Acetic Acid: What is acetic acid made of? | Properties | Uses

Chemical Formula

• Pyroligneous Acid – C₅H₄O₂
• Acetamide – CH₃CONH₂
• Sodium Methoxide – CH₃ONa

• Methyl Cyanide – CH₃CN
• Silicon Dioxide – SiO₂
• Hydrazoic Acid – HN3

Acetic Acid has been known since ancient times as vinegar. About 10% of vinegar is acetic acid. It was first built in 1720 by Stahl in an impure form. It was first obtained in pure form by Lavosier in 1786 by oxidation of ethyl alcohol.

In 1845, Kolbe synthesized it from its elements. Its molecule formula C₂H₄O₂ and structure formula are as below.

Industrial Preparation of Acetic Acid

The following are the major methods of industrial manufacture of acetic acid.

Synthesize Acetic Acid from Acetylene

In this method, firstly acetylene gas are obtained by the action of water with calcium carbide.

After this, acetaldehyde is obtained by flowing acetylene gas in a mixture of 42% sulfuric acid and 1% mercuric sulfate at 60°C heat. Thus obtained acetaldehyde is mixed with air vapours and flowing at high pressure and 70°C temperature with catalyst (vanadium pent oxide or manganese acetate) and the result we get oxidation of acetaldehyde. It gives Acetic Acid.

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂

C₂H₂ + H₂O → CH₃CHO

2CH₃CHO + O₂ → 2CH₃COOH

The Determination of Acetic Acid in Pyroligneous Acid

Pyroligneous acid obtained from distillation of wood contains about 10% acetic acid in addition to water, 2-4% methyl alcohol and about 0.5% acetone.

To obtain acetic acid from pyroligneous acid, it is heated in a copper vessel. The vapours obtained from it flow into the milk of lime.

The vapour of acetic acid obtained by heating pyroligneous acid reacts with milk of lime to form calcium acetate. Due to being non-volatile, calcium acetate remains in the milk of lime.

The impurities of methyl alcohol, acetone etc. are dissociated as vapors. Calcium acetate is filtered and separated. After drying it is distilled with concentrated sulphuric acid to obtain acetic acid of about 75% concentration.

In order to obtain pure and anhydrous acetic acid from an aqueous solution of acetic acid, sodium hydroxide is first neutralized by aqueous solution of acetic acid.

When this solution evaporates, solid sodium acetate is obtained. Crystals from its aqueous solution yield crystals of sodium acetate. After distillation of these crystals with concentrated sulfuric acid, acetic acid of about 99% purity is obtained.

A small amount of water is also present in it. To remove small amounts of water, cool it to 16. On doing this, acetic acid is converted into a solid state.

Water remains in a fluid state. Small amounts of water are filtered or separated by any other suitable method. In this way pure and anhydrous acetic acid is obtained.

Pure and anhydrous acetic acid is obtained in the solid state similar to ice. Hence, it is called glacial acetic acid.

Because there is a substantial difference in the boiling point of water and the boiling point of acetic acid.

Hence, the efficient distillation method can also be used to obtain glacial acetic acid from aqueous solutions of acetic acid.

Nowadays due to the advancement of efficient distillation methods, glacial acetic acid is mainly made from this method by the aqueous solution of acetic acid.

Acetic Acid by Fermentation

Nowadays acetic acid is made in commercial quantities mainly from vinegar. In addition to water, vinegar contains about 6% Acetic acid, some other organic acids, some ester and some other substances.

Pure and anhydrous acetic acid can be made from the effective distillation of vinegar.

In India, vinegar is made by fermentation of sugarcane juice. In other countries vinegar is made by fermentation of ethyl alcohol.

When an aqueous solution of ethyl alcohol is left open in the air, it ferments and aqueous solution of acetic acid (vinegar) is obtained. Bacteria called mycoderma aceti(Mother of vinegar) are present in the air, which reach an aqueous solution of ethyl alcohol for their growth and convert it to vinegar by fermentation.

