetic acid production, primarily in facilities integrated with ethanol processing where acetaldehyde serves as an intermediate. [81] [22] Over-oxidation leads to byproducts such as formic acid and CO₂, alongside minor amounts of methyl acetate , methanol , and high-boiling residues like succinic acid derivatives, which are separated via distillation . The process's energy intensity stems largely from the upstream generation of acetaldehyde , often via dehydrogenation of ethanol , making it less competitive than direct carbonylation routes in modern contexts. [79] [82] Ethylene oxidation The vapor-phase oxidation of ethylene to acetic acid provides a direct synthetic route for acetic acid production, utilizing ethylene derived from petroleum sources as the primary feedstock. This method involves the catalytic reaction of ethylene with oxygen in the presence of water vapor , yielding acetic acid as the main product alongside minor amounts of acetaldehyde . The process is noted for its potential integration with ethylene oxide production facilities, leveraging shared ethylene supply chains to enhance efficiency. [83] A key industrial implementation is the Showa Denko process, a one-step vapor-phase method commercialized in Japan in 1997 with an initial capacity of 100,000 tons per year at the Chiba plant. This process employs a supported palladium catalyst, often augmented with heteropoly acids and promoters like selenium or tellurium to minimize carbon dioxide formation. The reaction proceeds according to C₂H₄ + ½O₂ → CH₃COOH, typically under conditions of 150–210 °C and approximately 9–10 bar pressure in a fixed-bed reactor, achieving ethylene conversions of around 7–10% per pass. Selectivity to acetic acid reaches 80–86%, with acetaldehyde as a coproduct at 8–10% and CO₂ at about 5%. This approach accounts for a minor portion of global acetic acid production, estimated at less than 5%, and has seen adoption in Japan and select European facilities for small- to medium-scale operations (50,000–100,000 tons per year). [84] [85] The mechanism of this process follows a Wacker-type pathway, where ethylene undergoes initial coordination and hydration with the palladium catalyst to form an acetaldehyde intermediate, facilitated by the CuCl₂ reoxidant in variants akin to the original Hoechst-Wacker system (PdCl₂–CuCl₂). The acetaldehyde is then subsequently oxidized to acetic acid under the reaction conditions, with water vapor playing a crucial role in promoting selectivity over complete combustion to CO₂. This stepwise mechanism allows for high yields while coproducing acetaldehyde , which can be recycled or separated downstream. The process offers advantages in reduced wastewater generation compared to liquid-phase alternatives, making it environmentally preferable for integrated petrochemical complexes. [85] [83] Propylene oxidation The vapor-phase catalytic oxidation of propylene to acetic acid represents a researched direct synthetic route for acetic acid production. This process involves the reaction of propylene with oxygen (or air) over metal oxide catalysts, including molybdenum-vanadium oxides, phosphomolybdate supported on silica, or zinc-promoted cobalt-molybdenum oxides. The reaction typically occurs at temperatures of 343–482 °C (650–900 °F), usually at atmospheric pressure, with steam frequently added as a moderator to enhance selectivity toward acetic acid. [86] [87] Acetic acid is the principal product, with reported selectivities ranging from 60% to 80% depending on the catalyst composition, reaction conditions, and use of promoters such as zinc. Key byproducts include carbon dioxide, acetaldehyde, acetone, and minor amounts of compounds such as acrolein or formaldehyde. The process has been patented (e.g., US3423455A and US3536755A) and studied in academic and industrial research but has not become a primary industrial route for acetic acid, which remains dominated by methanol carbonylation. Additionally, acetic acid forms as a byproduct in the commercial production of acrylic acid via the partial oxidation of propylene. Biological fermentation Biological fermentation represents a key microbial route for acetic acid production, leveraging bacteria to convert renewable substrates into acetate under controlled conditions. This process encompasses both oxidative and anaerobic pathways, enabling the utilization of diverse feedstocks such as ethanol , sugars, and syngas , and contributes to sustainable production alternatives. [88] In the oxidative pathway, bacteria of the genus Acetobacter perform incomplete oxidation of ethanol to acetic acid in an aerobic environment. The reaction proceeds as follows: C H X 3 C H X 2 O H + O X 2 → C H X 3 C O O H + H X 2 O \ce{CH3CH2OH + O2 -> CH3COOH + H2O} CH X 3 ​ CH X 2 ​ OH + O X 2 ​ ​ CH X 3 ​ COOH + H X 2 ​ O This two-step enzymatic process involves membrane-bound alcohol dehydrogenase converting ethanol to acetaldehyde, followed by aldehyde dehydrogenase forming acetic acid. [88] Acetobacter species thrive in acidic, oxygen-rich conditions, making this pathway central to vinegar production, where ethanol from prior alcoholic fermentation (e.g., from fruits or grains) is oxidized to yield acetic acid concentrations of 4-18%. This method accounts for approximately 5-20% of vinegar-related output, with optimized strains achieving acetic acid yields of 90-95% from ethanol. [89] The anaerobic pathway, in contrast, utilizes acetogenic bacteria like Clostridium thermoaceticum (now classified as Moorella thermoaceticum ) via the Wood-Ljungdahl pathway to fix carbon dioxide into acetic acid. This reductive acetyl-CoA route condenses a methyl group (from CO₂ reduction via tetrahydrofolate) with carbon monoxide (from CO₂ reduction) to form acetyl-CoA , which is then cleaved to acetate . The simplified overall reaction is: 2 C O X 2 + 4 H X 2 → C H X 3 C O O H + 2 H X 2 O \ce{2 CO2 + 4 H2 -> CH3COOH + 2 H2O} 2 CO X 2 ​ + 4 H X 2 ​ ​ CH X 3 ​ COOH + 2 H X 2 ​ O Particularly suited for syngas fermentation , this process converts mixtures of CO, H₂, and CO₂—derived from biomass gasification or industrial off-gases—into acetic acid under strictly oxygen-free conditions. [90] Yields can reach up to 90% conversion of CO₂ to acetate , with stoichiometric production of 3 moles of acetate per mole of glucose in hexose fermentation . [91] Typical conditions for biological acetic acid fermentation include temperatures around 30°C for oxidative processes and pH ranges of 3