| HS Code | |
| Productname | Acetic Anhydride |
| Iupacname | Ethanoic anhydride |
| Casnumber | 108-24-7 |
| Ecnumber | 203-957-4 |
| Unnumber | 1715 |
| Molecularformula | C4H6O3 |
| Molecularweight | 102.09 g/mol |
| Appearance | Colorless liquid |
| Odor | Pungent, vinegar-like |
| Density | 1.08 g/cm3 at 20 °C |
| Meltingpoint | -73.1 °C |
| Boilingpoint | 139.8 °C |
| Flashpoint | 49 °C closed cup |
| Autoignitiontemperature | 316 °C |
| Solubility | Reacts with water; miscible with ether, ethanol, and benzene |
| Refractiveindex | 1.3900 at 20 °C |
| Vaporpressure | 4 mmHg at 20 °C |
| Hazardclass | 8 |
| Packinggroup | II |
| Signalword | Danger |
As an accredited Acetic Anhydride factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Acetic anhydride supplied in 200 kg sealed steel drums, labeled corrosive and flammable, with moisture-resistant lining; store dry. |
| Container Loading (20′ FCL) | Acetic Anhydride in 20′ FCL: UN 1715, Class 8 corrosive liquid, sealed drums, moisture-controlled, securely lashed for hazardous transport. |
| Shipping | Acetic anhydride (UN 1715, Class 8, PG II) ships as a corrosive liquid under DOT/IMDG/IATA hazardous-materials rules. Use sealed, compatible containers, labeled and placarded. Keep dry, cool, upright, and segregated from water, oxidizers, acids, bases, and heat. Provide emergency response information and trained handling. |
| Storage | Store acetic anhydride in a cool, dry, well-ventilated, fire-resistant area away from heat, sparks, open flames, and direct sunlight. Keep containers tightly closed and clearly labeled, using corrosion-resistant, compatible materials. Protect from moisture, alcohols, amines, bases, and oxidizers. Ground and bond during transfer, provide secondary containment, and follow local regulations for flammable, corrosive liquids. Store separately from incompatible substances. |
| Shelf Life | Acetic anhydride shelf life is typically 2 years if stored tightly sealed in a cool, dry place away from moisture. |
In batch reactor trains used for acetylsalicylic acid (ASA) production, salicylic acid is charged with acetic anhydride at a molar ratio of 1.00:1.05 to 1.00:1.20 and a catalytic quantity of 85% phosphoric acid or concentrated sulfuric acid at 0.2-0.5 wt% relative to salicylic acid. The reaction mass is held at 80-90°C for 45-90 min under reflux in glass-lined vessels fitted with anchor or retreat-curve agitators; tip speeds are maintained at 1.5-3.0 m/s to ensure heat transfer without vortex entrainment. Water content in the reactor is controlled below 0.5 wt% because hydrolysis of acetic anhydride to acetic acid reduces yield and causes batch-to-batch variability. After the hold period, residual acetic anhydride is quenched with purified water, the exotherm is managed by jacket cooling, and the mixture is cooled to 10-20°C to crystallize ASA. Filtration is followed by washing at a final filtrate conductivity below 50 µS/cm and vacuum drying at 60-70°C. Free salicylic acid is controlled below 0.25 wt% per the USP monograph for aspirin, and residual solvent specifications are set by pharmacopoeial limits. In the parallel acetylation of p-aminophenol to paracetamol, acetic anhydride is added at a molar ratio of 1.00:1.05 to an aqueous reaction mixture held at 60-80°C; the product crystallizes directly from the neutralized batch, and residual 4-aminophenol is limited to 0.005 wt% under the BP monograph. The processing conflict is the narrow operating window between reaction completion and hydrolysis of the acetylated product, so cooling rates and residual water limits are more critical than raw stoichiometry.
