Acetic Acid 

Acetic acid, systematically known as ethanoic acid, is one of the most important organic acids in both industrial chemistry and biological systems. While most commonly recognized as the main component of vinegar, its applications extend far beyond the kitchen. It is a precursor for paints, adhesives, packaging materials, and synthetic fibers.

In chemical nomenclature, it belongs to the carboxylic acid family and serves as a fundamental building block in organic synthesis.

1.0Chemical Structure and Formula

The chemical formula for acetic acid is CH₃​COOH. It is the second simplest carboxylic acid (after formic acid).

Its molecular structure comprises a methyl group (CH₃​) attached to a carboxyl group {-COOH}. The -COOH  group is responsible for the molecule's acidic properties.

The acidity arises from the molecule's ability to ionise upon dissolution in an aqueous solution. During this process, the hydrogen atom within the carboxyl group's hydroxy-OH  part dissociates as a proton t{H}⁺  or, more accurately, forms a hydronium ion H₃O⁺ in water, according to the reversible reaction:

$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}+{\mathrm{H}}^{+}$

The resulting species is the acetate ion, CH₃COO^- ion.

Molecular Data

• IUPAC Name: Ethanoic Acid
• Molar Mass: 60.05 g/mol
• Hybridization: The carbonyl carbon is sp₂ hybridized, while the methyl carbon is sp³ hybridized.

Functional Group and Hybridization

The carboxyl group is the functional group of acetic acid, responsible for its acidic properties. The carbon atom in the carboxyl group is sp² hybridized, forming one double bond with oxygen and a single bond with a hydroxyl group.

2.0Physical Properties

Understanding the physical nature of acetic acid is crucial for its handling and storage in industrial settings.

• Appearance: Pure acetic acid is a clear, colorless liquid.
• Odour: It possesses a distinct, pungent, and sour smell (characteristic of vinegar).
• Boiling Point: 118°C.
• Melting Point: 16.6°C.
• Solubility: It is a polar protic solvent. It is miscible (mixes in all proportions) with water, alcohol, ether, and glycerol.
• Density: 1.05 g/cm³ (slightly denser than water).

What is Glacial Acetic Acid?

The term "Glacial Acetic Acid" refers to anhydrous (water-free) acetic acid. The name derives from its physical property of freezing into ice-like crystals at temperatures slightly below room temperature (approx. 16.6°C). In this concentrated form, it is highly corrosive and requires careful handling.

Preparation of Acetic Acid (CH₃COOH)

Industrially, acetic acid is made by the carbonylation of methanol. The process involves three key reactions:

1. CH₃OH + HI → CH₃I + H₂O
   Methanol reacts with hydrogen iodide to form methyl iodide.
2. CH₃I + CO → CH₃COI
   Methyl iodide then combines with carbon monoxide to produce acetyl iodide.
   (This step requires a metal carbonyl complex as a catalyst.)
3. CH₃COI + H₂O → CH₃COOH + HI
   Acetyl iodide reacts with water to yield acetic acid, regenerating HI.

Overall, the process converts methanol into acetic acid through an iodide intermediate and catalytic carbonylation.

3.0Chemical Properties

Acetic acid is classified as a weak acid. Unlike strong acids (like hydrochloric acid) that fully dissociate in water, acetic acid only partially dissociates.

Dissociation in Water

When dissolved in water, the equilibrium exists as follows:

$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}(\mathrm{aq})\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{CO}{\mathrm{O}}^{-}(\mathrm{aq})+{\mathrm{H}}^{+}(\mathrm{aq})$

Here, $C{H}_{3}CO{O}^{-}$ is the acetate ion.

Important Chemical Reactions

1. Reaction with Bases (Neutralization)

As a weak acid, it reacts with strong bases such as sodium hydroxide to form a salt and water.

$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaOH}\to \mathrm{C}{\mathrm{H}}_3\mathrm{COONa}(\mathrm{Sodium}\mathrm{Acetate})+{\mathrm{H}}_2\mathrm{O}$

This reaction is often used in titration labs to determine the concentration of vinegar.

