Ethanoic Acid - Properties, Preparation, Structure, Formula, Uses and Ethanoic Acid as a Solvent

We've all tasted pickles at some point in our lives! That sharp tanginess has always piqued my curiosity. I noticed that it is a transparent sour liquid called vinegar that attracts the tastes of all. 

But have you ever wondered what vinegar's chemical composition is, and why it is used in pickles? 

Ethanoic acid is what it is.

It is known to kill and thus terminate the growth of microorganisms in pickles, making it the magical ingredient that makes pickles last longer! In the eighth century, a Persian alchemist named Jabir Ibn Hayyan (Geber) produced concentrated acetic acid from vinegar through distillation.

TABLE OF CONTENTS

• Nature and Molecular Formula
• Structure
• Preparation
• Physical Properties
• Chemical Properties and Reactions
• Ethanoic Acid as a Solvent
• Uses
• Practice Problems
• Frequently Asked Questions - FAQ

Nature and Molecular Formula

Ethanoic acid, colloquially known as acetic acid, is an organic compound and the second simplest form of carboxylic acid following formic acid (HCOOH). The IUPAC name of acetic acid is ethanoic acid and its molecular formula can be written as C₂H₄O₂ or CH₃COOH. 

Ethanoic acid is sour to taste, colourless and has a pungent odour. The molecular weight of ethanoic acid is 60.05 g mol^-1. Vinegar is a solution of acetic acid in water and it is formed after adding 5-8% of acetic acid by volume, in water. Vinegar is mostly used as a preservative in pickles. This acid has a variety of commercial applications, and its global demand is approximately 6.5 million metric tons per year.

Structure

• Ethanoic acid contains a methyl group bound to the carboxylic acid functional group. The structure of ethanoic acid comprises the following atoms: two carbons, four hydrogens and two oxygens (CH₃COOH). The alpha carbon, that is, the carbon of the methyl group, is sp² hybridised.
• 

• In the solid state, ethanoic acid consists of a chain of molecules wherein individual molecules are connected to each other via hydrogen bonds. Ethanoic acid in solid state is composed of a number of linked molecules, each connected through hydrogen bonds with each other. 
• Dimers of acetic acid in its vapour phase exist at an approximate temperature of 120^oC.
• Dimers of acetic acid can also be found in the liquid phase in its dilute solution. when it is present in a dilute solution. These dimers are adversely affected by solvents that promote hydrogen bonding. The dimers of acetic acid are negatively affected by certain solvents that lead to hydrogen bonding.
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• The structure of acetic acid is given as CH₃(C = O)OH, or CH₃COOH

Preparation

Commercially, acetic acid is produced by a chemical method involving the carbonylation of methanol. However, the entire process of production of ethanoic acid can be divided into two broad categories: Chemical Method and Fermentation.

Chemical Method

• Methanol Carbonylation Process (Monsanto Process)

In this process, methanol and carbon monoxide are made to react with each other in the presence of a Rhodium-based catalyst at a temperature of 150-200OC and a pressure of 30-50 bar. The major product formed is acetic acid, along with by-products like formic acid or formaldehyde.

CH₃OH + HI → CH₃I + H₂O

CH₃I + CO → CH3_COI

CH₃COI + H₂O → CH₃COOH + HI

• Oxidation of Acetaldehyde

Acetaldehyde is oxidised at 150^OC and 55 bar pressure in the presence of a chromium-based catalyst to form acetic acid.

2 CH₃CHO + O₂ → 2CH₃COOH

• Oxidation of Ethylene

Additionally, a direct conversion process of ethylene to acetic acid can also be performed at high temperatures and pressure in the presence of lead-platinum or lead catalyst.

C₂H₄ + O2 CH₃COOH

• Oxidation of Petroleum Hydrocarbons

Hydrocarbons obtained from petroleum industries, for example, light naptha can also be used to produce acetic acid in the presence of cobalt or chromium acetate catalysts. They are readily oxidised by oxygen or even air to give peroxides, which decompose to give acetic acid.

2C₄H₁₀ + 5O₂ → 4CH₃COOH + 2H₂O

Fermentation

Fermentation is mainly used for the production of vinegar from alcoholic foodstuff. In this, the agricultural feed, apple cider, wine, fermented grain, potato mash, rice, and malt is first treated with yeast, followed by acetic acid bacteria (Acetobacter) to form the end-product of acetic acid. There are three fermentation processes known and used, namely Orleans, Tinkle and Submerged.

