Ethanoic Acid

Ethanoic acid (also commonly known as Acetic acid) is a two-carbon acid and hence is the second member of the carboxylic acid family after methanoic acid (which is a one-carbon carboxylic acid). Although a correct and scientifically valid IUPAC name for this acid is ethanoic acid, you may also find its common name, acetic acid, mentioned as its IUPAC name at a lot of places.

A solution of 5-8% of Ethanoic Acid in water is known as vinegar, which is a common household substance used in kitchens. Ethanoic Acid freezes during winters and forms a glacier-like appearance; therefore it is also sometimes referred to as glacial acetic acid and is a very common laboratory chemical.

Structure of Ethanoic Acid

The chemical formula of Ethanoic Acid is CH₃COOH which can also be written as CH₃CO₂H or C₂H₄O₂ in the condensed form. Its molecular mass (or molar mass) is 60.05 g/mol.

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Ethanoic acid has the second simplest possible structure of a carboxylic acid after Methanoic acid. It comprises a methyl (–CH₃) group attached to the carboxylic acid functional group (–COOH). However, some may also argue that its structure is formed by a linkage between an acetyl group (–CH₃CO) and a hydroxyl group (–OH). The various forms in which the molecular structure of Ethanoic acid can be represented are given below:

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It has sp2 hybridisation. Usually, Ethanoic Acid exists as a dimer in liquid and vapour state due to the intermolecular hydrogen bonding which occurs between two molecules of ethanoic acid. The more electronegative oxygen atom of the carboxylic acid group attracts the electron cloud of the lesser electronegative hydrogen atom, thereby forming a hydrogen bond.

Physical Properties of Ethanoic Acid

Pure Ethanoic Acid is a colourless clear liquid which has a very pungent and characteristic odour:

• The flash point of Ethanoic Acid is 39°C.

• The density of Ethanoic Acid is 1.05 g/cm³.

• The boiling point of Ethanoic Acid is 118°C and it’s melting point is 16°C.

• Ethanoic Acid has one hydrogen bond donor and two hydrogen bond acceptor atoms.

• The solubility of pure Ethanoic Acid in water is >100 mg/mL at 25⁰C. It implies that it is highly soluble in water in all proportions.

• Ethanoic Acid is completely soluble in organic solvents such as carbon tetrachloride and carbon disulfide. It is miscible with organic solvents such as ethyl ether, benzene, acetone, glycerol and ethanol.

• The vapor pressure of Ethanoic Acid is 15.7 mm Hg at 250C.

• The LogP of Ethanoic Acid is -0.17.

• Ethanoic Acid is generally stable at normal laboratory storage temperature and other conditions.

• The viscosity of Ethanoic Acid is 1.056 mPa-s at 250C.

• The surface tension of Ethanoic Acid is 27.10 mN/m at 250C.

• The heat of combustion of Ethanoic Acid is 874.2 kJ/mol.

• The heat of evaporation of Ethanoic Acid is 23.36 at 250C.

• The pH of 1.0 Molar solution of Ethanoic Acid is 2.4.

• The pKa (dissociation constant) of Ethanoic Acid is 4.76 at 250C.

Chemical Properties of Ethanoic Acid (Reactions of Ethanoic Acid)

Esterification Reaction: When any carboxylic acid reacts with any alcohol, it leads to the formation of another class of chemical compounds named esters. This reaction which leads to the formation of esters is known as the esterification reaction. Given below is an example presented when the Ethanoic Acid (a carboxylic acid) reacts with Ethanol (ethyl alcohol) resulting in the formation of ethyl ethanoate (an ester):

CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃

(Ethanoic acid) (Ethanol) (Ethyl Ethanoate)

The new class of chemical compounds formed by this reaction (Esters) has a characteristic fruity smell by which they can be easily identified. Esters are commercially used as synthetic flavoring agents in the food industry and for their good fragrance in the perfume industry. Apart from this, one of the main uses of esters is in the production of soaps. When esters are reacted with any base (particularly alkalis), they give a carboxylic acid salt which is the basic molecular structure of soaps. This reaction is known as the saponification reaction. Higher molecular weight esters are generally preferred to carry out this reaction. A general elucidation of a saponification reaction is given below:

RCOOR’ + NaOH → RCOO–Na+ + R’OH

(An ester) (Sodium hydroxide) (Soap) (Alcohol)

Reaction with Base: Ethanoic Acid is a weak acid. Like any acid, it reacts with a base to form one molecule of salt and one molecule of water. An example of this reaction is given below where Ethanoic acid is reacting with Sodium Hydroxide (a base) to form sodium ethanoate and water:

CH₃COOH + NaOH → CH₃COONa + H₂O

(Ethanoic Acid) (Sodium (Sodium (water) hydroxide) ethanoate)

This salt (Sodium ethanoate) which is formed by this reaction has many industrial applications such as in the textile industry to carry out neutralization of sulfuric acid (used to clean fibers), in the food industry as a preservative and a mild seasoning and flavoring agent, and as a buffering agent along with Ethanoic acid (commonly known asthe acetate buffer) to keep the pH of a medium constant.

Reaction with Carbonates: When Ethanoic acid is reacted with carbonates or hydrogen derivatives of carbonates (hydrogen carbonates, also commonly known as the bicarbonates), it leads to the formation of salt, and carbon dioxide and water as by-products. Examples in which Ethanoic acid is reacted with sodium carbonate and sodium bicarbonate are given below:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

(Ethanoic Acid) (Sodium (Sodium (carbon (water carbonate) ethanoate) dioxide)

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O

(Ethanoic Acid) (Sodium (Sodium (carbon (water) bicarbonate) ethanoate) dioxide)

Formation of Carboxylic Acid Derivatives: Although a weak carboxylic acid, Ethanoic acid reacts with a number of chemicals to form certain acid derivatives, characterized as a separate class of organic chemicals named carboxylic acid derivatives. These include acid chlorides, acid anhydrides, esters and amides.

Acid Chlorides: When ethanoic acid reacts with thionyl chloride (SOCl2), it leads to the formation of acetyl chloride. This reaction can be elucidated as follows:

CH₃COOH + SOCl₂ → CH₃COCl +HCl + SO₂

(Ethanoic Acid) (Thionyl (Acetyl (Hydrochloric (Sulfur Chloride) Chloride) Acid) Dioxide)

Acid Anhydride: When Ethanoic Acid reacts with an acid chloride in the presence of a base, it leads to the formation of acetic anhydride. This reaction can be elucidated as follows:

CH₃COOH + CH₃COCl → (CH₃CO)₂O + HCl

(Ethanoic Acid) (Acetyl (Acetic (Hydrochloric Chloride) Anhydride) Acid)

Esters: This reaction has already been described under the Esterification reaction.

Amides: When Ethanoic acid reacts with methylamine in the presence of DCC (Dicyclohexylcarbodiimide), it leads to the formation of dimethylamine. The reaction can be elucidated as follows:

CH₃COOH + NH₂CH₃ → CH₃CONHCH₃ + H₂O

(Ethanoic Acid) (Methyl (N-methyl (Water) Imine) Acetamide)

Methods of Preparation of Ethanoic Acid

The most common method of preparation of Ethanoic acid is the carbonylation of methanol. In this reaction, methanol is reacted with carbon monoxide in the presence of metal carbonyl as a catalyst which results in the formation of the acetic acid.

Uses of Ethanoic Acid

• It is used as a solvent for various reactions which require aprotic solvent.

• It is used in the production of vinegar, esters, and synthetic polymers.

• It is used as a blood cell lysing agent in laboratories.

• It is used as an antifungal agent.