Question

How do you count for the following observations? (a) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why ? Write a balanced redox equation for the reaction. (b) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas $\mathrm{HCl}$, but if the mixture contains bromide then we get red vapour of bromine. Why?

Short answer

Alcoholic KMnO4 is used for controlled oxidation to benzoic acid; sulphuric acid oxidizes bromides to bromine due to higher oxidizing power.

Step-by-step solution

Understanding the Effect of Different Media

In part (a), we need to understand why alcoholic potassium permanganate is preferred over alkaline or acidic potassium permanganate for oxidizing toluene to benzoic acid. Alkaline or acidic potassium permanganate may over-oxidize toluene, whereas alcoholic potassium permanganate provides a controlled oxidation, yielding primarily benzoic acid without further oxidation.

Writing the Redox Equation

The balanced redox reaction of oxidizing toluene (C_7H_8) to benzoic acid (C_6H_5COOH) using potassium permanganate (KMnO_4) in an alcoholic solution can be written as:$$3C_7{H}_8+4KMn{O}_4+4H_{2}O\to 3C_6{H}_{5}COOH+4Mn{O}_2+4KOH$$

Analyzing the Acidic Reaction with Halides

In part (b), concentrated sulfuric acid reacts differently with chlorides and bromides. With chlorides, the reaction produces HCl gas, and this is because chloride ions are a weaker reducing agent and simply release chloride ions as hydrochloric acid fumes. With bromides, sulfuric acid acts as a stronger oxidizing agent and oxidizes bromides to bromine vapor due to the higher oxidation power required, producing red bromine vapor.

Key concepts

Potassium Permanganate

Potassium permanganate (KMnO extsubscript{4}) is a chemical compound known for its strong oxidizing properties. This makes it widely used across various oxidation reactions.
It exists in solutions of different mediums such as acidic, alkaline, or alcoholic, and each influences the oxidation outcome. - **Alkaline Potassium Permanganate**: Often results in over-oxidation, which means that more products are formed, which may not always be desired.
- **Acidic Potassium Permanganate**: Like alkaline solutions, it can be too aggressive, leading to unwanted by-products.
- **Alcoholic Potassium Permanganate**: This is more conservative, allowing for controlled oxidation. This is particularly important when oxidizing organic compounds such as toluene to benzoic acid.

Redox Reaction

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons from one substance to another. In these reactions, the substance that gains electrons is reduced, while the one that loses electrons is oxidized.
Redox reactions are fundamental in both chemistry and biology, playing crucial roles in energy transfer and chemical synthesis.In the oxidation of toluene to benzoic acid, potassium permanganate serves as the oxidizing agent, accepting electrons, while toluene is oxidized, losing electrons. The balanced equation for this redox process is:$$3C_7{H}_8+4KMn{O}_4+4H_{2}O\to 3C_6{H}_{5}COOH+4Mn{O}_2+4KOH$$
This equation showcases three main events: oxidation of toluene, reduction of permanganate, and the preservation of mass and charge.

Benzoic Acid Synthesis

The synthesis of benzoic acid from toluene is a classic example of an oxidation reaction where potassium permanganate is used in an alcoholic medium.
During this transformation, the methyl group of toluene is oxidized to the carboxyl group, resulting in benzoic acid, a compound with valuable applications in food preservation and pharmaceuticals.
Choosing the right oxidizing conditions is essential to prevent further oxidation beyond the carboxyl group. The controlled nature of alcoholic potassium permanganate avoids over-oxidation, efficiently transforming toluene into benzoic acid.

Sulfuric Acid Reactions

Sulfuric acid ( H_2SO_4 ) is a strong acid often used in reactions involving halogens. When it reacts with halides, its role depends on the type of halide present. - **Reactions with Chlorides**: Sulfuric acid reacts with chloride salts to release HCl gas, which is colorless. The chloride ion is a poor reducing agent, hence no visible reaction beyond the HCl formation occurs. - **Reactions with Bromides**: In contrast, bromides being better reducing agents lead to a more vigorous reaction. Sulfuric acid oxidizes bromide ions to bromine, a darker, red vapor, which is easily noticeable.
This highlights the dual role of sulfuric acid as both an acid and an oxidizing agent depending on the halide involved.

Toluene Oxidation

Toluene oxidation involves converting the methyl group of toluene into a carboxyl group, resulting in benzoic acid.
This process primarily uses potassium permanganate because of its strong oxidative potential. - **Mechanism**: In an alcoholic medium, toluene is oxidized by potassium permanganate, avoiding further oxidation thanks to the controlled nature of the reaction environment.
- **Significance**: This oxidation is critical in industrial chemistry, where benzoic acid is used as a key intermediate in manufacturing plasticizers, perfumes, and dyes.
The ability to efficiently and selectively oxidize toluene to benzoic acid is a significant achievement in industrial organic chemistry, benefiting various applications.