Benzoic Acid

Benzoic acid is an organic acid with the chemical formula C₆H₅CO₂H. It is a simple aromatic carboxylic acid, since it contains a carboxyl group bonded to a benzene ring. It got its name from gum benzoin, which served for a long time as the only source of benzoic acid.

Benzoic Acid Structure

Its molecule comprises a benzene ring which is linked to a carboxyl group. Further, the molecule is made of 7 carbon atoms, 6 hydrogen atoms and 2 oxygen atoms.

Physical properties of benzoic acid

• It is colorless in solid-state.
• Its crystal structure in the solid state is monoclinic.
• Its melting point is 395 K.
• Its boiling point is 523 K.
• Its molecular weight is 122.12 g/mol.
• Its density is 1.27 g /cm³ at 15 degrees Celsius.
• It has a pleasant odor owing to the presence of an aromatic ring.
• When heated to 130 degrees Celsius, its density decreases to 1.075 gm per cubic centimeter.

Chemical properties of benzoic acid

• It is soluble in water. Its solubility in water at 25 degrees Celsius is 3.44 g/L and at 100 degrees Celsius 56.31 g/L.
• It is also soluble in carbon tetrachloride, acetone, benzene, and alcohol.
• Its dissociation constant (pKa) is 4.2
• Its carboxyl group, as well as the aromatic ring, can take part in reactions.

Laboratory Preparation of Benzoic Acid

i) From Grignard reagent

Bromobenzene is converted into benzoic acid by the carboxylation of the intermediate phenylmagnesium bromide (Grignard reagent). The steps are as follows;

• The Grignard reagent (phenylmagnesium bromide) is prepared from bromobenzene and magnesium wherein diethyl ether is used as a solvent.
• The phenyl magnesium bromide is then reacted with CO₂ and then undergoes hydrolysis to form benzoic acid.

ii) Oxidation of benzyl compounds

It can be produced through the oxidation of benzyl derivatives, benzyl alcohol and benzyl chloride.

Industrial Preparation of Benzoic Acid

In the industries, it is produced on a large scale by partial oxidation of toluene using oxygen. Cobalt or manganese naphthenates is used as a catalyst in this method. In this process, benzotrichloride is reacted with calcium hydroxide in water. Iron or iron salts are used as catalysts here. It produces calcium benzoate which is then acidified to form benzoic acid.

Benzoic Acid Chemical Reactions

i) Formation of Salt

Owing to the presence of a carboxyl group, which is acidic, benzoic acid tends to react with bases like sodium hydroxide to form a salt, sodium benzoate C₆H₅COO^-Na⁺ (NaC6H5COO).

ii) Formation of Esters

Esters are formed when benzoic acid is reacted with alcohol in the presence of sulphuric acid, which acts as a dehydrating agent.

iii) Formation of Acid Halides

Benzoic acid forms acid halide, benzoyl chloride, upon reacting with thionyl chloride (SOCl₂) or Phosphorus pentachloride PCl₅.

iv) Sulfonation

Benzene ring's sulfonation takes place when benzoic acid reacts with the fuming sulphuric acid H₂SO₄. Here, the functional group SO₃H replaces the hydrogen atom located at meta-position to the carboxylic acid group.

v) Nitration

During nitration, when sulphuric acid acts as a catalyst, the nitro NO₂ group occupies a meta-position to the position of the carboxyl group in the benzene ring.

vi) Halogenation

In this reaction, chlorine (Cl₂) reacts with benzoic acid in the presence of ferric chloride (FeCl₂), which acts as catalyst, to form meta chlorobenzoic acid (Cl-C₆H₄-COOH).

vii) Decarboxylation

In this, carbon dioxide is removed. Sodium carbonate reacts with sodium benzoate using soda-lime and forms benzene.

viii) Reduction

Benzoic anhydride converts into benzoic acid and benzyl alcohol upon reacting with sodium borohydride and followed by acid hydrolysis.

Uses of Benzoic Acid

In food: It is used as a preservative in foods like soft drinks, fruit juices, pickles, etc. Processed food contains benzoic acid in order to prevent the growth of bacteria, mould, yeast, etc. It does not change the flavor or odor of the food items. However, it is used in acidic foods as its antifungal and antibacterial properties are effective even at low pH. So, it is good as a preservative in an acidic environment. In an alkaline medium, it breaks into ions and thus cannot perform the role of a preservative.

In medicine and healthcare: It is used in drugs as a preservative. It is also found in baby products, soaps, detergents, skin products, and more.

In treatment: It is used to prevent bacterial infections including ringworm and athlete's foot. It is produced as hippuric acid in the liver. This acid is not toxic and is used to treat skin irritation caused by insect bites, burns, and more. It also helps in the treatment of infections of the urinary tract and bronchitis.

Other uses of benzoic acid

• It is also used as a precursor for benzoyl chloride.
• It is also required in the preparation of dyes and insect repellents.
• It is used in the manufacturing process of phenol.

Precautions while using Benzoic Acid

Although benzoic acid is non-toxic under normal circumstances, it still causes health risks, so, the following precautions could be taken while handling it.

• Wash your hands properly after using it
• The area should be well ventilated
• Avoid contact with skin, eyes, clothes, etc.
• Minimize generation of dust and also avoid breathing it

Storage of Benzoic Acid

The container should be tightly closed and kept in a cool, dry and well-ventilated area. It should not be close to heat, stoves, and incompatible substances like oxidizing agents and strong bases.