2. Reaction with Carbonates (The "Volcano" Reaction)

This is the classic science fair reaction. Acetic acid reacts with bicarbonates or carbonates to release Carbon Dioxide gas.

$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaHC}{\mathrm{O}}_3\to \mathrm{C}{\mathrm{H}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\uparrow$

The rapid release of $C{O}_2$ gas causes the characteristic fizzing.

3. Esterification (Fisher Esterification)

When heated with alcohol in the presence of an acid catalyst (like H₂​SO₄​), acetic acid forms an ester. Esters are sweet-smelling compounds used in perfumes.

$\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{COO}{\mathrm{C}}_2{\mathrm{H}}_5(\mathrm{Ethyl}\mathrm{Acetate})+{\mathrm{H}}_2\mathrm{O}$

• Reactants: Acetic Acid (sour) + Ethanol (alcohol smell)
• Product: Ethyl Acetate (sweet, fruity smell, often like pear or nail polish remover).

4.0Production Methods

Acetic acid is produced via two main pathways: synthetic chemical processes and biological fermentation.

A. Carbonylation of Methanol

This is the dominant industrial method (accounting for over 75% of global production). Methanol reacts with carbon monoxide in the presence of a catalyst (such as rhodium or iridium).

$C{H}_{3}OH+CO\to C{H}_{3}COOH$

Notable processes include the Monsanto process and the Cativa process.

B. Biological Fermentation (Oxidative Fermentation)

For food-grade vinegar, acetic acid is produced by the bacterial fermentation of ethanol. Bacteria from the genus Acetobacter oxidize ethanol in the presence of oxygen.

$C_2{H}_{5}OH+O_2\to C{H}_{3}COOH+H_{2}O$

This method is strictly required for vinegar sold for culinary use in many jurisdictions, as synthetic acetic acid cannot be labelled as "vinegar."

5.0Glacial Acetic Acid vs. Dilute Acetic Acid

Students often confuse these two terms. The difference lies purely in water concentration.

6.0Major Uses and Applications

The versatility of acetic acid makes it a staple in various sectors.

A. Industrial Applications (Chemical Manufacturing)

The largest use of acetic acid is as a raw material for other chemicals.

• Vinyl Acetate Monomer (VAM): Used to make Polyvinyl Acetate (PVA), which is essential for wood glue, adhesives, and paints.
• Purified Terephthalic Acid (PTA): Acetic acid is used as a solvent in the production of PTA, which is the raw material for PET plastics (water bottles) and polyester fibers (clothing).
• Acetic Anhydride: A major reagent used to produce cellulose acetate (film and textiles) and aspirin.

B. Food Industry

• Vinegar: Household vinegar is typically a 4-8% solution of acetic acid. It is used for flavoring, pickling, and marinades.
• Preservative (E260): It acts as an acidity regulator. Lowering the pH of food prevents the growth of bacteria such as Clostridium botulinum and Salmonella.

C. Medical and Pharmaceutical

• Antiseptic: Dilute solutions are used to treat ear infections (otitis externa) due to their antibacterial and antifungal properties.
• Cervical Cancer Screening: Acetic acid is used in visual inspection with acetic acid (VIA) to detect cervical abnormalities.
• Wart Removal: High concentrations are used to treat common warts.

D. Household Uses

• Descaling Agent: Excellent for removing limescale (calcium carbonate) from kettles, showerheads, and coffee machines.
• Cleaning: Used as a streak-free cleaner for windows and mirrors.

7.0Health and Safety Hazards

While safe in dilute forms (like vinegar), concentrated acetic acid is a hazardous substance.

• Corrosive: Glacial acetic acid can cause severe skin burns and permanent eye damage.
• Inhalation: Vapors are highly irritating to the nose, throat, and lungs. Long-term exposure to high levels can damage the respiratory tract.
• Flammability: While dilute acetic acid is not flammable, concentrated acetic acid is combustible above 39°C.
• Handling: Always wear protective gloves and eyewear when handling concentrations higher than 10%.