In the presence of 18-crown-6, potassium permanganate dissolves in benzene to give "purple benzene," a useful reagent for oxidizing alkenes in an aprotic environment. Use a drawing of the complex to show why KMnO₄ dissolves in benzene and why the reactivity of the permanganate ion is enhanced.

Verified step by step guidance

1. Understand the role of 18-crown-6: 18-crown-6 is a crown ether, a cyclic compound with six oxygen atoms that can coordinate with a potassium ion (K⁺). This coordination stabilizes the potassium ion and makes it soluble in nonpolar solvents like benzene.
2. Draw the complex: Represent the 18-crown-6 molecule as a ring structure with six oxygen atoms. Show the potassium ion (K⁺) in the center of the ring, coordinated to the oxygen atoms through ion-dipole interactions. This forms a stable complex, [K(18-crown-6)]⁺.
3. Explain the solubility in benzene: The [K(18-crown-6)]⁺ complex is soluble in benzene because the crown ether shields the ionic nature of the potassium ion, making the complex more compatible with the nonpolar benzene solvent. The permanganate ion (MnO₄⁻) remains as a counterion, which also becomes soluble due to the association with the [K(18-crown-6)]⁺ complex.
4. Discuss the enhanced reactivity of MnO₄⁻: In an aprotic environment like benzene, the permanganate ion (MnO₄⁻) is not stabilized by hydrogen bonding (as it would be in a protic solvent). This lack of stabilization increases its electrophilicity, making it a more reactive oxidizing agent for alkenes.
5. Summarize the oxidation process: When alkenes are introduced to the 'purple benzene' solution, the highly reactive MnO₄⁻ ion oxidizes the alkenes, typically forming diols or other oxidized products depending on the reaction conditions. The aprotic environment ensures that the reaction proceeds efficiently without competing side reactions.

Key Concepts

Crown Ethers

Crown ethers, like 18-crown-6, are cyclic compounds that can selectively bind cations due to their unique ring structure. The size and shape of the crown ether allow it to encapsulate specific ions, such as potassium, effectively solubilizing them in nonpolar solvents like benzene. This interaction enhances the solubility of otherwise insoluble salts, facilitating their use in organic reactions.

Potassium Permanganate (KMnO4)

Potassium permanganate is a strong oxidizing agent commonly used in organic chemistry. In the presence of crown ethers, its solubility in nonpolar solvents increases, allowing it to participate in reactions that would otherwise be limited by its solubility. The permanganate ion (MnO4-) can undergo various redox reactions, making it valuable for oxidizing alkenes to form carbonyl compounds.

Aprotic Solvent Environment

An aprotic solvent is one that does not donate protons (H+) and can stabilize ions without forming hydrogen bonds. In this environment, the reactivity of certain reagents, like permanganate, is enhanced because the solvent does not interfere with the reaction mechanism. This allows for more efficient oxidation of alkenes, as the permanganate ion remains more reactive and less hindered by solvent interactions.