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What is Benzoic Acid? Structure, Properties, Reactions

Aug 10, 2022
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Benzoic Acid

Benzoic acid has a history from the 16th century. In 1556, the legendary clairvoyant Nostradamus first described benzoic acid as the dry distillation of gum benzoin. Later on, many other chemists identified it. But it was Friedrich Wöhler and Justus von Liebig who determined the chemical structure.

Benzoic acid is the simplest benzene ring containing carboxylic acid. Its most ordinary natural source is gum benzoin. This gum benzoin is a resin found in the bark of trees of the genus Styrax. Benzoic acid is a commonly seen product in many industrial and household applications. It has a certain pleasant smell.

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The following article will let you know more about the aromatic compound benzoic acid.

Benzoic Acid Manufacture

In laboratories, benzoic acid is produced by reacting Toluene and oxygen at high temperatures with a catalyst of cobalt and manganese salts.

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Its first industrial concoction was the hydrolysis of benzo trichloride to calcium benzoate. This calcium benzoate is further treated with hydrochloric acid to form benzoic acid.

Nowadays, this method has been completely replaced by the air oxidation of Toluene. Synthesis by oxidation avoids the issue of product impurity with chlorinated byproducts.

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The chemical reaction representing the oxidation of Toluene to produce benzoic acid is given below:

 

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Benzoic Acid Manufacture

 

Benzoic Acid Structure

It is an aromatic compound with molecular or structural benzoic acid formula C6H5COOH and an empirical benzoic acid formula C7H6O2. The benzoic acid structure consists of a carboxylic acid group connected to a benzene ring.

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The benzoic acid structure comprises six hydrogen atoms, seven carbon atoms, and two oxygen atoms. Six carbon rings have alternate single and double covalent bonds and the -COOH group. The benzoic acid structure is given below:

Benzoic Acid Structure

 

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The crystalline benzoic acid structure is monoclinic and has a planar molecular shape, with a 1.72 D dipole moment in dioxane.

Benzoic Acid Properties

The chemical benzoic acid formula is C7H6O2, having a molecular mass of 122.12 g/ mol. The other physical and chemical properties of benzoic acid are given below:

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Physical Properties:

  • In a solid state, benzoic acid is a colorless solid with a crystalline appearance.
  • It has a faint and pleasant odor.
  • The density of benzoic acid at 15℃ is 1.266 g/cm³.
  • The benzoic acid melting point is 395 K, while its boiling point is 523 K.
  • The crystal benzoic acid structure is monoclinic.
  • Benzoic acid has a crystalline nature.

Chemical Properties:

  • Benzoic acid has a high solubility in hot water while poor solubility in cold water.
  • It can easily dissolve in organic solvents like acetone, benzene, alcohol, liquid ammonia, ether, CCl₄, etc.
  • The acid dissociation constant, i.e., pKa, of benzoic acid is 4.2 in water.
  • It shows many reactions like nitration, halogenation, and sulfonation and gives their respective substituted products.
  • The carboxyl group can undergo reactions such as forming acid halides, esters, and salts.

Chemical Reactions of Benzoic Acid

1. Nitration Reaction:

In the presence of sulphuric acid (which acts as a catalyst), when benzoic acid reacts with a nitrating agent, the nitro (NO2) group occupies the meta position relative to the position of the carboxyl group in the benzene ring. As a result, nitro-substituted benzoic acid, i.e., O2N-C6H4-COOH, is formed.

The meta group took over the ortho or para position in this reaction because the -COOH group shows the -I effect.

2. Halogenation Reaction:

When benzoic acid is treated with halogens, in the presence of ferric chloride (FeCl3) that acts as a catalyst, meta-substituted halo benzoic acid is formed.

Like in the case of chlorine (Cl2), meta chlorobenzene acid (Cl-C6H4-COOH) is formed.

3. Sulfonation Reaction:

When benzoic acid is treated with fuming sulfuric acid (H2SO4), the benzene ring’s sulfonation occurs. In this reaction, the functional group SO3H replaces the H- atom in the meta position of benzoic acid. So, the product is termed meta-sulfonic derivative benzoic acid.

4. Formation of Acid Halide:

Benzoic acid is treated with Phosphorus pentachloride (PCl5) or Thionyl chloride (SOCl2), and benzoyl chloride is formed. It is an example of acid or acyl halides.

Benzoyl chloride is very reactive, and therefore, when it responds with ammonia (NH3) or an amine like methyl amine (CH3NH2) gives benzamide (C6H5CONH2).

5. Formation of Esters:

When benzoic acid reacts with any alcohol, in the presence of a dehydrating agent such as sulphuric acid (H2SO4), it undergoes esterification. As a result, pleasant-smelling salts are produced.

