Benzoic Acid: Formula, Structure, Properties, Uses and Tests

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Preparation of Benzoic Acid

Laboratory Method

Benzyl chloride: Benzoic acid is obtained by oxidation of benzyl chloride by alkaline KMnO4.

2KMnO₄ + 2KOH → 2K₂MnO₄ + H₂O + O

C₆H₅CH₂Cl + KOH → C₆H₅CH₂OH + KCl

C₆H₅CH₂OH → C₆H₅CHO → C₆H₅COOH

Using KOH in the above reaction, potassium salt of benzoic acid is obtained at the end of the reaction. Once it is acidified by SO₂ gas, benzoic acid is released.

Method used

To make Benzoic Acid in the laboratory, take 5 gram benzyl chloride, 4 gram sodium carbonate, 8 gram KMnO₄ and about 200 ml water in a round flask. Apply a reflux condenser to the flask and heat it until the pink color of KMnO₄ fades.

After cooling the flask mixture to about 80°C, the SO₂ gases flow in it until the precipitate of MnO₂ completely dissolves. Crystals of Benzoic Acid dissociate upon cooling the mixture. After filtering them, washing them and drying them, pure benzoic acid is obtained.

Benzyl alcohol, benzaldehyde or Toluene: benzoic acid is obtained by oxidizing any of these three compounds with acidic or alkaline KMnO₄ or acidic K₂Cr₂O₇.

C₆H₅CH₃ → C₆H₅CH₂OH → C₆H₅CHO → C₆H₅COOH

Phenyl cyanide: Phenyl cyanide is also called cyanobenzene or benzonitrile. Benzoic acid is obtained from its acid water decomposition.

C₆H₅CN + 2H₂O → C₆H₅COOH + NH₃

Benzoyl Chloride: Benzoic acid is obtained by reaction of benzoil chloride with water.

C₆H₅COCl + H₂O → C₆H₅COOH + HCl

Benzotrichloride: benzoic acid is obtained by reaction of benzotrichloride with aqueous NaOH or KOH.

Physical Properties

Benzoic acid is a white solid crystal. Its melting point is 122°C. It is less soluble in cold water but more soluble in hot water, ethanol and ether. Its vapour irritates the nose and throat.

Chemical Properties

Acidic character: It reacts with bases to form salts. Example:

C₆H₅COOH + NaOH → C₆H₅COONa + H₂O

C₆H₅COOH + NaHCO₃ → C₆H₅COONa + H₂O + CO₂

Benzoic acid is a stronger acid than acetic acid. The reason for this is that the carboxyl group in benzoic acid is associated with the minus-denatured phenyl group. Hence the amount of partial positive charge on H of carboxyl group in benzoic acid is high.

The carboxyl group in acetic acid is associated with the methyl-methyl group. Therefore, the amount of partial positive charge on H of carboxyl group in acetyl acid is less. Hence benzoic acid is a stronger acid than acetic acid.

Esterification: Like other acids, benzoic acid also reacts with alcohols to form esters. Example:

C₆H₅COOH + C₂H₅OH → C₆H₅COOC₂H₅ + H₂O

Decarboxylation: Decarboxylation occurs when benzoic acid is heated with soda lime like other carboxylic acids.

C₆H₅COOH + NaOH → C₆H₅COONa + H₂O

C₆H₅COONa + NaOH → C₆H₆ + Na₂CO₃

Reaction with phosphorus pentachloride: Like other carboxylic acids, benzoic acid reacts with PCl₅ to form acid chloride.

C₆H₅COOH + PCl₅ → C₆H₅COCl + POCl₃ + HCl

Reducation: It is reduced by H₂ / Ni, Na / Alcohol, LiAlH₄ or NaBH₄ to form benzyl alcohol.

C₆H₅COOH + 4H → C₆H₅CH₂OH + H₂O

Replacement reactions of nuclei: The carboxylic group is a meta directing group. Hence meta products are derived from its helogenation, nitration, sulphonisation.

Benzoic Acid uses

It is used as a medicine and in the manufacture of medicines.

It is used in the manufacture of dyes called aniline blue.

sodiuam benzoate is used in preservation of many foods like fruits, pickles, tomato sauce, fruit juice, etc.

Benzoic Acid Tests

This gives a normal NaHCO₃ test of carboxylic acid. Therefore, on adding a saturated solution of NaHCO₃ to benzoic acid, CO₂ gas exits with intense bubbling.

Specific tests are as follows.

Addition of ferric chloride(FeCl3) to a neutral solution of benzoic acid results in a camel hair-like buff precipitate.

Flammable vapor of benzene is obtained when benzoic acid is heated with soda lime.