Before cellulose pulp is esterified, the moisture content is reduced to below 1 wt% and the alpha-cellulose content is specified above 95%. Pre-treatment with glacial acetic acid is carried out in a sigma-blade kneader at 30-40°C, followed by addition of acetic anhydride at a dry-cellulose-to-anhydride weight ratio of 1:3.0 to 1:3.5 and sulfuric acid catalyst at 0.5-2.0 wt% on cellulose. The initial exotherm is controlled below 35°C to prevent oxidative degradation and localized charring; after the primary cellulose triacetate dope clarifies, the batch is ramped to 50-60°C and held until the degree of substitution exceeds 2.9. Hydrolysis is then conducted with water/acetic acid at 60-80°C to reduce the degree of substitution to 2.4-2.5, which is the acetone-soluble window required for filter tow and fibre spinning. The precipitated flake is washed to remove sulfate residues, stabilized, and dried to below 0.5% moisture. Acetyl content is determined by ASTM D871-96, and intrinsic viscosity is monitored as an indirect measure of molecular weight for downstream spinning dope filtration. For cellulose acetate filter tow, the flake is dissolved in acetone at 25-30 wt% solids, filtered through sintered-metal media, and dry-spun through multi-hole spinnerets in heated cabinets; triacetin is metered into the opened tow at 5-10 wt% on dry fibre before plug-making. The limit of this hydrolysis pathway is that over-hydrolysis below DS 2.3 irreversibly reduces acetone solubility and raises filtration pressure, while under-hydrolysis leaves dichloromethane-soluble triacetate domains that impair downstream triacetin uptake.
| Degree of substitution | Acetyl content | Solvent behaviour | Typical downstream route |
|---|---|---|---|
| 2.4-2.5 | 39.1-40.0 wt% | Soluble in acetone, ethyl acetate | Filter tow, spectrally clean fibre |
| 2.8-3.0 | 42.5-44.8 wt% | Soluble in dichloromethane/methanol | High-modulus film, high-solids coating |
The reactor charge for acetylation of softwood or plantation radiata pine is typically kiln-dried material at 6-8% equilibrium moisture content, loaded into a vacuum-pressure vessel, and subjected to a preliminary vacuum of 0.2 bar absolute before anhydride flooding. Pressure is raised to 10-14 bar and temperature to 120-140°C; hold times run from 2-8 h depending on cross-section and permeability, because acetylation is diffusion-limited rather than intrinsically slow. Target modification is a weight percent gain of 20-25%, corresponding to acetyl content near 20-21 wt% of dry modified wood. The by-product acetic acid is stripped under vacuum, condensed, and recovered for regeneration into acetic anhydride; residual free acid is removed during post-curing until the product exits with a mild, non-corroding odour profile. Modification of hydroxyl sites reduces equilibrium moisture content at 65% RH and 20°C from roughly 12% in untreated controls to below 6%; radial swelling under liquid-water exposure is reduced by 70-80%. Standards used for qualification include EN 335 use-class assignment, EN 350 durability classification, and EN 113 basidiomycete decay testing, where modified material typically falls in durability class 1 with mass loss below 5% after the prescribed exposure period. The critical boundary is that thick sections above 50-75 mm may retain unconverted core if the cycle is truncated; manufacturers therefore specify maximum board thickness and schedule non-destructive core drilling to verify weight gain on production trials. Acetylated wood is not automatically strength-graded for structural applications, and end-use approval requires separate bending and fastener tests under the relevant national timber code.
TAED production is distinguished by the need to drive acetylation to full substitution while keeping free acetic acid low enough for detergent granule storage. Ethylenediamine is reacted with acetic anhydride at a molar ratio of at least 4.0:1 to force formation of the tetraacetyl derivative, with staged dosing because the first acetylation to N,N'-diacetylethylenediamine occurs rapidly at 70-80°C, whereas the final two acetyl groups require reflux at 110-125°C for several hours. The product is crystallized from acetic acid/water mixtures, washed with deionized water, and vacuum-dried at 80°C; specification limits for moisture are typically below 0.5 wt%, residual free acetic acid below 0.5 wt%, and purity above 99%. Residual acetic acid is the main fabrication problem because it promotes caking of compacted detergent powders and contributes to warehouse odour, so wash cycles are designed around conductivity and pH targets rather than fixed time. Particle size is controlled to a median diameter of 500-800 µm by crystallization temperature and seed addition; oversize is milled and fines recycled. In wash liquor, TAED reacts with perborate or percarbonate to generate peracetic acid at 20-60°C, which is the functional bleaching species in low-phosphate and oxygen-based detergent formulations. Compliance for detergent use is anchored to the EU Detergents Regulation (EC) No 648/2004 and biodegradability screening under OECD 301B; detergent producers additionally set transport stability limits for compacted granules at 40°C and 75% RH to prevent premature activator loss.