C₂H₅OH + O₂→ CH₃COOH + H₂O

Ethanoic acid is also generated from an anaerobic fermentation by certain bacteria that have the ability to directly convert sugar into acetic acid.

C₆H₁₂O₆→ 3CH₃COOH

Physical Properties

• At room temperature, ethanoic acid being a lower carboxylic acid is present in the liquid state.
• It is sour to taste and has a very pungent odour.
• Ethanoic acid is completely soluble in water. The solubility of acids in water decreases with an increase in their molecular weight.
• The freezing point of acetic acid is 290 K; beyond this temperature, it crystallises to form glacial acetic acid. The boiling point of ethanoic acid is 391 K.
• The density of acetic acid is 1.05 g cm^-3. It is denser than water which has a density of 1 g cm^-3.
• The pH of ethanoic acid is 2.4 at 1 M concentration in the solution.
• In its liquid form, acetic acid is a polar, protic solvent, with a dielectric constant of 6.2. Acetic acid, which is a polar protic solvent has the dielectric constant value of 6.2.

Chemical Properties and Reactions

Ethanoic acid is a weak acid. However, it reacts with a variety of compounds to form useful end-products.

• The carboxyl functional group in ethanoic acid can be ionised in the following way, forming H+ ion and thus causing its acidic nature. 

CH₃COOH ⇌ CH₃COO^- + H⁺

• Ethanoic acid, being an acid, reacts with the base sodium hydroxide to form the salt sodium ethanoate and water.

This sodium ethanoate can be used in the production of methane gas by making it react with soda lime.

• Sweet-smelling ester ethyl ethanoate can be formed on the reaction of ethanoic acid with ethanol. This reaction occurs in an acidic medium and is known as esterification.

• Ethanoic acid also reacts with metals like Na and K to form various metal ethanoates with the evolution of hydrogen gas.

• Ethanoic acid reacts with sodium carbonate and also sodium bicarbonate to evolve carbon dioxide and form sodium acetate.

• Ethanoic acid reacts with phosphorus halides like PCl₅, PCl₃ or even SOCl₂ to form ethanoyl chlorides.
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• Acetic acid on reaction with ammonia forms an ammonium salt which on strong heating forms ethanamide. Amides can not be prepared from carboxylic acids and amines unless the ammonium salt is heated strongly to dehydrate it.
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• Potassium salt of ethanoic acid (CH₃COOK) on undergoing electrolysis forms ethane. 

Ethanoic Acid as a Solvent

Ethanoic acid in the liquid state is a hydrophile (readily dissolves in water) loves water and hence easily dissolves in water and is a polar protic solvent. A mixture of acetic acid and water is similar to a mixture of ethanol and water. In this way, acetic acid and water mixtures are comparable to ethanol and water mixtures. Ethanoic acid has a dielectric constant of 6.2 and hence can dissolve polar inorganic salts as well as non-polar compounds, such as oils, and non-polar solutes. Acetic acid also forms miscible mixtures with hexane, chloroform, and several oils. However, it does not form miscible mixtures with long-chain alkanes (such as octane). Hexane, chloroform, and a number of oils can all be mixed with acetic acid. But it is not miscible with long-chain alkanes (such as octane).

The desirable solvent properties of acetic acid, along with its ability to form miscible mixtures with both polar and non-polar compounds, make it a very important industrial solvent. Acetic acid is a very significant industrial solvent due to its advantageous solvent characteristics and capacity to mix miscibly with both polar and non-polar chemicals. It is widely used in the industrial preparation of dimethyl terephthalate. It is extensively utilised in the production of dimethyl terephthalate (DMT).

Uses

• One of the most popular uses of ethanoic acid is in the production of food-grade vinegar. This vinegar is used in the preparation of food.
• As ethanoic acid can react with metals, its product, metal ethanoate, is used in the printing industry.
• Acetates are also used in the making of photographic films and in the textile industry as synthetic fibres.
• The esters formed from acetic acid are used in the production of solvents for paints and resins.
• In our body, the acetyl group derived from acetates is used in the formation of coenzymes which forms the core of many metabolic processes in the body.
• A diluted version of ethanoic acid is used as a descaling agent in the household.
• Acetic acid is used as an antiseptic due to its antibacterial qualities.
• The manufacture of rayon fibre involves the use of ethanoic acid.
• It is employed to treat cancer by its direct injection into the tumour.
• Acetic acid is the major constituent of vinegar; it finds use in the pickling of many vegetables.
• Ethanoic acid is hugely used in rubber manufacturing. It is also used in the manufacture of various perfumes.
• It is widely used in the production of VAM (vinyl acetate monomer).
• It produces acetic anhydride. Acetic acid undergoes a condensation reaction together, and the product formed is acetic anhydride.