C₂H₅OH + O₂ → CH₃COOH + H₂O

The method of making vinegar from ethyl alcohol is called quick vinegar method. In this method, a large keg of wood is used. This casks have wooden planks at the top and bottom. In between these two planks fill the clipping of vinegar-soaked beach wood. Some holes remain in the casks walls so that the fluid present in the casks remains in contact with air.

Approximately 10% aqueous solution of ethyl alcohol is slowly added upward in the casks. The temperature of the casks is kept at about 30-35°C. The fluid obtained from the bottom of the keg is slowly poured over it from time to time. After about 1 week vinegar is obtained from the lower part of the keg(casks).

Preparation of Acetic Acid by other method

Other major methods of making acetic acid –

Ethyl Alcohol

Acetic acid is obtained by oxidation of the acidic potassium dichromate solution of ethyl alcohol.

C₂H₅OH → CH₃CHO → CH₃COOH

Methyl Cyanide

Acetic acid is obtained from water decomposition by dilute acid of methyl cyanide.

CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl

Ethyl Acetate

Acetic acid is obtained from water decomposition by dilute acid of ethyl acetate.

CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH

Acetamide

Acetic acid is obtained by the action of nitrous acid on Acetamide.

CH₃CONH₂ + HNO₂ → CH₃COOH + N₂ + H₂O

Acetic acid is also obtained from the water decomposition of acetamide.

Sodium Methoxide

Sodium acetate is obtained by passing carbon mono oxide at 200° temperature and 8 atmospheric pressure on sodium methoxide.

CH₃ONa + CO → CH₃COONa

Acetic acid is obtained after distillation by adding sulfuric acid to sodium acetate.

CH₃COONa + H₂SO₄ → CH₅COOH + NaHSO₄

Physical Properties of Acetic Acid

It is a colorless, strong-smelling and corrosive liquid. The intense smell of vinegar is due to the presence of acetic acid in it. Its boiling point is 118°C. It freezes like snow at 16.5°C. It is soluble in water, alcohol and ether. It is heavier than water and its relative density is 1.045 at 16°C.

Due to the excess of hydrogen bonding in carboxyl group, their boiling points are higher than the boiling point of alcohol with the same molecular weight. Example: Both acetic acid and n propyl alcohol have molecular weights of 60, while acetic acid has a boiling point of 118°C and n – propyl alcohol has a boiling point of 97°C.

Chemical Properties of Acetic Acid

A molecule of acetic acid contains a methyl group and a carboxyl group. Hence, it exhibits reactions of methyl and carboxyl group. Due to the presence of methyl group, it exhibits only the halogenation reaction.

The carboxyl group participates in all its remaining reactions. These reactions of it represent the normal reactions of the carboxyl group. It does not have a formyl group present. Therefore, like formic acid, it does not exhibit detrimental properties.

Following are its major chemical reactions.

Reaction with Bases: acetic acid is a weak monobasic acid. And ionizes in aqueous solutions, forming cations of acetic ion (CH₃COO^–) and hydrogen. Due to its acidic properties it reacts with bases to form salts.

CH₃COOH + NaOH → CH₃COONa + H₂O

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

CH₃COOH + NH₃ → CH₃COONH₄

Acetamide(CH₃CONH₂) is obtained by heating ammonium acetate(CH₃COONH₄).

CH₃COONH₄ → CH₃CONH₂
+ H₂O

Carboxylic Acid

1. Introduction Carboxylic Acid

2. Nomenclature Carboxylic Acid

3. Structure Carboxylic Acid

Esterification: Just as the acid and base reacts to form salts and water, similarly the reaction of acid and alcohol makes ester and water. This reaction is called esterification. The esterification action of carboxylic acid is often carried out in the presence of concentrated sulfuric acid. It works as both anhydrous and catalyst.

