Benzoic Acid - Structure & Properties

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

What is Benzoic Acid?

Benzoic Acid (C6H5COOH) is a carboxylic acid in which the benzene ring is attached to the carboxylic acid group. Since it contains an aromatic ring, benzoic acid, is called as an aromatic carboxylic acid, and due to that ring, this compound gets its pleasant odor.

Structure

Empirical formula for benzoic acid is C7H6O2 and its molecular formula is C6H5COOH. It is otherwise known as benzene carboxylic acid. It contains 7 atoms of carbon, 2 atoms of oxygen and 6 atoms of hydrogen. Its structure contains a six-carbon ring with alternate single and double bonds and –COOH group attached to that ring which makes it as benzoic acid.  

Properties of Benzoic acid

Benzoic acid is a 

  • Colorless solid

  • Crystalline in nature

  • Molar mass is 122.12 g/mol

  • Melting point is 395 K

  • Boiling point is 523K 

  • Insoluble in water but readily soluble in organic solvents like benzene, alcohol and ether. 

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 salt

In benzoic acid, we have carboxyl group, which is acidic in nature and so it reacts with a base like sodium hydroxide to produce a salt, sodium benzoate (C6H5COO-Na+), which is an ionic compound and when sodium benzoate is treated with an acid, (such as HCl) it forms the reactant back, that is Benzoic acid.

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Formation of esters-

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

Benzoic acid            Ethyl alcohol       Ethyl Benzoate 

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).

 Image result for benzoic acid to benzamide conversion


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.

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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.

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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).

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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.

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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.


Borane DMS is selectively used to reduce carboxylic acid to alcohol. 

In case of Rosenmund's reduction, benzoic acid is first converted into benzoyl chloride, which is then treated with palladium and barium sulphate (BaSO4) as the catalyst, it gets reduced to benzaldehyde.

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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.

Image result for benzoic acid anhydride formation

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 it is on controlled oxidation it gets converted into benzaldehyde.

Image result for benzoic acid to benzaldehyde


Uses of benzoic acid

  • Some esters of benzoic acid acts as plasticizers

  • Benzoic acid and its salt sodium benzoate 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.

  • 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.