Benzoic Acid: Structure, Uses, Physical & Chemical Properties

Benzoic acid is an organic compound in which a carboxyl group is attached to a benzene ring. It is a colourless crystalline compound with the chemical formula C₆H₅COOH.

• The salt of benzoic acid is used as a preservative in many foods and cosmetics.
• The pleasant aroma of the compound is because of the aromatic benzene ring which is attached to the carboxyl group.
• The commercial production of benzoic acid can be obtained through the partial oxidation of toluene with oxygen, catalyzed by either manganese or cobalt naphthenates. 
• Since the benzoic acid chemical formula is C₆H₅COOH, its Carboxylic acid functional group (-COOH) is considered to be a strong electron-withdrawing group.
• When the Carboxylic acid attaches itself with the Benzene ring and retreats electron density from Benzene to yield positive sites at ortho- and para-positions, it can be said that Benzoic acid produces resonating structures.

Key Terms: Benzoic Acid, Benzoin, Partial Oxidation, Toluene, Oxygen, Grignard Reagent, Benzyl Compound, Hydrolysis, Benzene

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What is Benzoic Acid?

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Benzoic acid is an organic compound. The benzoic acid chemical formula is C₆H₅COOH.

• Benzoic Acid comprises a carboxyl group that is attached to a benzene ring.
• Thus, benzoic acid is often known as an aromatic carboxylic acid.
• Benzoic Acid is a compound that exists in the form of a crystalline, colourless solid under normal conditions.
• The term ‘benzoate’ is usually referred to esters and salts of C₆H₅COOH.
• The empirical benzoic acid formula is C₇H₆O₂.
• The benzoic acid structure typically has a carboxylic acid group connected to a benzene ring.
• The benzoic acid structure has six hydrogen atoms, seven carbon atoms, and two oxygen atoms.
• The six carbon rings in the structure have alternate single and double covalent bonds, alongside the -COOH group. 

Benzoic Acid

Chemical Data of Benzoic Acid

The chemical data of benzoic acid:

Benzoic acid’s commercial production can be acquired via partial oxidation of toluene with oxygen, catalyzed by manganese or cobalt naphthenates. The salts of Benzoic Acid are often used as food preservatives.

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Production of Benzoic Acid

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Benzoic acid is prepared by the industrial method, laboratory synthesis method, by hydrolysis, Grignard reagent, and oxidation of benzyl compound. 

• Initially, the only source of benzoic acid was gum benzoin.
• Benzoic acid is commercially produced by the method of partial oxidation of toluene and oxygen, with cobalt naphthenates or manganese acting as a catalyst. 
• Benzoic acid is also prepared by the reaction of calcium hydroxide and tri-chlorotoluene, with water as the catalyst, and also by reacting hydrochloric acid with the product of calcium benzoate.

There are several types of Reactions of Benzoic Acid. 

Some include:

Hydrolysis

Upon hydrolysis of benzonitrile and benzamide, it then converts into benzoic acid. It can help convert it to the conjugate base in acid or basic conditions.

Grignard reagent

Bromobenzene can be converted into benzoic acid by the help of the carboxylation of the intermediate phenylmagnesium bromide.

Oxidation of Benzyl Compounds

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

Nitration Reaction

In the presence of sulphuric acid (a catalyst), the benzoic acid undergoes a reaction with a nitrating agent. The nitro (NO₂) group possesses the meta position relative to the position of the carboxyl group found in the benzene ring. Thus, the nitro-substituted benzoic acid, i.e., O₂N-C₆H₄-COOH, is formed.

Halogenation Reaction

When benzoic acid reacts with halogens, under the presence of ferric chloride (FeCl₃) in the form of a catalyst, the meta-substituted halo benzoic acid is created.

Sulfonation Reaction

When benzoic acid reacts with fuming sulfuric acid (H₂SO₄), the sulfonation of the benzene ring takes place. Here, the functional group SO₃H substitutes the H- atom in the meta position of benzoic acid. So, the product that is formed is known as meta-sulfonic derivative benzoic acid.

Acid Halide Formation

Benzoic acid, when reacted with Phosphorus pentachloride (PCl₅) or Thionyl chloride (SOCl₂), forms benzoyl chloride. It is a prime instance of acid or acyl halides.

Formation of Esters

When benzoic acid is reacted with any sort of alcohol, under the presence of a dehydrating agent like sulphuric acid (H₂SO₄), it then undergoes a process known as Esterification. Thus, it forms pleasant-smelling salts.

Salt Formation

The carboxyl group in benzoic acid is known to be acidic. Thus, it is treated with alkaline compounds, an example includes sodium hydroxide (NaOH), to yield its salt derivative called sodium benzoate. This occurs due to a neutralization reaction.