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CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Halogenation: The hydrogen atoms of the methyl radical are displaced one by one from the chlorine or bromine atoms when chlorin or bromin is flowed in acetic acid in the presence of red phosphorus.

CH₃COOH + Cl₂ → CH₂ClCOOH + HCl

CH₂ClCOOH + Cl₂ → CHCl₂COOH + HCl

CHCl₂COOH + Cl₂ → CCl₃COOH + HCl

Similar reaction with chlorin and bromin occurs in the presence of red phosphorus of other carboxylic acid. In this reaction the hydrogen atoms present on the adjacent carbon atom of the carboxylic acid group i.e. α-hydrogen atom are displaced by the halogen atoms. This reaction is called Hell Volhard Zelinsky reaction (HVZ Reaction).

Reaction with PCl₅: It reacts with PCl₅ to form acetyl chloride (CH₃COCl). In this reaction, the -OH group of the -COOH group is displaced from the chlorine atom.

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

Same reaction is also occur with PCl₃ and SOCl₂.

Dehydration: When heated acetic acid alone or in the presence of anhydrous agents such as phosphorus pentoxide, one of its two molecules dissociates and produces acetic anhydrid.

Reduction: Lithium is reduced by aluminum hydride to form ethanol.

CH₃COOH – LiAlH4 → CH₃CH₂OH

It can also be reduced by H₂ / Ni, Na / alcohol and NaBH₄.

Reactions to salts: Methane is obtained by heating sodium acetate with soda lime.

CH₃COONa + NaOH → CH₄ + Na₂CO₃

Ethane is obtained after the legal decomposition of sodium or potassium acetate.

2CH₃COONa + 2H₂O → C₂H₆ + 2CO₂ + 2NaOH + H₂

Acetone is obtained by heating calcium acetate alone.

(CH₃COO)₂Ca → CH₃COCH₃ + CaCO₃

Acetaldihyde is obtained by mixing calcium phormate with calcium acetate.

(CH₃COO)₂Ca + (HCOO)₂Ca → 2CH₃COCH₃ + CaCO₃

Methyl bromide is obtained by reacting with bromine in a dry CCl₄ solution of silver salts of acetic acid. This reaction is called Hunsdiecker Reaction.

CH₃COOAg + Br₂ → CH₃Br + CO₂ + AgBr

Schmidt Reaction: It reacts with hydrejoic acid in the presence of concentrated sulfuric acid to form methyl amin. This reaction is called Schmidt Reaction.

CH₃COOH + N₃H → CH₃NH₂ + CO₂ + N₂

Acetic Acid Uses

As reagent in the laboratory

Used in making acetone and esters, which are used in making Perfume dyes plastic and medicine.

In the manufacture of artificial fibers called cellulose acetate and polyvinyl acetate.

making white and artificial vinegar

which is an oxidizing agent in making tetra acetate.

Rubber quivering

Acetic Acid Tests

Acetic acid and acetate ion are tested by the following methods –

Acetic acid has a vinegar-like odor. Vinegar smells when heated with concentrated sulfuric acid.

In a neutral solution of acetic acid, the color of the solution turns red by adding a few drops of the neutral solution to the ferric chloride.

On heating with a few drops of ethyl alcohol and concentrated sulfuric acid, ethyl acetate is formed which has a sweet smell like fruit.

The solutions of acetic acid and its salts do not show the reducing properties. Hence, acetic acid does not reduce ammonia silver nitrate solution or Fehling solution.

Cacodyl Test: Upon mixing and evaporating arsenious acid in an acetic acid solution neutralized by potassium dioxide, a substance called Cacodyl oxide [(CH₃)₄As]O₂ is formed which has a strong odor.

In addition to the above tests, acetic acid also gives a general test of carboxylic acid (NaHCO₃). Effervescences are obtained due to the formation of CO₂ gas upon the addition of NaHCO₃ solution to acetic acid.