CH₃COOH — Acetic Acid

Acetic acid (CH₃COOH) is one of the simplest and most important carboxylic acids. It is a colorless, corrosive liquid known for its characteristic pungent odor and sour taste, commonly recognized as the key component of vinegar. Vinegar typically contains 4–8% acetic acid by volume. In its concentrated form, it is known as glacial acetic acid because it solidifies at temperatures below 16.6°C, forming ice-like crystals.

Acetic acid is widely used in the production of synthetic fibers, plastics, solvents, and dyes. It also serves as a chemical reagent and food preservative. Biologically, acetic acid is a product of fermentation and a central intermediate in metabolism, forming part of the acetyl group in acetyl-CoA within living cells.

Acetic acid has the molecular formula \(CH_3COOH\), consisting of a methyl group (CH₃–) attached to a carboxyl group (–COOH). The carbon atom in the carboxyl group forms a double bond with one oxygen atom and a single bond with a hydroxyl (–OH) group.

The molecule is polar due to the presence of the carboxylic group, which allows it to form strong hydrogen bonds. In the liquid state, acetic acid molecules often exist as dimers connected through hydrogen bonds:

This hydrogen bonding explains its relatively high boiling point compared to other compounds of similar molecular weight.

Acetic acid can be prepared both naturally and synthetically.

1. Natural Fermentation (Biological Method)

Acetic acid is produced by the aerobic oxidation of ethanol by Acetobacter bacteria. This process occurs during the fermentation of alcohol to vinegar:

This biological process has been used for centuries in vinegar production.

2. Synthetic Methods

• From Acetaldehyde: Ethanal (acetaldehyde) is oxidized in the presence of a catalyst like manganese acetate or cobalt acetate to produce acetic acid.

\(CH_3CHO + [O] \rightarrow CH_3COOH\)

• By Oxidation of Ethanol: Ethanol is oxidized by potassium dichromate (K₂Cr₂O₇) and sulfuric acid.

\(C_2H_5OH + 2[O] \rightarrow CH_3COOH + H_2O\)

• Carbonylation of Methanol (Monsanto Process): The most important industrial method involves reacting methanol with carbon monoxide in the presence of a rhodium-iodide catalyst.

\(CH_3OH + CO \xrightarrow{Rhodium/Iodine} CH_3COOH\)

• Appearance: Colorless liquid with a pungent smell and sour taste.
• Boiling and Melting Points: High due to hydrogen bonding and dimer formation.
• Solubility: Miscible with water, alcohol, and ether due to polarity.
• Acidic Nature: Acetic acid ionizes in aqueous solution to form acetate ions:

\(CH_3COOH \leftrightarrow CH_3COO^- + H^+\)

• Reaction with Bases: Reacts with bases to form acetate salts and water:

\(CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O\)

• Reaction with Metals: Reacts with active metals such as magnesium and zinc to liberate hydrogen gas:

\(2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2\)

• Reaction with Carbonates and Bicarbonates: Produces carbon dioxide, water, and acetate salts.

\(CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + CO_2 + H_2O\)

• Esterification: Reacts with alcohols in the presence of concentrated sulfuric acid to form esters.

\(CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O\)

• Dehydration: On heating with phosphorus pentoxide (P₂O₅), it yields ketene (CH₂=C=O).

\(CH_3COOH \xrightarrow{P_2O_5} CH_2CO + H_2O\)

• Food Industry: Used as a preservative (E260) and flavoring agent in pickles, sauces, and vinegar.
• Textile Industry: Used in dyeing and printing processes to fix dyes to fabrics.
• Plastic and Polymer Production: Essential in the manufacture of polyvinyl acetate and cellulose acetate, which are used to make adhesives, fibers, and plastics.
• Pharmaceuticals: Used as an intermediate in the synthesis of aspirin and other medicinal compounds.
• Chemical Industry: Acts as a reagent for the production of acetic anhydride, acetate esters, and solvents.
• Household Cleaning: Dilute acetic acid is an effective descaling and disinfecting agent.
• Biochemical Role: Serves as a precursor for acetyl-CoA in metabolism, connecting glycolysis and the citric acid cycle.

While dilute acetic acid (vinegar) is safe for consumption, concentrated or glacial acetic acid is highly corrosive. It can cause burns on contact with skin and severe damage to eyes and mucous membranes. Inhalation of vapors may irritate the respiratory tract.

When handling concentrated acetic acid, use protective gloves, goggles, and ensure good ventilation. In case of contact, wash the affected area thoroughly with water. Acetic acid should be stored in tightly closed containers away from strong oxidizers, bases, and direct sunlight.

Despite its hazards in concentrated form, acetic acid remains a vital compound in both industry and daily life due to its versatility and low environmental toxicity.