Acetic Acid is a key organic compound with widespread utility in a number of industrial applications. Characterized by its colorless appearance and pungent smell, Acetic Acid is a crucial building block in the organic synthesis process and a significant contributor to food and beverage flavorings. This article delves into the industrial applications of Acetic Acid, discussing its role as a chemical intermediary, as well as in areas like the production of vinegar, textiles, and pharmaceuticals, among others.
What is Acetic Acid?
Acetic Acid, also known as ethanoic acid, is a clear, colorless liquid with a pungent vinegar-like smell. It is an organic compound with the chemical formula CH3COOH. Acetic Acid is one of the simplest carboxylic acids widely used in various industries and everyday applications. Acetic Acid is primarily known for its presence in vinegar and is responsible for the sour taste and distinctive smell. Vinegar is a diluted solution of Acetic Acid, typically containing around 4-8% Acetic Acid by volume. Acetic Acid is produced both through natural fermentation processes, as seen in vinegar production, and through chemical synthesis methods on an industrial scale. Its versatile nature, sour taste, and distinct odor make it an essential compound in various fields, from food and beverages to chemicals and manufacturing.
Source: Wikipedia
How is Acetic Acid Produced?
Acetic Acid is produced both synthetically and by bacterial fermentation.
The most common synthetic methods used are the carbonylation of methanol, acetaldehyde oxidation, and ethylene oxidation. Other critical processes are:
- Methyl formate isomerization
- Conversion of syngas to Acetic Acid
- Gas phase oxidation of ethylene and ethanol
Carbonylation of Methanol
Methanol (CH3OH) and carbon monoxide (CO) are the primary reactants for the carbonylation process. A suitable catalyst facilitates the reaction between methanol and carbon monoxide.
Step 1: Formation of Methyl Iodide (CH3I)
Methanol reacts with iodine (I2) to form methyl iodide (CH3I):
CH3OH + I2 → CH3I + HI
Step 2: Formation of Acetyl Iodide (CH3COI)
Methyl iodide then reacts with carbon monoxide to produce acetyl iodide (CH3COI):
CH3I + CO → CH3COI
Step 3: Formation of Acetic Acid (CH3COOH)
Acetyl iodide is hydrolyzed with water (H2O) to form Acetic Acid and hydrogen iodide (HI):
CH3COI + H2O → CH3COOH + HI
Methanol carbonylation can be achieved through two well-known processes: the Monsanto process and the Cativa process, each utilizing different catalysts under controlled temperature and pressure conditions.
Monsanto Process
The Monsanto process, also known as the rhodium-catalyzed process, is an important industrial method for producing Acetic Acid from methanol and carbon monoxide. The Monsanto process employs a rhodium-based catalyst, often in rhodium iodide complexes. Methyl iodide (CH3I) is added as a promoter to enhance the reaction rate. Methanol and carbon monoxide react in the presence of the rhodium catalyst to form Acetic Acid.
Cativa Process
The Cativa process, also known as the iridium-catalyzed process, is an alternative method for the carbonylation of methanol to produce Acetic Acid. The Cativa process employs an iridium-based catalyst, typically iridium iodide complexes. The reaction conditions for the Cativa process are similar to the Monsanto process, with high pressure (30-60 atmospheres) and moderate temperatures (150-200°C). Similarly, a promoter, usually methyl iodide, is added to enhance the reaction rate. Methanol and carbon monoxide react in the presence of the iridium catalyst, resulting in the formation of Acetic Acid. The iridium catalyst facilitates the insertion of carbon monoxide into methanol, producing Acetic Acid.
Acetaldehyde Oxidation
Before the Monsanto process, Acetic Acid was mostly generated by oxidizing acetaldehyde. The oxidation of acetaldehyde to Acetic Acid (CH₃COOH) can be carried out using an oxidizing agent, such as atmospheric oxygen or a chemical oxidant like potassium permanganate (KMnO₄) or chromic Acid (H₂CrO₄). The specific chemical reactions involved depend on the choice of an oxidizing agent.
2CH₃CHO + O₂ → 2CH₃COOH
In this reaction, two acetaldehyde molecules react with one oxygen molecule to form two Acetic Acid molecules. Catalysts are often used to enhance the reaction rate and selectivity of the oxidation process. Common catalysts employed include metal catalysts like copper or silver or metal oxide catalysts like palladium or platinum.
Ethylene Oxidation
The oxidation of ethylene to produce Acetic Acid is a commercially important process. The primary method used for this purpose is the Wacker process, named after the German company Wacker Chemie, which developed the technique.
