Benzoic Acid: Definition, Formula, Structure, Preparation, Chemical Reactions, Uses, Hydrolysis of Anhydrides, Physical and Chemical Properties

The delectable strawberries that some of us indeed love, could contain as much as 29mg/kg of benzoic acid. In fact why only strawberries! Even prunes, cranberries, yogurt and spices like cloves, salvia, thyme and nutmeg contain this famous aromatic carboxylic acid called benzoic acid.

It is also an antiseptic and fungistatic agent to prevent the growth of bacteria and fungi and cure skin diseases.

It indeed finds great utility in the food, medicine, and personal care industries. Hence, it is essential to understand the chemistry of this acid.

TABLE OF CONTENTS

• Formula and Structure of Benzoic Acid
• Physical Properties of Benzoic Acid
• Chemical properties of Benzoic Acid
• Preparation of Benzoic Acid
• Chemical Reactions of Benzoic Acid
• Hydrolysis of Anhydrides
• Uses
• Practice Problems
• Frequently Asked Questions - FAQ

Formula and Structure of Benzoic acid

Benzoic acid is a white or colourless solid-state organic chemical compound with the formula

It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, which was for a long time its only source. The gum benzoin is a balsamic resin obtained from the bark of several species of trees in the genus Styrax.

Benzoic acid occurs naturally in many plants and serves as an intermediate in the biosynthesis of many secondary metabolites. It consists of a carboxyl group attached to a benzene ring. Therefore, benzoic acid is also called aromatic carboxylic acid. This compound is attained as a crystalline, colourless solid under normal conditions.

The empirical formula denoting benzoic acid is and the molecular formula is denoted by . It is also called benzene carboxylic acid.

It has seven carbon atoms, six hydrogen atoms, and two oxygen atoms. It is a ring of six carbons attached with alternate bonds with a group attached to it in one of the vertices. The group makes it benzoic acid.

Physical Properties of Benzoic acid

• Benzoic acid has a colourless appearance in its solid state..
• It exhibits crystalline structure which is monoclinic.
• The presence of the aromatic ring gives this compound a faintly pleasant odour.

Chemical Properties of Benzoic acid

• The properties are based on the fact that the structure is of the aromatic ring and an acidic carboxyl group connected to the ring structure.
• The benzene ring of the benzoic acid undergoes electrophilic substitution reactions like nitration, halogenation, and sulfonation. These reactions lead to the generation of ether, halides, and salts.
• The acid dissociation constant (pK_a) of benzoic acid is determined as 4.2.
• The reactions can target both the benzene/aromatic ring and the acidic carboxyl group.
• It is insoluble in water; however, it is soluble in organic solvents like benzene, ether, carbon tetrachloride, acetone, and alcohol.

Preparation of Benzoic acid

The commercial production of benzoic acid is done via the partial oxidation of toluene with oxygen, catalyzed by manganese or cobalt naphthenates.

Another industrial method of preparing benzoic acid is by reacting tri-chlorotoluene with calcium hydroxide in the presence of water, and the treatment of the calcium benzoate product with hydrochloric acid.

Generally, carboxylic Acids are prepared in the ways stated below. Let’s check some specific methods of preparing benzoic acid which is the simplest aromatic carboxylic acid.

• Aromatic Carboxylic acids can be prepared by oxidation of primary and secondary Alkyl benzenes using reagents like alkaline or acidic KMnO4 and chromic acid, H₂CrO₄. Tertiary alkyl benzenes do not undergo oxidation.

• Cumene also undergoes oxidation to give benzoic acid

• The unsaturated alkyl chain attached to the benzene ring is also oxidized to carboxylic acids, the cleavage of -C = C- takes place to -COOH. Hence 1-Phenyl-prop-2-ene also undergoes oxidation to to produce benzoic acid.

• Benzoic acid can also be prepared from 1-Propyne benzene and acetophenone separately upon oxidation.

• Amides upon hydrolysis produce benzoic acid.

