Acetic Acid

Acetic acid is a colourless organic compound with a distinct sour taste and pungent smell, most commonly recognised as the primary component of vinegar apart from water. Its chemical formula is CH₃COOH, and it belongs to the class of organic compounds known as carboxylic acids. Acetic acid plays an essential role in industrial chemistry, biology, and daily human life. It is one of the simplest carboxylic acids, yet its applications range from food preservation to polymer manufacturing. Its characteristic acidic properties arise from the presence of the carboxyl group (–COOH), which releases hydrogen ions in aqueous solutions.

Chemical Structure and Properties

Acetic acid is a weak monoprotic acid, meaning it can donate one proton (H⁺) per molecule in an aqueous solution. Structurally, it consists of a methyl group (CH₃–) attached to a carboxyl group (–COOH). The systematic IUPAC name is ethanoic acid, and it has a molecular mass of 60.05 g/mol.
Key physical and chemical properties include:

• Molecular formula: CH₃COOH
• Boiling point: 118.1°C
• Melting point: 16.6°C
• Density: 1.049 g/cm³ (at 25°C)
• pKa value: 4.76 at 25°C
• Appearance: Colourless liquid with a sharp, vinegar-like odour

Pure acetic acid is often called glacial acetic acid because it solidifies just below room temperature, forming ice-like crystals. It is miscible with water, alcohol, and many other organic solvents due to its polar nature and ability to form hydrogen bonds.
Chemically, acetic acid exhibits the typical reactions of carboxylic acids, including esterification with alcohols to form esters, reaction with bases to form salts (acetates), and reduction to form aldehydes or alcohols.

Historical Background

Acetic acid has been known since ancient times, primarily as vinegar obtained through the fermentation of wine. The word “vinegar” derives from the Old French term vinaigre, meaning “sour wine.” Ancient civilisations such as the Babylonians and Egyptians used vinegar as a preservative and medicinal agent.
In the 8th century, the Persian alchemist Jabir ibn Hayyan is believed to have concentrated acetic acid from vinegar through distillation. By the 16th century, the method of dry distillation of wood produced impure acetic acid, known as pyroligneous acid. The modern industrial production of acetic acid began in the 19th century with the development of synthetic routes involving acetaldehyde oxidation.

Methods of Production

Acetic acid can be produced both biologically and synthetically.
1. Biological Production (Fermentation): The most traditional method is through the oxidation of ethanol by Acetobacter bacteria in the presence of oxygen. This is a two-step process:

• C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (Fermentation of sugars to ethanol)
• C₂H₅OH + O₂ → CH₃COOH + H₂O (Oxidation of ethanol to acetic acid)

This method is still widely used for vinegar production and is considered natural fermentation.
2. Synthetic Production: Modern industrial synthesis of acetic acid involves chemical routes that are more efficient for large-scale manufacturing. The main synthetic methods include:

• Methanol Carbonylation (Monsanto and Cativa Processes): The reaction of methanol with carbon monoxide in the presence of a catalyst (rhodium or iridium complexes) to yield acetic acid.CH₃OH + CO → CH₃COOH
  The Monsanto process, developed in the 1960s, and its successor, the Cativa process, are the predominant methods used today due to their high efficiency and selectivity.
• Oxidation of Acetaldehyde: Acetaldehyde is oxidised in the presence of metal catalysts (such as manganese or cobalt acetate) to produce acetic acid.
• Butane Oxidation: Liquid-phase oxidation of butane with oxygen at high temperature and pressure forms acetic acid along with by-products like formic and propionic acids.

Globally, over 15 million tonnes of acetic acid are produced annually, with synthetic methods accounting for more than 90% of production.

Industrial and Commercial Applications

Acetic acid has wide-ranging industrial and domestic applications, making it one of the most versatile organic acids.
1. Food Industry: It is primarily used as vinegar, containing about 4–8% acetic acid, for seasoning and food preservation. Its antimicrobial properties prevent spoilage by inhibiting bacterial growth.
2. Chemical Industry:

• Vinyl Acetate Monomer (VAM): Used to produce polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH), essential components in adhesives, paints, and coatings.
• Acetic Anhydride: Employed in the production of cellulose acetate (used in photographic films, textiles, and plastics) and aspirin.
• Esters of Acetic Acid: Such as ethyl acetate and butyl acetate, which are widely used as solvents in inks, coatings, and perfumes.

3. Textile Industry: Acetic acid is utilised in dyeing and printing processes to adjust pH and fix dyes to fabrics.
4. Pharmaceuticals: It serves as a reagent in the synthesis of several pharmaceuticals, including aspirin and paracetamol. Dilute acetic acid solutions are also used as antiseptics and ear drops to treat infections.
5. Laboratory Use: In analytical chemistry, it functions as a solvent and pH regulator. Glacial acetic acid is commonly used in titration of weak bases.

Biological Role and Safety Considerations

Acetic acid occurs naturally in the metabolic pathways of many organisms. It is an intermediate in the Krebs cycle, where acetyl-CoA is oxidised to release energy. Small quantities of acetic acid are also found in fruits, milk products, and fermented foods.
Although dilute acetic acid (as in vinegar) is safe for consumption, concentrated or glacial acetic acid is corrosive and can cause severe burns upon contact with skin or mucous membranes. Inhalation of vapours may irritate the respiratory tract, and ingestion can damage internal tissues. Hence, proper safety precautions, such as gloves and eye protection, are required when handling concentrated acetic acid.

Physical Behaviour and Acidic Nature

In aqueous solution, acetic acid partially ionises as follows:CH₃COOH ⇌ CH₃COO⁻ + H⁺
Being a weak acid, only about 1% of molecules ionise in water, making it weaker than mineral acids such as hydrochloric or sulphuric acid. The conjugate base, acetate ion (CH₃COO⁻), forms salts with metals like sodium, potassium, and calcium. These acetates have numerous applications in industry and medicine.

Environmental Impact and Sustainability

Acetic acid is biodegradable and poses minimal environmental risk when diluted. However, large-scale industrial emissions, particularly from methanol carbonylation, must be managed to prevent air pollution. The trend in recent years has been toward greener production methods, including bio-based acetic acid derived from biomass fermentation, aligning with sustainable chemistry principles.
Acetic acid can also act as a precursor in bioplastic production, offering an eco-friendly route to polymer synthesis. Moreover, its use in replacing more toxic acids and solvents underlines its role in environmentally conscious chemical manufacturing.

Significance in Everyday Life

Beyond industrial and biological importance, acetic acid features prominently in daily life. From household cleaning (where vinegar acts as a natural disinfectant and descaler) to culinary uses (as a preservative and flavouring agent), its versatility is unmatched. Its role in traditional medicine, such as in treating sore throats or skin infections, has been documented across cultures for centuries.
In modern scientific research, acetic acid is used to maintain pH balance in buffers, sterilise surfaces, and prepare reagents for DNA extraction and histology.

Economic and Global Context

China, the United States, and Western Europe are leading producers of acetic acid. The compound’s global demand continues to grow, especially due to its critical role in polymer and solvent production. Market trends indicate increasing preference for bio-acetic acid, derived from renewable feedstocks like ethanol and agricultural residues, due to environmental regulations and sustainability goals.
The global acetic acid market value is projected to exceed USD 15 billion by the early 2030s, reflecting its indispensable role in both consumer goods and industrial sectors.