Ethylene (C2H4) is typically obtained from cracking hydrocarbon feedstocks, such as ethane or naphtha, through a steam cracking process. The produced ethylene serves as the starting material for Acetic Acid production. In the Wacker process, ethylene is oxidized using a combination of oxygen and a PdCl2 catalyst to form acetaldehyde.
The primary reaction involved is C₂H₄ + O₂ → CH₃CHO.
The oxidation reaction is typically carried out under controlled conditions to optimize ethylene conversion to acetaldehyde. The acetaldehyde produced in the previous step is further oxidized to Acetic Acid (CH₃COOH) using oxygen or air as the oxidizing agent. The reaction can be catalyzed by metal catalysts, such as palladium or rhodium, supported on a suitable substrate.
CH₃CHO + O₂ → CH₃COOH
The acetaldehyde oxidation step is a crucial part of the process, as it converts the intermediate compound into the desired final product, Acetic Acid. The ethylene and acetaldehyde oxidation steps are typically carried out at elevated temperatures and pressures, optimized to achieve the desired reaction rate and selectivity.
Conversion of Syngas to Acetic Acid
Syngas is reacted with a copper-zinc-aluminum oxide catalyst to produce methanol (CH₃OH) through syngas-to-methanol (STM) or methanol synthesis. The reaction typically proceeds according to the following equation:
CO + 2H₂ → CH₃OH
The copper-zinc-aluminum oxide catalyst helps facilitate the reaction by providing the necessary active sites for converting carbon monoxide (CO) and hydrogen (H₂) into methanol. Gas phase oxidation of ethylene and ethanol. Carbonylation refers to introducing a carbonyl group (C=O) into a molecule. This reaction is carried out in the presence of a homogeneous rhodium catalyst. The specific catalyst used is often a rhodium complex.
The overall reaction of methanol carbonylation can be represented as follows:
CH₃OH + CO → CH₃COOH
The rhodium catalyst enables the carbonylation reaction, where methanol and carbon monoxide react to form Acetic Acid. The two-step process, consisting of syngas-to-methanol conversion and methanol carbonylation, allows for the efficient and selective production of Acetic Acid from syngas. The process is well-established and widely used in the industry due to its high yield and productivity.
Bacterial Fermentation
Bacterial fermentation is another important method for producing Acetic Acid. The two main types of bacterial fermentation are oxidative fermentation and anaerobic fermentation.
Oxidative Fermentation
Acetic Acid bacteria of the genus Acetobacter have played a significant role in vinegar production throughout human history. These bacteria can convert alcohol into Acetic Acid, with the overall chemical reaction being:
C₂H₅OH + O₂ → CH₃COOH + H₂O
In this reaction, ethanol (alcohol) and oxygen combine to form Acetic Acid and water. When a dilute alcohol solution, such as apple cider, wine, fermented grain, malt, rice, or potato mashes, is inoculated with Acetobacter bacteria and provided with sufficient oxygen, it undergoes fermentation. It gradually transforms into vinegar over a few months.
The Acetobacter bacteria metabolize the alcohol in the solution, utilizing it as a carbon and energy source, and produce Acetic Acid as a byproduct. The process requires the presence of oxygen, which the bacteria use to oxidize ethanol.
Anaerobic Fermentation
Certain species of anaerobic bacteria, such as Clostridium and Acetobacterium, can convert sugars directly into Acetic Acid without producing ethanol as an intermediate.
The chemical equation represents this direct conversion process:
C₆H₁₂O₆ → 3CH₃COOH
In this reaction, a sugar molecule (glucose, represented as C₆H₁₂O₆) is directly converted into three molecules of Acetic Acid (CH₃COOH).
These acetogenic bacteria can also produce Acetic Acid from one-carbon compounds such as methanol, carbon monoxide, or a mixture of carbon dioxide and hydrogen. The reaction can be represented as:
2CO₂ + 4H₂ → CH₃COOH + 2H₂O
In this case, carbon dioxide and hydrogen combine to produce Acetic Acid and water. These acetogenic bacteria have the potential to produce Acetic Acid more efficiently than ethanol-oxidizing bacteria like Acetobacter because they can utilize less costly inputs, including carbon monoxide or carbon dioxide and hydrogen gas. However, it is essential to note that the acid tolerance of Clostridium bacteria is generally lower compared to Acetobacter strains. Even the most acid-tolerant Clostridium strains are limited in producing vinegar at high concentrations. Typically, the vinegar produced by Clostridium strains reaches concentrations of only a few percent, while Acetobacter strains can make vinegar with concentrations up to 20%.
