Question

Explain how you would prepare benzoic acid from benzene. Use any inorganic reagents.

Short answer

To prepare benzoic acid from benzene, follow these steps: 1) Nitrate benzene with HNO₃ and H₂SO₄ to form nitrobenzene. 2) Reduce nitrobenzene using H₂ and a nickel catalyst or iron filings and hydrochloric acid to form aniline. 3) Diazotize aniline with HNO₂ (formed from NaNO₂ and HCl) to yield benzenediazonium chloride. 4) Hydrolyze benzenediazonium chloride with water at high temperature to produce benzoic acid, N₂, and HCl.

Step-by-step solution

Nitration of Benzene

To start the process, we will first convert benzene into nitrobenzene. This step involves a nitration reaction. Nitration of benzene is an electrophilic aromatic substitution reaction, which occurs in the presence of a nitrating mixture, i.e., concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄). The reaction can be represented as: Benzene + HNO₃ → Nitrobenzene + H₂O

Reduction of Nitrobenzene

Now, we need to reduce nitrobenzene to form aniline. The reduction of nitrobenzene can be done by using a reducing agent, such as hydrogen gas (H₂) in the presence of a nickel catalyst or by using iron filings and hydrochloric acid. The reaction can be represented as: Nitrobenzene + 6[H] → Aniline + 2H₂O

Diazotization of Aniline

In this step, we will convert aniline into a diazonium salt. Diazotization is the process of converting primary aromatic amines into diazonium salts by treating them with nitrous acid (HNO₂), which is formed by reacting sodium nitrite (NaNO₂) with a mineral acid (such as HCl). The reaction can be represented as: Aniline + HNO₂ → Benzenediazonium Chloride + 2H₂O

Hydrolysis of the Diazonium Salt

Finally, we will hydrolyze the benzenediazonium chloride to form benzoic acid. This step can be done by heating the diazonium salt with water under high temperature. The diazonium salt will undergo a reaction called hydrolysis to form benzoic acid. The reaction can be represented as: Benzenediazonium Chloride + H₂O → Benzoic Acid + N₂ + HCl After completing all these steps, we have successfully prepared benzoic acid from benzene using various inorganic reagents.

Key concepts

Nitration of Benzene

The nitration of benzene is the initial and vital step in the conversion pathway to benzoic acid. It starts with an electrophilic aromatic substitution reaction. To initiate this reaction, a mixture of concentrated nitric acid (HNO₃) and sulfuric acid (H₂SO₄) is needed. This mixture creates the nitronium ion ( NO₂⁺ ) which is the active electrophile that attacks the benzene ring.
When benzene, a stable ring structure with electrons, encounters this strong electrophile, it leads to the formation of nitrobenzene.

• Begin by adding benzene to a carefully prepared mix of HNO₃ and H₂SO₄.
• Ensure the temperature is maintained below 55°C to prevent side reactions.
• The reaction conditions favor the substitution of a hydrogen atom in benzene with a nitro group ( NO₂ o), thus producing nitrobenzene and a molecule of water (H₂O).

Reduction of Nitrobenzene

In the second step, the nitrobenzene produced from the nitration process undergoes reduction. This means the nitro group is transformed into an amino group, converting nitrobenzene to aniline. This reduction is crucial as it introduces an amine group into the benzene ring, essential for subsequent reactions.
The reduction can be done using different techniques:

• Use a combination of hydrogen gas ( H₂ ) and a nickel catalyst to catalyze the reaction efficiently.
• Alternatively, employ iron filings along with hydrochloric acid (HCl) to achieve the same result.

During the reduction, nitrobenzene accepts hydrogen to form aniline and water. The balanced chemical equation summarizing this is: Nitrobenzene + 6[H] → Aniline + 2H₂O.
This reaction perfectly sets the stage for diazotization.

Diazotization

The third step hinges upon a chemical transformation called diazotization. This process involves converting the amine group of aniline into a powerful intermediate called a diazonium salt.
For this transformation, aniline is treated with nitrous acid (HNO₂), which is created in situ by reacting sodium nitrite (NaNO₂) with a mineral acid like hydrochloric acid (HCl).

• Ensure to carry out this reaction in an ice-cold environment to stabilize the reactants.
• The reaction results in the formation of benzenediazonium chloride, a diazonium compound ideal for further functional transformations.

This reaction can be represented by the equation: Aniline + HNO₂ → Benzenediazonium Chloride + 2H₂O.
Diazotization is frequently used due to the versatility of diazonium salts in organic synthesis.

Hydrolysis of Diazonium Salts

The last segment involves the hydrolysis of the formed diazonium salt to finally yield benzoic acid.
Diazonium salts are sensitive to hydrolysis, especially under the influence of heat, and can decompose to liberate nitrogen gas, thus transforming the substituent on the benzene ring.
For achieving the desired benzoic acid:

• The benzenediazonium chloride is heated with water.
• This causes the diazonium group to hydrolyze, forming benzoic acid and releasing nitrogen gas and hydrochloric acid as byproducts.

The overall reaction can be simply stated as: Benzenediazonium Chloride + H₂O → Benzoic Acid + N₂ + HCl. This hydrolysis opens up numerous options for further chemical manipulation, showcasing how a simple diazonium salt can lead to a variety of end products, like benzoic acid in this context.