Practice Problems

Q1. A liquid was mixed with ethanol and a drop of concentrated H2SO4 was added. A compound with a fruity smell was formed. The liquid was:

A. HCHO

B. CH₃COCH₃

C. CH₃CHO

D. CH₃COOH

Answer: Esters have a fruity smell. We know carboxylic acid mixed with alcohol in presence of

conc. H₂SO₄ gives ester. Hence, the liquid is acetic acid. So, option C) is the correct answer.

Q2. Consider the following reaction.

It is an example of:

A. Nucleophilic substitution reaction

B. Nucleophilic addition only

C. Nucleophilic addition followed by elimination reaction

D. 1,2-Elimination only

Answer: The given reaction is an esterification reaction. In an esterification reaction, first, a nucleophilic addition reaction takes place followed by the elimination of a water molecule. S, option C) is teh correct answer.

Q3. When ethanoic acid is treated with aqueous NaHCO₃ , CO₂ is liberated. The C of CO₂ comes from

A. NaHCO₃ 

B. COO-

C. CH₃COOH

D. None of the above

Answer: 

CH₃COOH + NaHCO₃ ⟶ CH3COO^-Na⁺ + H₂CO₃

H₂CO₃H₂O +CO₂

When ethanoic acid reacts with aqueous sodium bicarbonate, sodium acetate and carbonic acid are formed. Carbonic acid further gives water and CO₂. Therefore, the C of CO₂ comes from bicarbonate. So, option D) is the correct answer.

Q4. Formic acid can be distinguished from acetic acid because formic acid:

A. Releases H₂ with sodium

B. Reduces ammoniacal AgNO₃

C. Gives ester with alcohol

D. Turns red litmus to blue

Answer: 

Option A): Formic acid cannot be distinguished from acetic acid as all carboxylic acids react with sodium to release H₂.

Option B): Formic acid cannot be distinguished from acetic acid as all carboxylic acids react with alcohols to give esters 

Option D): Formic acid cannot be distinguished from acetic acid cannot be distinguished as all carboxylic acids turn blue litmus red. 

Option C): When ammoniacal AgNO₃ is heated with an aldehyde, it forms a silver mirror. HCOOH (formic acid) contains both an aldehyde and carboxylic group, so it gives Tollens test and the silver complex [Ag(NH3)₂]⁺ gets reduced to give a silver mirror. Whereas acetic acid does not give Tollens test (reduces ammoniacal AgNO₃). So, option C) is the correct answer.

In the following set of reactions, product D is:

So, option D) is the correct answer.

Q5. How can ethanol and ethanoic acid be differentiated?

Answer: The difference between ethanol and ethanoic acid depending on their physical and chemical properties is given in the table below. 

Frequently Asked Questions - FAQ

What type of acid is ethanoic acid?
Answer: CH₃COOH is a weak acid. It undergoes complete dissociation only when it is reacted with a strong base. Hydrochloric acid is a much stronger acid than acetic acid. Acetic acid is a weak acid because it only partially dissociates into its constituent ions when dissolved in water.

What is the harmful effect of acetic acid?
Answer: Exposure to more concentrated solutions of acetic acid When we are exposed to high concentrations of acetic acid that is around (>25%), it can cause corrosive damage. Breathing vapours with high levels of acetic acid can cause irritation of the eyes, nose and throat, cough, chest tightness, headache, fever and confusion. A cough, chest tightness, headache, fever, and confusion can all result from breathing in high-acetic-acid vapours, as might irritation of the eyes, nose, and throat.

What is glacial acetic acid?
Answer: An undiluted solution of acetic acid is commonly referred to as glacial acetic acid. It forms crystals which appear like ice at temperatures below 16.6^oC. It has a wide range of applications as a polar protic solvent. In the field of analytical chemistry, glacial acetic acid is widely used in order to estimate substances that are weakly alkaline.

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