6. Formation of Salts:

The carboxyl group in benzoic acid is acidic. Therefore, it reacts with alkaline compounds such as sodium hydroxide (NaOH) to produce its salt derivative known as sodium benzoate. It happens because of a neutralization reaction.

When this sodium benzoate reacts with an acid, such as hydrochloric acid, then it gives benzoic acid back.

7. Decarboxylation of Benzoic Acid

When sodium salt of benzoic acid reacts with soda lime, it gives benzene as the major product. Together with benzene, carbon dioxide is released in the form of sodium carbonate.

The removal of carbon dioxide is termed decarboxylation.

8. Reduction of Benzoic Acid

When benzoic acid reacts with lithium aluminum hydride (LiAlH4) and is followed by acidic hydrolysis, it gives benzyl alcohol as the product, which lacks selectivity.

In this reaction, sodium borohydride (NaBH4) cannot be used in place of LiAlH4. It is because LiAlH4 is a weak reducing agent, while NaBH4 is a strong reducing agent.

9. Dehydration Reaction

When two molecules of benzoic acid or benzoyl chloride are treated with dehydrating agents such as acetic anhydride or phosphorus pentoxide P2O5, it forms anhydride with the removal of a water molecule.

On treating benzoic acid with acetic anhydride (AC2O), the product obtained will be benzoic acid anhydride.

10. Reduction of Benzoic Anhydride

When the benzoic anhydride, a benzoic acid derivative, is treated with sodium borohydride (NaBH4) followed by acidic hydrolysis, it is converted into benzyl alcohol and benzoic acid.

11. Controlled Oxidation of Benzyl Alcohol

On reacting benzoic acid with lithium aluminum hydride (LiAlH4), the final product obtained is benzyl alcohol. If this reaction is controlled, then the product obtained will be benzaldehyde.

Usage of Benzoic Acid

Benzoic acid is manufactured in large quantities. Various reasons across a diverse range of industries increase the industrial usage of benzoic acid. Some of the usages of benzoic acid are given below:

  • Some esters of benzoic acid act as plasticizers.
  • It is used in the production of phenol.
  • Benzoic acid and sodium derivatives of benzoic acid are used in the food industry.
  • It is used in ointments to cure skin diseases like skin irritation, insect bites, sunburns, etc.
  • In many food items like pickles, sauces, soft drinks, salad dressings, barbecue, etc., benzoic acid is used to prevent the growth of microorganisms.
  • It is used in the making of fragrances.
  • It also acts as a pH adjuster.
  • Benzoic acid is used as one of the ingredients in manufacturing cosmetics items like lipstick, etc.
  • The sodium derivative of benzoic acid is used to lower glycine levels in the blood.
  • One of the uses of benzoic acid is that it acts as a precursor to benzoyl chloride.
  • As it does not alter the taste and odor of food items, benzoic acid is highly used as a preservative.
  • It is mostly used in acidic foods as it has more antibacterial and antifungal effects even at a very little pH.
  • It is used as a preservative in an acidic medium because, in an alkaline medium, it splits into ions and loses its nature as a preservative.
  • One of the components of mouthwash, toothpaste, and face wash creams is benzoic acid.
  • Benzoic acid is also used to manufacture dyes and insect repellents.
  • In the food sector, benzoic acid and its salts are frequently utilized as food preservatives. 

Conclusion

Benzoic acid is used in several daily activities.. It is not highly soluble in water. However, when the temperature rises, the solubility of this chemical in water increases (as in the case with most compounds). It is a natural component of numerous plants, animals, and berries. As a result, it can also be found in many food products, including milk products such as yoghurt.

The above discussion concludes that benzoic acid is a colorless crystalline solid with the chemical formula C6H5COOH, i.e., a carboxyl group is attached with a benzene ring. Most of the benzoic acid produced today is synthetic.

Frequently Asked Questions

1. Why is benzoic acid soluble in hot water and not in cold water?

Benzoic acid is a non-polar compound. The presence of bulk amounts shows low solubility with room-temperature water molecules due to the presence of bulk. The reason it shows low solubility in cold water is only because of the presence of the carboxylic group in it. As the temperature increases, the length of hydrogen bonds forms with water molecules, and ionization occurs. It increases the solubility of benzoic acid in hot water.

2. Is benzoic acid harmful to use?

Benzoic acid is used in foods and medicines and is of low hazard at the concentrations used. It is generally categorized as slightly hazardous in industrial use with no special transport requirements and biodegradable. Eating it or rubbing it in your eyes would not be advised, and workers have to be protected from daily exposure to dust which would be irritating, etc.

3. What are the medicinal uses of benzoic acid?

Benzoic acid aids in preventing infection produced by bacteria. Salicylic acid, a benzoic acid derivative, assists the body hut rough or discarded skin cells.

Salicylic acid and benzoic acid topical is a combination medicine used to treat skin irritation and eczema, fungal infections, insect bites, or inflammation caused by burns.

Benzoic acid

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