Acetylation of native starch in aqueous suspension requires pH-stat-controlled addition of acetic anhydride into a 35-45 wt% solids slurry at 25-40°C, with the pH held at 8.0-10.0 by metered sodium hydroxide. The purpose of the pH-stat is to neutralize liberated acetic acid without allowing the slurry to exceed the gelatinization threshold; temperature excursions above 50°C or pH above 10.5 produce swollen granules that blind centrifuge screens and raise dryer load. Acetic anhydride addition is limited to a level that yields acetyl groups below 2.5 wt% in the finished starch ester, matching the food additive specification of EU E 1420 under Commission Regulation (EU) No 231/2012 and the acetyl ester limit of 21 CFR 172.892. The reaction is completed within 1-2 h, after which the slurry is neutralized to 5.5-6.5, washed in a countercurrent centrifuge battery or hydrocyclone stack to remove sodium acetate, and dried to below 14% moisture. The resulting acetylated starch is used in retorted sauces, fruit fillings, dairy desserts, and frozen gravies where native starch would retrograde or produce a short, gel-like texture. Processors specify the degree of substitution not by acetyl content alone but by Brabender or RVA viscosity profiles, paste clarity, and freeze-thaw stability; this is because even small differences in esterification uniformity create measurable changes in hot viscosity and cold storage syneresis.
Triacetin obtained from direct esterification of glycerol with acetic anhydride is specified less by single-pass conversion than by final acidity, water content, and Gardner colour. Glycerol is charged with acetic anhydride at a molar ratio of 1.00:3.05 to 1.00:3.15 and a methanesulfonic acid catalyst loading of 0.1-0.5 wt%; the batch is initially held at 50°C, then ramped to 110-120°C under reflux, and finally stripped under vacuum below 200 mbar to remove acetic acid and residual water. Reaction time is 2-4 h after the final temperature is reached, and the crude ester is passed through a wiped-film evaporator or vacuum distillation column to meet an assay of 99.0% or higher, free acidity below 0.1% as acetic acid, and moisture below 0.2%. In filter-tow manufacture, triacetin is applied at 5-10 wt% on fibre dry basis through a post-spinning spray or kiss-roll system before the tow is bulked into bales; distribution is monitored by near-infrared scanning of crimped tow because uneven plasticizer causes hardness variation in finished filter rods. The same ester is used as a solvent in coating and ink systems where low volatility and high flash point are required. Regulatory references include 21 CFR 184.1901 for triacetin as a food additive and E 1518 under Commission Regulation (EU) No 231/2012; industrial users additionally monitor iodine colour number and ester content by gas chromatography. The primary operational boundary is the presence of mono- and diacetin impurities: levels above 1.0% reduce plasticizing efficiency and can create tacky deposits on tow guides.
The reaction of primary aromatic amines with acetic anhydride proceeds rapidly in aqueous acetic acid, but the batch is held for 2-4 h to complete conversion of residual aniline before crystallization. Aniline is charged with acetic anhydride at a molar ratio of 1.00:1.05 to 1.00:1.10, and the temperature is controlled between 60°C and 100°C because the addition is exothermic and the boiling point of the water/acetic acid mixture limits the upper range. After acetylation, excess anhydride is quenched with water, the reaction mass is neutralized to 4.5-5.0 with sodium hydroxide, and acetanilide is crystallized, filtered, washed, and dried to a flake or powder with a melting point of 113-115°C and purity above 99.5%. The product is an isolable intermediate for chlorosulfonation, nitration, and subsequent sulfa-drug or dye chemistry; in sulfanilamide routes, residual aniline is controlled below 0.1% because it carries into the final API and fails pharmacopoeial purity tests. Process equipment is specified in glass-lined steel or 316L stainless because hot acetic acid and acetic anhydride are corrosive to carbon steel, and pumps, seals, and transfer lines are rated for both acid service and the crystallization slurry. The limitation is that acetanilide is a protected-amino intermediate rather than a finished end-product, so supply-chain specifications are written around downstream nitration or chlorosulfonation performance, particularly moisture and free aniline content.