Industrial Uses of Acetic Acid
Acetic Acid As A Chemical Intermediary
Acetic Acid is used as a versatile chemical reagent in producing different compounds. The largest single use of Acetic Acid is in the production of vinyl acetate monomer (VAM), which is a critical component in the production of polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH). These materials are widely used in adhesives, paints, coatings, textiles, and films. Another significant application of Acetic Acid is in the production of acetic anhydride. Acetic anhydride is used primarily as a reagent in synthesizing cellulose acetate, which finds applications in producing textiles, cigarette filters, and photographic film fibers.
Vinegar Production
Acetic Acid is the main component of vinegar and is responsible for its distinctive sour taste. Vinegar production involves ethanol fermentation by Acetic Acid bacteria, typically of the Acetobacter or Gluconobacter genus. The process starts with converting sugars or carbohydrates in fruits, grains, or other organic materials into ethanol through alcoholic fermentation, usually facilitated by yeast. Fermentation can occur naturally or with the addition of specific yeast strains. Once ethanol is produced, it serves as the substrate for the Acetic Acid bacteria. These bacteria oxidize the ethanol into Acetic Acid in the presence of oxygen, a process known as acetous fermentation. This fermentation occurs in a controlled environment, typically in large vessels or barrels. During fermentation, the Acetic Acid bacteria convert the ethanol to Acetic Acid, producing vinegar.
Food Preservative and Flavoring Agent
Acetic Acid is used as a food preservative and flavoring agent in various food products, such as sauces, pickles, dressings, and condiments. Acetic Acid’s sour taste and antimicrobial properties make it a valuable ingredient in food preservation and flavor enhancement across various culinary applications.
Textile Industry
Acetic Acid finds application in the textile industry, particularly in dyeing and printing.
| Dye Fixation | Acetic Acid is commonly employed to fix dyes onto fabrics. When fabrics are dyed with certain types of dyes, such as reactive dyes, acid dyes, or some natural dyes, Acetic Acid is used as an acidifying agent to adjust the pH of the dye bath. The acidic environment created by Acetic Acid helps fix dyes onto the fabric fibers, enhancing color fastness and preventing the dyes from bleeding or fading. |
| pH Regulator | Acetic Acid acts as a pH regulator in dye baths. It helps maintain the optimum pH level required for dyeing certain types of fibers or specific dye classes. Different dyes and fibers have unique pH requirements for successful dyeing. Acetic Acid can be used to lower the pH of the dye bath when necessary. |
| Cleaning and Degreasing | Acetic Acid is also used in the textile industry for cleaning and degreasing processes. It helps remove impurities, residues, and excess dyes from fabrics, improving their quality and appearance. |
| Textile Printing | In textile printing, Acetic Acid is used as a component of printing pastes or as a pre-treatment chemical. It helps prepare the fabric for printing by adjusting the pH and enhancing the absorption of dyes or pigments onto the fabric’s surface. |
Pharmaceuticals
Acetic Acid is used in the pharmaceutical industry to produce various drugs and pharmaceutical compounds. It is used as a solvent for the formulation of certain medications. Acetic Acid is also utilized as a starting material or reagent in synthesizing various pharmaceutical drugs and active pharmaceutical ingredients (APIs). It can be involved in different chemical reactions, such as esterification, acetylation, and condensation reactions, to produce specific drug molecules.
Cleaning Agent
Acetic Acid can be used as a cleaning agent due to its antibacterial properties and its ability to dissolve mineral deposits, grime, and stains.
| Surface Cleaning | Acetic Acid is effective in cleaning various surfaces, including countertops, glass, tiles, and fixtures. It can help remove dirt, grease, and mineral buildup from these surfaces. Diluted Acetic Acid solutions are often used for general cleaning purposes. |
| Stain Removal | Acetic Acid is useful in removing certain types of stains, such as hard water stains, lime deposits, and rust stains. Its acidic nature helps break down and dissolve these stains, making them easier to remove. |
Photography
Acetic Acid is commonly used as a component of the stop bath solution in film photography. After the film is exposed and developed in a developer solution, the stop bath is used to halt the development process and prevent further chemical reactions. The stop bath containing Acetic Acid helps neutralize the alkaline developer, ensuring the development process stops at the desired time.