Decarboxylation of Benzoic Acid

When soda lime is reacted with the sodium salt of benzoic acid, it produces benzene as the major product. Coupled with benzene, carbon dioxide can be released in the form of sodium carbonate. Carbon dioxide removal is known as the process of decarboxylation.

Reduction of Benzoic Acid

When benzoic acid is reacted with the presence of lithium aluminium hydride (LiAlH₄) and is then followed by the process of acidic hydrolysis, it produces benzyl alcohol as the product, lacking selectivity.

Dehydration Reaction

If two molecules of benzoic acid or benzoyl chloride react with dehydrating agents like acetic anhydride or phosphorus pentoxide P₂O₅, it gives rise to anhydride with water molecule removal.

Benzoic Anhydride Reduction

If benzoic anhydride, which is a benzoic acid derivative, reacts with sodium borohydride (NaBH₄) and is then followed by acidic hydrolysis, it gets converted into benzyl alcohol and benzoic acid.

Controlled Oxidation of Benzyl Alcohol

Upon reaction with benzoic acid with lithium aluminium hydride (LiAlH₄), the final product that can be produced is benzyl alcohol. In case this reaction is controlled, the product acquired will be benzaldehyde.

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Structure of Benzoic Acid

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The molecular formula of benzoic acid is C₆H₅COOH. It contains 7 carbon atoms, 6 hydrogen atoms, and 2 oxygen atoms. The empirical benzoic acid formula is C₇H₆O₂.

Benzoic Acid Structure

The structural formula of benzoic acid is made up of six rings of carbon where the single and double bonds exist alternatively and the benzene ring is extended with an attachment of – COOH or carboxyl functional group. 

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Properties of Benzoic Acid

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The several properties of Benzoic Acid include:

Physical Properties of Benzoic acid

The Physical Properties of Benzoic Acid are:

• It is colorless and crystalline in nature.
• It has a faintly pleasant odor.
• The crystal structure of benzoic acid is monoclinic. 

Chemical Properties of Benzoic Acid

The Chemical Properties of Benzoic Acid are:

• Molar mass or molecular weight of 122.12g/mol
• Density at 15°c is 1.27g/cm^3.
• The boiling point is 523k and the melting point is 395k.
• It can dissolve in organic solvents such as alcohol, ether, and benzene but is insoluble in water.
• The reaction can take place either in the aromatic ring or in the carboxyl group. 
• Reactions such as nitration, sulfonation, and halogenation take place in the benzene ring whereas reactions such as the formation of salts, acid halides, and esters in the carboxyl group. These reactions lead to the modification of the carboxyl group as well as the benzene ring. 
• It can dissolve easily in carbon tetrachloride, benzene, alcohol, and acetone.

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Uses of Benzoic Acid

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The uses of Benzoic Acid are:

• It is used for the production of phenol
• It is used in the production of insect repellants, dyes, cosmetics, and toothpaste.
• It is also used as ointment to treat fungal skin diseases, insect bites, skin irritations, fungal infections, etc. 
• There are some specific esters of benzoic acid that act as a plasticizer.
• As it acts as a pH adjuster, it is used in the production of fragrances.
• Its preservative properties are the maximum in the acidic medium.
• Sodium benzoate which is a sodium salt of benzoic acid is used to control the glycerine level of the blood.
• It is also used in toothpaste, mouthwash and face creams. 

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Things to Remember

• The chemical formula of benzoic acid is C₇H₆O₂ or C₆H₅COOH
• The faintly pleasant odour of the compound is due to the presence of the aromatic ring.
• The solubility of benzoic acid in water increases with the increase in temperature.
• Benzoic acid is the primary compound of benzoyl chloride.
• Benzyl compounds such as benzyl alcohol and benzyl chloride are easily oxidized to benzoic acid.
• It is used for the production of phenol, insect repellants, dyes, cosmetics, and toothpaste.
• It is also used as an ointment to treat fungal skin diseases, insect bites, skin irritations, fungal infections, etc. 

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Previous Year Questions

• Which of the following compounds has the lowest boiling point? [NEET 1994]
• Which of the following has zero dipole moment? [NEET 1996]
• Identify Z in the sequence of reactions, CH₃CH₂CH [NEET 2014]
• The compound that is not a Lewis acid, is?
• The pKa of a weak acid HA and pKb of a weak base BOH are [KEAM]
• Which of the following alkane cannot be made in good yield by Wurtz reaction ? [NEET 2020]
• An alkyl bromide (X)(X) reacts with Sodium in ether to form 4, 54,5 -diethyl [KCET 2013]
• Ethyl benzene cannot be prepared by [KCET 2011]
• Conversion of benzene to acetophenone can be brought by [KCET 2013]
• Cycloalkane formed when 1, 41,4 -dibromopehtane is heated with Sodium [KCET 2013]
• In electrophillic aromatic substitution reaction, the nitro group [KCET 2009]