Acetic Acid

Acetic Acid is a synthetic carboxylic acid. It is a monocarboxylic acid with two carbon atoms. It is a clear, colorless organic liquid that has a pungent odor like vinegar. is a solution of acetic acid and water that contains 5%-20% acetic acid by volume. An undiluted solution of acetic acid is known as glacial acetic acid. It forms ice-like crystals when the temperature falls below 16 degrees Celsius.

The IUPAC name of acetic acid is ethanoic acid. Its formal name is also ethanoic acid, but it is usually referred to as acetic acid, which is derived from the Latin word acetum, which means vinegar.

Acetic Acid Formula I Structure of Acetic Acid

In the solid-state, in the chain of molecules, the molecules are connected to each other by hydrogen bonds. Its chemical formula is CH₃COOH or its structure is given by CH₃(C=O) OH, or CH₃CO₂H.

It is a carboxylic acid that contains a methyl group (CH₃) connected to a carboxyl group. We can also say that the acetyl group (CH₃CO) is connected to the hydroxyl group (OH). It has sp² hybridization and occurs as a dimer in a liquid or vapor state due to intermolecular hydrogen bonding.

Lewis structure of Acetic Acid

It is clear from the below Lewis structure of acetic acid that it has six single bonds and one double bond. Further, each oxygen atom contains two lone pairs (total four electrons) of electrons. Each dash shows one bond in which two electrons are bonded to each other.

Physical Properties of Acetic Acid

It is a colorless liquid with a pungent smell.
Its melting point is 289K and its boiling point is 391K at STP.
Its molar mass is 60.052 g/mol and its density in the liquid form is 1.049 g.cm.^-3
It is a polar, protic solvent in liquid form with a dielectric constant of 6.2.
It has a corrosive nature. So, it can cause blisters when comes in contact with the skin.

Chemical Properties of Acetic Acid

The carboxyl functional group in acetic acid can cause ionization of the compound, such as CH₃COOH ? CH3COO^- + H⁺
The release of the proton due to ionization of acetic acid is the reason for the acidic quality of acetic acid.
Its acid dissociation constant (pKa) in a solution of water is 4.76.
Its conjugate base is acetate (CH₃COO^-).
The pH of its solution of 1.0M concentration is 2.4, so it does not dissociate completely.

Acetic Acid Reactions

Acetic acid can undergo all the chemical reactions of other carboxylic acids as described below:

Decomposition
At a temperature of more than 440 degrees Celsius, it undergoes decomposition to produce either methane and carbon or water and ethenone as shown below:
CH₃COOH + heat → CO₂ + CH₄
CH₃COOH + heat → H₂C=C=O + H₂O
Reaction with metals
Few metals such as magnesium, zinc, and iron undergo corrosion when exposed to acetic acid. Acetate salts are formed in these reactions. For example, when ethanoic acid reacts with magnesium, magnesium acetate is formed with the release of hydrogen gas as shown below:
2CH₃COOH + Mg → Mg (CH₃COO)₂ (magnesium acetate) + H₂
Reaction with alkalis
Acetic acid when reacts with alkalis forms acetate salts as shown below:
CH₃COOH + KOH → CH₃COOK + H₂O
Reaction with carbonates
Acetic acid forms acetate salts when reacts with carbonates. Carbon dioxide and water are also formed.
2CH₃COOH + Na₂CO₃ (sodium carbonate) → 2CH₃COONa + CO₂ + H₂O
CH₃COOH + NaHCO3 (sodium bicarbonate) → CH₃COONa + CO₂ + H₂O

Uses of Acetic Acid

Acetic acid has numerous uses. Some of the major uses of acetic acid are as follows:

It is used in the manufacturing of dyes, plastics, silk, perfumes, and more.
It is also used as a table vinegar.
It is used as a local irritant in medicines.
It is used as a coagulation agent in the rubber industry.
It is required in the formation of acetone, acetate, esters, etc.
It is important for the production of VAM (vinyl acetate monomer).
It is also used in the treatment of cancer.
It is an important industrial solvent because of its desirable solvent properties and its ability to form miscible mixtures with both polar and nonpolar compounds. So, it is widely used to prepare dimethyl terephthalate (DMT) on a large scale or in industries.
It is also used as a micro bacterial disinfectant.

Glacial Acetic Acid

Glacial acetic acid refers to an acetic acid that contains very less water (less than 1%). It is also known as anhydrous (water-free) acetic acid. It is called glacial as it tends to solidify at low temperatures less than 16 degrees Celsius.

It can be formed by dripping acetic acid solution over a frozen "stalactite" of acetic acid. The pure acid sticks to the glacial acetic acid, whereas, the impurities flow with the liquid. Acetic acid is a weak acid; considered safe to drink in vinegar. However, glacial acetic acid is corrosive in nature, so it can harm the skin on contact.

Preparation of Acetic Acid

Acetic acid is produced through the carbonylation of methanol. The three steps involved in this method as described below:

CH₃OH (methanol) + HI (hydrogen iodide) →CH₃l (methyl iodide intermediate) + H₂O
CH₃I + CO (carbon monoxide) →CH₃Col (acetyl iodide)
CH₃COI + H₂O → CH₃COOH (acetic acid) + HI

In the first step, a methyl iodide intermediate is formed when methanol reacts with hydrogen iodide. In the second step, the intermediate (methyl iodide) reacts with carbon monoxide to form acetyl iodide. In the third step, the acetyl iodide is reacted with water that leads to the formation of acetic acid along with the hydrogen iodide.

Other Methods of Acetic Acid Preparation

By oxidation:

The oxidation of acetaldehyde by using some naphthalene salts of chromium, cobalt, manganese, etc., results in the formation of acetic acid.
O₂ + 2CH₃CHO → 2CH₃COOH
The oxidation of ethylene (C₂H₄) by using palladium catalyst and a heteropoly acid also results in the formation of acetic acid, as shown below:
O₂ + C₂H₄ → CH₃COOH
Furthermore, some anaerobic bacteria can also convert sugar into acetic acid directly through anaerobic fermentation without involving the formation of ethanol intermediate, as shown below:
C₆H₁₂O₆ → 3CH₃COOH

Interesting Facts about Acetic Acid

It is the second simplest carboxylic acid, after formic acid.
It is mainly used in vinegar, and in the formation of cellulose acetate and polyvinyl acetate.
It is used as a food additive for flavor and to maintain acidity.
Around 6 metric tons of acetic acid are used worldwide in a year.
Most of the acetic acid is produced by using petrochemical feedstock.
It forms acetate when ionizes at physiological pH.
Acetic acid bacteria like Acetobacter and Clostridium acetobutlicum also produce acetic acid.
It is also produced by fruits when they ripen.
The acetyl group when joined to coenzyme A, acts as a holoenzyme that is used in the metabolism of fat and carbohydrates.