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Acetic anhydride, CAS 108-24-7, is supplied as a liquid acetylation reagent with molecular formula C4H6O3 and molecular weight 102.09 g/mol. Commercial product designations include industrial acetylation grade, low-iron pharmaceutical-synthesis grade, and ACS reagent grade. Bulk industrial acetylation grade is commonly specified at assay ≥ 98.5 % by GC area normalization, with acetic acid carryover at or below 0.50 % and water below 0.10 %; both impurities reduce per-pass acetylation efficiency and increase aluminum or stainless steel corrosion when the liquid is exposed to atmospheric moisture. At 20 °C, density is 1.080–1.085 g/cm³ by ASTM D4052, closed-cup flash point is 49 °C, freezing point is −73.1 °C, boiling point at 101.3 kPa is 139.8 °C, and vapor pressure is approximately 0.4 kPa. The liquid has a dynamic viscosity of approximately 0.91 mPa·s at 20 °C and is soluble in ethanol, ethyl acetate, and toluene, but it hydrolyzes in water and reacts with alcohols, primary or secondary amines, and strong oxidizers.
Industrial production of acetic anhydride commonly proceeds by thermal cracking of acetic acid to ketene at 700–750 °C under reduced pressure, followed by absorption of ketene in acetic acid in a packed absorber. The absorber temperature is maintained below 50 °C to limit polymerization and color formation; unreacted ketene is recycled to the absorption tower. The crude anhydride is then distilled, and a center cut is drawn to meet the acetic acid and water limits for industrial acetylation grade. Because ketene is acutely toxic and cannot be stored economically over long distances, the production train is usually integrated with downstream acetylation capacity. This integration reduces long-distance anhydride transport but makes supply reliability sensitive to cracker, absorber, and distillation bottlenecks.
| Parameter | Typical specification | Test method |
|---|---|---|
| Assay, as C4H6O3 | ≥ 98.5 % by GC | Manufacturer normalized GC |
| Acetic acid | ≤ 0.50 % | ASTM D1613 |
| Water | ≤ 0.10 % | ASTM E203 |
| Color, Pt-Co | ≤ 10 | ASTM D1209 |
| Density at 20 °C | 1.080–1.085 g/cm³ | ASTM D4052 |
Acetyl chloride produces hydrogen chloride as the leaving group; this creates a more corrosive and chloride-contaminated acetylation stream and usually requires alkaline scrubbing and chloride removal from finished esters. Acetic anhydride generates acetic acid, which can be neutralized or recovered by distillation and does not introduce chloride residues. Ketene is a gas with high acute inhalation toxicity and is generated on-site because of storage and transport constraints; acetic anhydride is a pumpable liquid that can be stored in bulk tanks. Acetic acid alone is generally insufficient for production-scale acetylation because its equilibrium generates water, limiting conversion unless water is continuously removed. The following table summarizes the principal process differences.
| Reagent | Physical state at 25 °C | Principal by-product | Relative reactivity | Critical process limitation |
|---|---|---|---|---|
| Acetic anhydride | Liquid | Acetic acid | Moderate | Water sensitivity; exothermic hydrolysis |
| Acetyl chloride | Liquid | Hydrogen chloride | High | Corrosive HCl gas evolution; chloride residues |
| Ketene | Gas | None directly | High | On-site generation; high acute toxicity |
| Acetic acid | Liquid | Water | Low | Equilibrium limited; requires water removal |
The theoretical acetyl transfer capacity of acetic anhydride is one acetyl group per molecule, equivalent to 42.2 % active acetyl by mass. This is lower than the active acetyl content of acetyl chloride by mass, but the absence of chloride residues and the ability to recover acetic acid favor acetic anhydride in cellulose ester and pharmaceutical acetylation. The reactivity difference also affects equipment selection: acetyl chloride handling commonly demands glass-lined or high-nickel alloy metallurgy combined with HCl scrubbing, whereas acetic anhydride can be processed in 316L stainless steel or glass-lined equipment with acetic acid recovery systems.