Benzonitrile undergoes acidic or basic hydrolysis to produce benzoic acid.

• Carboxylation of Grignard’s reagent also produces benzoic acid.

• Acidic Hydrolysis of esters produces corresponding carboxylic acid and alcohols.

Chemical Reactions of Benzoic acid

Benzoic acid undergoes several reactions involving cleavage of -C-bond. These reactions are stated below.

• Esterification: On heating under reflux, a solution of methyl alcohol and benzoic acid containing a small amount of sulfuric acid, for about an hour a fruity smelling substance, methyl benzoate which is known as an ester is formed. Yield will increase ion continuous removal of water from the reaction mixture.

• Formation of Acyl chlorides: Benzoic acid reacts with reagents like to produce benzoyl chloride.

• Benzoic acid reacts with ammonia to form benzamide.

• Benzoic acid can be reduced only by  or diborane,  reduces carboxylic acids to primary alcohol.

• Benzoic acid undergoes decarboxylation using soda-lime to give benzene.

• Benzoic acid undergoes Electrophilic substitution reactions. -COOH is a meta directing and deactivating group towards electrophilic substitution reaction. So nitration, bromination etc, occurs at the meta position with respect to the -COOH group.

• Benzoic acid does not undergo Friedel-Crafts reaction as the catalyst aluminum chloride (Lewis acid) gets bonded to the carboxyl group.

Hydrolysis of Anhydrides

Acyl halides when hydrolysed give carboxylic acids. Acyl halides are more readily hydrolysed with an aqueous base to give carboxylate ions. Carboxylate ions on acidification produce corresponding carboxylic acids.

Uses of Benzoic acid

• The major use of benzoic acid is in the production of phenol.
• This compound is used in ointments that prevent or treat fungal skin diseases.
• Benzoic acid and its sodium derivative are used as a preservative in the food industry.
• Benzoic acid is an ingredient in many cosmetic products, such as lipsticks and finds use in many other personal care products.
• It is also used as a precursor to benzoyl chloride.
• Benzoic acid is one of the major components of toothpaste, mouthwash, and face wash creams.
• Benzoic acid finds utility in the manufacture of dyes and insect repellants.

Practice Problems

Q.1. The major product formed in the following reaction is:

Solution: Since benzoic acid is the meta directing and deactivating group, thus meta Bromo-benzoic acid is formed.

Q.2. Which is the best method for carrying out the following reaction?

A) 

B) , , 

C) , , 

D) , , 

Solution:

Hence option B is correct.

Q.3. Why does the potassium salt of benzoic acid doesn't undergo Kolbe’s electrolysis reaction to give biphenyl?

Answer: The reason is the intermediate phenyl radical formed is highly unstable. This is the reason why it doesn't undergo Kolbe’s reaction.

Q.4. Benzoic acid is treated with sodium borohydride. The compound obtained is:

1. Benzyl alcohol
2. Benzaldehyde
3. Toluene
4. None of the above

Answer: Benzoic acid doesnot get reduced by sodium borohydride  (NaBH₄). Hence answer would be Option D. Benzoic acid on reaction with lithium aluminium hydride undergoes reduction gives benzyl alcohol.

Q.5. What is the product of the following reaction?

Answer: Acyl derivative of benzoic acid is obtained.

Frequently Asked Questions - FAQ

Question 1. What happens when Benzoic acid is heated?

Answer. When benzoic acid is heated in the presence of a dehydrating agent like P₂O₅ it forms benzoic anhydride.

Question 2. What is the negative impact of benzoic acid on skin?

Answer. If someone is exposed to Benzoic Acid in larger concentrations, it may generate rashes or hives due to skin allergy.

Question 3. Is benzoic acid polar?

Answer. Only the carboxylic acid group (- COOH) is polar present in the benzoic acid molecule is polar, the bulk of the benzoic acid molecule is no-polar i.e., the benzene ring part. It is only the carboxylic group that is polar. This is the reason why benzoic acid dissolves only slightly in cold water.

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