Benzoic Acid (C7H6O2): Preparation, Properties And Uses

 Benzoic Acid

Benzoic acid is an organic compound with the chemical formula C₆H₅CO₂H. It is a solid that is crystalline in appearance, similar to white needles. A natural source of benzoic acid is gum benzoin, which comes from certain tree barks. However, benzoic acid can be synthesized by chemical means.

Benzoic acid is an organic compound because it contains carbon and it is also an aromatic carboxylic acid. It is aromatic because it has a benzene ring in its chemical structure. Benzene is aromatic because it has alternating double bonds between each carbon. It’s classified as a carboxylic acid because it has a carbon group in its structure, which is the COOH group.

 Benzoic acid occurs naturally in many plants and serves as an intermediate in the biosynthesis of many secondary metabolites.  Salts of benzoic acid are used as food preservatives. Benzoic acid is an important precursor for the industrial synthesis of many other organic substances. The salts and esters of benzoic acid are referred to as benzoates.

Properties

Synthesis Of Benzoic Acid From Benzene, Benzaldehyde, Benzonitrile, Benzamide, Benzyl Chloride

Preparation From Benzaldehyde

• 20 g of benzaldehyde are treated in a thick-walled vessel with a cold solution of 18g of potassium hydroxide in 12 g of water.
• The mixture is mixed until a permanent emulsion is formed.
• The mixture in the thick-walled vessel is then tightly closed by a cork and allowed to stand for at least 6 hours or overnight.
• To the crystalline paste (potassium benzoate) separating out, water is added until a clear solution is obtained from which benzyl alcohol is extracted by repeatedly shaking with ether.
• After the evaporation of the ether the residue is subjected to distillation whereby benzyl alcohol passes over at 206^oC (Yield about 8g).
• The benzoic acid is precipitated from the alkaline solution on acidifying with hydrochloric acid and purified by crystallization from hot water yielding product, which melts at about 121^oC.

Preparation From Benzonitrile

• A mixture of 5ml or 5g of benzonitrile and 75 ml of 10% aqueous sodium hydroxide (NaOH) is boiled under a reflux condenser until oil drops of benzonitrile disappear.
• The condenser is subsequently detached and boiled in the open flask for a few minutes in order to remove free ammonia.
• The solution is then cooled and neutralized by adding concentrated hydrochloric acid until precipitation of benzoic acid is complete.
• The crystals of benzoic acid are collected by filtration.
• The crystals are then washed with cold water and dried.
• Benzoic acid is purified by crystallization from hot water, yielding a product which melts at 121^oC.

Preparation From Benzyl Chloride

• 5g of benzyl chloride and 4 g of anhydrous sodium carbonate in 50 ml of water are mixed in a round flask attached to a reflux condenser and boiled gently.
•  A solution of between 6-8g of potassium permanganate in 150 ml of water is gradually dropped in from a dropping-funnel pushed through the top of the condenser.
• Within a period of about 2-3 hours, the pink color of the permanganate will have disappeared and replaced by a mass of dark brown precipitate of manganese dioxide.
• When the liquid is cold, a stream of sulfur dioxide is passed until the manganese dioxide is dissolved as manganese sulfate.
• The liquid is allowed to cool and the benzoic acid which separates, is filtered, washed with a little cold water and re-crystallized from hot water, yielding a final product with a melting point of around 121^oC.

Preparation From Toluene

• Industrially, benzoic acid can be prepared by employing oxygen gas for the partial oxidation of toluene. This process usually employs manganese or cobalt naphthenates as catalysts.

Preparation From Benzene

• Convert benzene to methyl benzene by reacting it with CH3Cl and anhydrous AlCl3. Oxidize methyl benzene with a powerful oxidizing agent like alkaline KMnO4. You will get benzoic acid.

Uses Of Benzoic Acid

• Benzoic acid is most commonly used in industrial manufacturing, to manufacture a wide variety of products such as perfumes, dyes, and insect repellents.
• Benzoic acid salt (sodium benzoate) is commonly used as a PH adjustor and preservative in food, preventing the growth of microbes to keep food safe.  It works by changing the internal PH of microorganisms to an acid state that is incompatible with their growth and survival.
• Production of phenol involves the use of benzoic acid.
• Benzoic acid is an ingredient in many cosmetic and personal care products such as lipsticks, facial cleanser and mouthwash.
• Because of its anti-fungal properties, it is an ingredient in topical antifungal cream, ointments and lotions that can help treat althlete’s foot and ringworm.   
• Benzoic acid is also present in various products including sauces, jams, jellies and candied fruits.