Properties of Acetic Acid
| Chemical formula | CH3COOH |
| Molecular weight (g/mol) | 60.052 |
| Odor | Strong vinegar-like smell |
| Appearance | Colorless liquid |
| Melting point(°C) | 16 – 17 |
| Boiling Point (°C) | 118 – 119 |
| Density (g/cm³) | 1.3 |
| Refractive Index | 1.371 |
| Vapor Pressure (mm Hg at 25 °C ) | 5.7 |
| Solubility | Solubility in water is relatively high, making it a miscible liquid in water. It is also soluble with polar organic solvents, while its solubility decreases in nonpolar solvents. |
| Stability | Stable under normal conditions but can degrade under acidic conditions or in the presence of light or heat. |
Acetic Acid Derivatives
Here are a few examples of the derivatives of Acetic Acid:
| Ethyl Acetate (CH3COOCH2CH3) | It is an ester formed by the reaction of Acetic Acid with ethanol. Ethyl acetate is commonly used as a solvent in various applications, such as paints, coatings, and flavorings. |
| Acetic Anhydride (C4H6O3) | It is formed by removing one molecule of water from two molecules of Acetic Acid. Acetic anhydride is a versatile chemical reagent widely used in organic synthesis, particularly for acetylation reactions. |
| Acetyl Chloride (CH3COCl) | It is formed by replacing the -OH group of Acetic Acid with a chlorine atom. Acetyl chloride is a reactive compound used for acylation reactions in organic synthesis. |
| Acetamide (CH3CONH2) | It is an amide derivative of Acetic Acid formed by replacing the -OH group with an amine group (-NH2). Acetamide is used as a precursor in various chemical reactions. |
| Acetophenone (C8H8O) | It is an aromatic ketone derived from Acetic Acid. Acetophenone is used in the production of fragrances, flavors, and pharmaceuticals. |
| Acetic Acid Ethyl Ester (Ethyl Acetate, CH3COOC2H5) | It is an ester formed by the reaction of Acetic Acid with ethanol. Ethyl acetate is widely used as a solvent and flavoring agent. |
Safety & Regulatory Considerations
Health Effects of Acetic Acid
Inhalation of Acetic Acid vapor can cause severe irritation to the eyes, mucous membranes, and upper respiratory tract. Long-term exposure to Acetic Acid vapor can lead to chronic bronchitis and other respiratory effects. Contact with concentrated Acetic Acid solutions, typically 80% or higher, can cause corrosive effects on the skin. It can result in severe burns and tissue destruction upon contact. Splashing glacial Acetic Acid into the eyes can cause permanent corneal opacity and damage to the eye. Prolonged exposure to Acetic Acid, such as inhalation or ingestion, can lead to tooth enamel erosion. Prolonged skin exposure to Acetic Acid vapor can result in skin darkening, cracking, and thickening. Workers exposed to Acetic Acid vapor for extended periods have reported thickening and blackening of the skin on their hands. Ingestion of corrosive concentrations of Acetic Acid can cause tissue destruction, perforation, and strictures in the throat, esophagus, and stomach. It can also lead to intestinal bleeding and damage to the heart and kidneys.
Exposure Limits
| OSHA PEL | The current Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for Acetic Acid is 10 ppm of air as an 8-hr time-weighted average (TWA) concentration. |
| NIOSH PEL | The National Institute for Occupational Safety Health (NIOSH) has established a recommended exposure limit(REL) of 10 ppm as an 8-hr TWA and 15 ppmas : short-term exposure limit (STEL). A STEL is a 15-min TWA exposure that should not be exceeded at any time during a workday [NIOSH 1992]. |
| ACGIH TLV | The National Institute for Occupational Safety Health (NIOSH) has established a recommended exposure limit(REL) of 10 ppm as an 8-hr TWA and 15 ppmas : a short-term exposure limit (STEL). A STEL is a 15-min TWA exposure that should not be exceeded at any time during a workday [NIOSH 1992]. |
Identification Numbers
| CAS Number | 64-19-7 |
| Chemical Name | Acetic Acid |
| RTECS No. | AF1225000 |
| DOT UN | 2789 29 (glacial Acetic Acid or Acetic Acid solutions that are more than 80% acid 2790 60 (aqueous solutions that are more than 10% but not more than 80% acid) |
Fun Facts About Acetic Acid
- In its natural form as vinegar, Acetic Acid has been known and used by humans for thousands of years. “Acetic” is derived from the Latin word “acetum,” which means vinegar.
- The Human Body naturally produces a small amount of Acetic Acid during digestion. It is a byproduct of the breakdown of carbohydrates and fats.
- Acetic Acid plays a role in the aging process of wines. Over time, Acetic Acid is formed through ethanol oxidation, contributing to aged wines’ complex flavors and aromas.