Storage in closed-loop systems is designed around the hydrolysis exotherm. Water ingress above roughly 0.10 % initiates a temperature rise, which accelerates further hydrolysis and can raise headspace pressure in sealed tanks if the vent is wetted or blocked. Field inspections of bulk stainless steel storage tanks identify bypass valve leakage and desiccant vent saturation as recurrent moisture sources at sites where ambient relative humidity exceeds 60 %. Nitrogen blanketing at 5–10 kPa gauge and vacuum/pressure relief devices set to manufacturer values are used; tank materials are typically 316L stainless steel or aluminum, while carbon steel is avoided because iron contamination accelerates color formation and acetic acid generated during hydrolysis attacks the metal surface. Acetic anhydride must be segregated from amines, strong oxidizers, strong bases, and water-based fire suppression media. The workplace exposure limit is commonly applied as a ceiling value of 5 ppm, and local exhaust ventilation is required where transfer pumps, vents, or sampling points can release vapor. Published data for this specific configuration is limited, so site-specific exposure monitoring is performed rather than relying only on default ventilation rates.
Cellulose acetylation is the largest single use of acetic anhydride. Cellulose sheet or wood pulp is pre-swollen with acetic acid, then reacted with acetic anhydride in the presence of sulfuric acid catalyst at initial temperatures below 50 °C. The acetylation exotherm is controlled by staged anhydride addition and jacket cooling; a rise above 60–70 °C can increase chain scission and lower solution viscosity. The degree of substitution is adjusted by the anhydride-to-anhydroglucose unit ratio, with commercial cellulose triacetate commonly targeted at DS 2.8–2.9 before partial hydrolysis to secondary cellulose acetate. The hydrolysis step uses dilute acetic acid and is terminated when the desired acetyl content and viscosity are reached; viscosity for cellulose acetate is measured by ASTM D871. Batch-to-batch variance is monitored by online refractive index or FT-NIR analysis of the acetylation liquor, and the acetic acid by-product is recovered in closed distillation loops. Low water content in the feedstock is critical because water consumes anhydride, alters the stoichiometric ratio, and reduces the attainable degree of substitution without a corrective anhydride addition.
In pharmaceutical synthesis, acetic anhydride serves as the acetyl donor for acetylsalicylic acid and paracetamol. For aspirin, salicylic acid is acetylated with acetic anhydride at 50–60 °C in a jacketed glass-lined reactor; residual anhydride is quenched with water after the reaction, and the exothermic quench is managed by controlled addition at temperatures below 40 °C. For paracetamol, p-aminophenol is acetylated in acetic acid or water; the anhydride consumption is monitored by the disappearance of the primary amine peak in HPLC or by titration of residual acetic acid. Low-iron pharmaceutical-synthesis grade is used where iron concentrations below 1 ppm are required to limit color formation in the finished API. Acetylation with acetic anhydride is preferred over acetyl chloride in these routes because the by-product is acetic acid rather than hydrogen chloride, which simplifies glass-lined reactor maintenance and avoids chloride residues.
Detergent-grade tetraacetylethylenediamine is produced by acetylation of ethylenediamine with acetic anhydride and acetic acid; the reaction is run under controlled temperature because the intermediate and product can discolor if local hot spots develop. Wood and starch acetylation consume smaller volumes, with acetic anhydride added to the substrate under vacuum or nitrogen to limit hydrolysis. In each case, acetic anhydride is selected over acetyl chloride when chloride-free products and recoverable acetic acid are required.
Thermal degradation of acetic anhydride is slow at ambient storage but increases with temperature and metal ion contamination. Color formation is commonly measured in Pt-Co units; industrial grades are often specified at ≤ 10 Pt-Co by ASTM D1209, while low-iron pharmaceutical-grade product may be specified at ≤ 5 Pt-Co. Acetic acid content is controlled because it correlates with increased conductivity and corrosivity toward stainless steel. Water content is measured by ASTM E203; acidity is measured by ASTM D1613. Hydrolysis is acid-catalyzed and base-catalyzed; the operational boundary for storage is therefore to keep the material dry and away from caustic scrubber carryover. Distillation or re-purification should not be attempted in carbon steel equipment because iron contamination raises the color number and generates sludge.
Acetic anhydride is shipped under UN 1715 as a corrosive liquid with Packing Group II. Its closed-cup flash point of 49 °C, lower explosion limit of 2.7 vol%, and upper explosion limit of 10.3 vol% place constraints on storage tank heating and ventilation. REACH-registered supply chains require safety data sheets with the current classification for acute toxicity, skin corrosion, and flammability; users must verify that the delivered grade matches the ordered specification because low-iron and ACS reagent grades differ mainly in trace metal, color, and water limits, not in acetylation stoichiometry.
