Propionic acid Formula: Reactions and Solved Examples

Propionic acid is a carboxylic acid where ethane is attached to a carboxyl group to form saturated fatty acid. It is identified as a liquid with a pungent odor similar to a body odor. It finds extensive use in industrial and household means as an antifungal and flavoring agent. It is produced industrially from ethanol or ethylene and carbon monoxide. Besides, it is also produced naturally on our body, on the skin or in the gastrointestinal tract.

Key Terms: Propionic acid, carboxylic acid, saturated fatty acids, Antifungal agent, Flavoring agent, Carbon Monoxide, Skin

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Propionic Acid

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Propionic acid is a carboxylic acid. The Chemical formula of Propionic acid is CH₃CH₂COOH or C₃H₆O₂. Due to the presence of single bonds, it comes under the category of saturated fatty acids. In natural fats, propanoic acid is in the form of esters of glycerol.

Propionic Acid

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Preparation of Propionic Acid

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The industrial or commercial preparation of propionic acid involves two methods:

• Hydrocarboxylation of ethylene with nickel carbonyl as the catalyst. Hydrocarboxylation is a chemical reaction in which carbon monoxide and water/alcohol are added to the alkene to produce esters.

•  Aerobic oxidation of propionaldehyde or propanal with salts of manganese or cobalt is used as catalysts. The reaction takes place in temperatures between 40oC to 50oC. 

• Biological methods of manufacturing propionic acid involve fermentation of sugar and fermentation of mash made out of cornmeal. The bacteria that results in fermentation is known as Propionibacterium. 

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Structure of Propionic Acid

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The structure of Propionic acid is monoclinic which has a carboxyl group (COOH) attached to an ethane molecule. 

Structure of Propionic Acid

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Chemical Properties of Propionic Acid

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Chemical formula: CH₃CH₂COOH of C₃H₆O₂.

Molecular weight: 74.08 g mol-1

Chemical Name: Propionic acid, Ethane Carboxylic acid

Solubility: Miscible in Ether, Ethanol and Trichloromethane (CHCl₃). Propionic acid is fairly soluble in water.

Refractive Index: 1.38

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Physical Properties of Propionic Acid

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Odor: In liquid form the compound emits a strong pungent and corrosive smell.

Appearance: Propionic acid appears as a clear and colorless oily liquid.

Melting Point: 141.15^oC

Boiling Point: -20.5^oC

Density: 0.98 g/cm

Vapour Pressure: 0.32 kPa (20^oC)

0.47 kPa (25^oC)

9.62 kPa (100^oC)

Combustibility: If exposed to fire, heat or oxidizing agents, propionic acid turns flammable. 

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Important Chemical Reactions of Propionic Acid

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Some of the important chemical reactions of propionic acids are as follows.

Reaction with metals and alkalis

On reacting with metal hydroxides, propanoic acid forms salts whereas on reacting with weak bases, along with water carbon dioxide is released.

CH₃CH₂COOH + NaOH → CH₃CH₂Na + H₂O

CH₃CH₂COOH + NaHCO₃ → CH₃CH₂Na + H₂O + CO₂

Reaction with Ammonia

The reaction of propionic acid with ammonia (NH₃) gives ammonium salts. The ammonium salt if further heated at high temperatures gives amides.

CH₃CH₂COOH + NH₃ ⇔ CH₃CH₂COONH₄ (Ammonium Propanoate)

CH₃CH₂COONH₄CH₃CH₂CONH₂ (Propanamide)

Reaction with PCl₅, PCl₃ and SOCl₂

CH₃CH₂COOH + PCl₅ → CH₃CH₂COCl + POCl₃ + HCl

3CH₃CH₂COOH + PCl₃ → 3CH₃CH₂COCl + H₃PO₃

CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl

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Uses of Propionic Acid

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• Propanoic acid suppresses the growth of fungus. Hence, it is used to kill molds as preservatives in foods.
• Salts of propanoic acid known as propionates are added as flavoring agents to enhance the taste.

Uses of Propionic Acid

• Used in the industrial manufacturing of polymers, modified fibers (cellulose acetate propionate), herbicides, rubber, pharmaceuticals.

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Hazards of Propionic Acid

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• Being a highly corrosive compound, wear a mask and glove before handling.
• Keep the compound away from heat/flames/sparks. Avoid smoking near the compound.
• If propionic acid falls on the skin, remove spoiled clothes and wash the affected area with water.
• If fumes of propionic acid are inhaled, move to an open space and inhale fresh air. Artificial oxygen must be provided if breathing gets difficult. Seek medical assistance if necessary.

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Things to Remember

• Propionic acid is a carboxylic acid where ethane is attached to a carboxyl group to form saturated fatty acid.
• Due to the presence of single bonds, it comes under the category of saturated fatty acids.
• Propionic acid is prepared industrially by hydrocarboxylation method and by Aerobic oxidation of propionaldehyde.
• Propionic acid is fairly soluble in water.
• Propanoic acid suppresses the growth of fungus. Hence, it is used to kill molds as preservatives in foods.

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