Acids, Bases and Salts

Benzoic Acid

Benzoic acid is an organic compound. Benzoic acid is present in our day-to-day life products ranging from food to cosmetics. It acts as a preservative for many products such as pickles, jams, lipsticks, face wash creams etc. It also acts as an essential precursor for many organic compound syntheses.

Benzoic acid

                                                                                     Benzoic acid

What is Benzoic Acid?

Benzoic acid is a white or colourless solid with the formula \(C_6H_5CO_2H\). Benzoic acid is a simple aromatic carboxylic acid as it consists of a carboxyl group attached to a benzene ring. The name benzoic acid is derived from gum benzoin, which was for a long-time the only source of benzoic acid. Benzoic acid occurs in plants naturally and also acts as an intermediate in the biosynthesis of many secondary metabolites. Salts of benzoic acid are in use as food preservatives.

Production of Benzoic Acid

Industrial Preparation

Benzoic acid is commercially produced in industries by partial oxidation of toluene with oxygen. Cobalt or manganese naphthenates acts as a catalyst in this process. This process of production of benzoic acid uses abundant materials and proceeds in high yield.

In the industrial process, there is a reaction of benzotrichloride with calcium hydroxide in water. It is done using iron or iron salts as a catalyst. The calcium benzoate as a result is converted into benzoic acid with hydrochloric acid (HCl). The product has a significant number of derivatives of chlorinated benzoic acid.

Laboratory Synthesis

Benzoic acid is cheap and readily available. The laboratory synthesis of benzoic acid is done for its pedagogical value. It is a common preparation process.

Benzoic acid is purified by recrystallization from water. This is because of its high solubility in hot water and poor solubility in cold water. This process usually gives a yield of around 65 per cent of benzoic acid.

By hydrolysis

On hydrolysis of benzonitrile and benzamide, it converts into benzoic acid. It can also convert into its conjugate base in acid or basic conditions.

From Grignard reagent

Bromobenzene is converted into benzoic acid by carboxylation of the intermediate phenylmagnesium bromide.

Oxidation of benzyl compounds

All benzyl derivatives, benzyl alcohol and benzyl chloride readily oxidized to benzoic acid.

Physical Properties of Benzoic Acid

  1. Benzoic acid has a colourless appearance in its solid-state.
  2. Benzoic acid has a crystalline nature.
  3. The crystal structure of benzoic acid is monoclinic.
  4. The presence of the aromatic ring in benzoic acid gives a faintly pleasant odour.
  5. At a temperature of \(130^oC\), the density of benzoic acid reduces to 1.075 grams per cubic centimetre.

Chemical Properties of Benzoic Acid

  1. The solubility of benzoic acid in water at \(25^oC\) and \(100^oC\) is 3.44 g/L and 56.31 g/L respectively.
  2. Benzoic acid is insoluble in water but soluble in benzene, carbon tetrachloride, acetone, and alcohols.
  3. The acid dissociation constant i.e., \(pK_a\) of benzoic acid is 4.2
  4. The reactions of benzoic acid can occur at the carboxyl group or the aromatic ring.
  5. It’s melting point is 395K.
  6. The boiling point of benzoic acid is 523K.
  7. The molar mass of benzoic acid is 122.12g/mol.

Chemical Reaction Related to Benzoic Acid

1)Formation of Salt

Benzoic acid has a carboxyl group, which is acidic and so it reacts with a base like sodium hydroxide to produce a salt, sodium benzoate \(C_6H_5COO^- and Na^+\). It is an iron compound and when sodium benzoate is treated with an acid such as HCI and it forms the reactant back.

2)Formation of Esters

When benzoic acid reacts with any alcohol in presence of sulfuric acid. Here sulfuric acid acts as a dehydrating agent. In reaction, it removes water molecule produced by the combination of acid and alcohol to form a pleasant-smelling ester, ethyl benzoate. This reaction is an esterification reaction.

3)Formation of Acid Halides

When benzoic acid reacts with thionyl chloride \(SOCI_2\) or Phosphorus pentachloride \(PCI_5\), it forms benzoyl chloride that is an example of acyl or acid halides.

Benzoyl chloride is highly reactive and so it reacts with an amine such as methylamine \(CH_3NH_2\) or with ammonia \(NH_3\) to form benzamide \(C_6H_5CONH_2\).

4)Sulfonation

When benzoic acid reacts with the fuming sulfuric acid \(H_2SO_4\) it leads to the sulfonation of the benzene ring. In this reaction, the functional group \(SO_3H\) replaces H- atom in the meta position to the carboxylic acid group on the benzene ring, and so the product is meta-sulfobenzoic acid.

5)Nitration

With a nitrating agent, sulfuric acid acting as a catalyst, the nitro \(NO_2\) group occupies the meta position (over ortho or para position) relative to the position of the carboxyl group in the benzene ring.

6)Halogenation

During the halogenation of benzoic acid, chlorine \(CI_2\) reacts with benzoic acid in presence of a catalyst ferric chloride \(FeCl_2\) to form meta chlorobenzoic acid \(CI-C_6H_4-COOH\).

7)Decarboxylation of Benzoic Acid

The term decarboxylation is the removal of carbon dioxide. Sodium carbonate \(Na_2CO_3\) on treating with the sodium salt of benzoic acid (sodium benzoate) using soda lime (a mixture of NaOH and CaO) forms benzene.

8)Reduction of Benzoic Anhydride

When the benzoic anhydride is treated with sodium borohydride, it is followed by acid hydrolysis. Thus, converts into benzoic acid and benzyl alcohol.

9)Controlled Oxidation of Benzyl Alcohol

When benzoic acid treats with lithium aluminium hydride it converts into benzyl alcohol. It is a controlled oxidation reaction it gets converted into benzaldehyde.

Uses of Benzoic Acid

Some important uses of \(C_6H_5COOH\) that is benzoic acid are as follows

  • The production of phenol incorporates the use of benzoic acid.
  • Benzoic acid is in use in ointments that prevent or treat fungal skin diseases.
  • \(C_6H_5COOH\) is in use as a preservative in the food industry.
  • Benzoic acid is in use as an ingredient in many cosmetic products, such as lipsticks etc.
  • Benzoic acid is also a precursor to benzoyl chloride.
  • One of the components of toothpaste, mouthwash, and face wash creams is benzoic acid.
  • Benzoic acid is also in use in the manufacture of dyes and insect repellents.

FAQs on Benzoic Acid

Question 1: What are the uses of benzoic acid?

Answer: The primary use of benzoic acid is in the industrial production of phenol (aromatic compound). This is done through a process i.e., oxidative decarboxylation. It should be noted that the ideal temperature under which this process should be carried out is in the range of 300 Celsius to 400 degrees Celsius. Benzoic acid and its salts are broadly in use in the food industry as food preservatives.

Question 2: Write in short about the solubility of benzoic acid in water.

Answer: Benzoic acid is not much soluble in water. Thou, the solubility of benzoic acid in water increases when the temperature is increased. At 0 degrees Celsius of temperature, the solubility of benzoic acid is 1.7 grams per litre in water. When benzoic acid is heated to 100 degrees Celsius, the solubility of benzoic acid in water increases to 56.31 grams per litre.

Question 3: How can benzoic acid be prepared?

Answer: Industrially, benzoic acid is prepared by employing oxygen gas for the partial oxidation of toluene. This process employs manganese or cobalt naphthenate as catalysts. Benzoic acid can also be prepared through the hydrolysis of benzamide and benzonitrile. Benzoic acid can also be made by oxidizing benzyl chloride or benzyl alcohol, or any other derivative of the benzyl group.

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