[Chemistry Class Notes] on Benzoic Acid Pdf for Exam

Benzoic acid is used in a number of daily activities and is a very commonly seen product in many industrial as well as household applications. The structure of benzoic acid is a carboxyl group attached to a ring of benzene. It is usually denoted using the formula C₆H₅COOH. It contains an aromatic ring and therefore is also referred to as aromatic benzoic acid. As can probably be understood from this, this is also the reason why this compound has a certain pleasant smell.

What is the Structure Of Benzoic Acid?

The molecular formula for benzoic acid is C₆H₅COOH and its empirical formula is C₇H₆O₂. Iot is made up of six atoms of hydrogen, 7 atoms of carbon and 2 atoms of oxygen. There are six carbon rings that have alternate single and double covalent bonds and the -COOH group with it.

What are the Physical Properties of Benzoic Acid?

The following properties can be observed for benzoic acid:

• It is a colourless solid crystalline compound in the solid state 

• Benzoic acid has a molar mass of 122.12 g/mol.

• The boiling point of benzoic acid is 523 Kelvin, while its melting point is 395 Kelvin.

• Benzoic acid is not soluble in water but it can dissolve in organic solvents.

 

Chemical Properties

These properties are based on an assumption that each molecule is made up of an aromatic ring and an acidic carboxyl group linked to that ring structure. This benzene ring can be subjected to some of the reactions like nitration, halogenation and sulfonation. Like this, the carboxyl group can undergo some of the reactions such as the formation of esters, acid halides and salts. In particular, these reactions imply modifications of the benzene ring as well as the carboxyl group.

Formation of Salts

The carboxyl group present in benzoic acid is acidic in nature, and thus, reacts with base compounds such as sodium hydroxide to produce a salt known as sodium benzoate (as per the neutralisation reaction). When sodium benzoate is treated with an acid, such as hydrochloric acid, then benzoic acid is formed back.

 

Formation of Esters

Benzoic acid can also undergo esterification and produce pleasant smelling salts if it reacts with any alcohol, with sulphuric acid acting as a dehydrating agent.

 

Formation of Acid Halides

When benzoic acid reacts with thionyl chloride (SOCl₂) or Phosphorus pentachloride (PCl₅), it forms benzoyl chloride that is an example of acyl or acid halides. Benzoyl chloride is highly reactive and so it reacts with amine such as methyl amine (CH₃NH₂) or with ammonia (NH₃) to form benzamide (C₆H₅CONH₂).

Sulfonation

When benzoic acid reacts with the fuming sulfuric acid (H₂SO₄)  it leads to the sulfonation of benzene ring, in which the functional group SO₃H replaces H- atom in the meta position to the carboxylic acid group on the benzene ring, and so the product is termed as meta-sulfobenzoic acid.

Nitration

With nitrating agent, sulfuric acid acting as a catalyst, the nitro (NO₂) group occupies the meta position (over ortho or para position) relative to the position of the carboxyl group in the benzene ring.

Halogenation

During the halogenation of benzoic acid, chlorine (Cl₂) reacts with benzoic acid in presence of a catalyst ferric chloride (FeCl₃) to form meta chlorobenzoic acid (Cl-C₆H₄-COOH).

Decarboxylation of Benzoic Acid

Decarboxylation is the removal of carbon dioxide as sodium carbonate (Na₂CO₃) by treating the  sodium salt of benzoic acid (sodium benzoate) using soda lime (mixture of NaOH & CaO) to form benzene.

Reduction of Benzoic Acid

When benzoic acid is treated with lithium aluminium hydride (LiAlH₄) followed by hydrolysis in presence of acid, we will get benzyl alcohol as the product and it lacks selectivity. We cannot use sodium borohydride (NaBH₄) for the reduction of carboxylic acids, since it is a weak reducing agent.

Dehydration Reaction

When 2 molecules of carboxylic acid or benzoyl chloride is treated with phosphorus pentoxide  P₂O₅ or acetic anhydride , which acts as a dehydrating agent, removes a water  molecule and forms anhydride. If benzoic acid is treated with acetic anhydride (Ac₂O), we will get benzoic acid anhydride as the product.

 

Reduction of Benzoic Anhydride

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

Controlled Oxidation of Benzyl Alcohol

When benzoic acid is treated with lithium aluminium hydride it is converted into benzyl alcohol and on controlled oxidation it gets converted into benzaldehyde.

Uses of Benzoic Acid

• Some esters of benzoic acid acts as plasticizers

• Benzoic acid and its salt sodium benzoate are used as food preservatives for many of the food items like soft drinks, barbecue sauces, pickles and salad dressings which prevents the growth of bacteria and fungi.

• Benzoic acid is used in cosmetics as it can specifically help to treat skin irritation, sun burns, insect bites, fungal infections and so on.It is used in the preparation of fragrances and also acts as a pH adjuster.

• It is mostly utilized in acidic foods as it has more antifungal and antibacterial properties even at very low pH. It is used as a preservative in acidic medium, but in alkaline medium it loses its activity of preservation since in alkaline medium it splits up into ions.

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  Benzoic acid is used as a preservative since its addition doesn’t alter the odor or taste of the food items.

• Its preserving quality delays the multiplication of microorganisms. Sodium salt of benzoic acid, sodium benzoate is used to lower the glycine levels in the blood, since an amide bond forms between the benzoate and glycine, and so there is hippuric acid formation which is excreted through urine.