Problem 54 - A Complete Resource Book in Chemistry for JEE Main

Question

Ethylene reacts with \(1 \%\) cold alkaline \(\mathrm{KMnO}_{4}\) to form (a) ethylene glycol (b) oxalic acid (c) HCHO (d) ethyl alcohol

Step-by-Step Solution

Answer

Ethylene reacts with cold alkaline \( \mathrm{KMnO}_{4} \) to form ethylene glycol.

Step 1: Understanding the Reaction

Ethylene ( ext{C}_2 ext{H}_4) is an alkene, which when reacted with cold dilute ext{KMnO}_4 (Potassium Permanganate in alkaline medium), undergoes hydroxylation. This process involves adding hydroxyl groups ( ext{OH}) to the carbon atoms of the double bond.

Step 2: Cold Alkaline ext{KMnO}_4 Reaction

Cold alkaline ext{KMnO}_4 is a reagent used for syn dihydroxylation, meaning it adds hydroxyl groups to the same side of the double bond in alkenes to form vicinal diols (glycols). The reaction results in the conversion of the double bond into two ext{C}- ext{OH} bonds.

Step 3: Applying the Reaction to Ethylene

In this context, ethylene ( ext{CH}_2= ext{CH}_2) will react with cold ext{KMnO}_4 and ext{OH}^- to produce ethylene glycol ( ext{HO}- ext{CH}_2- ext{CH}_2- ext{OH}), a vicinal diol.

Step 4: Conclusion

Based on the cold alkaline ext{KMnO}_4 reaction of ethylene, the product formed is ethylene glycol, which corresponds to option (a).

Key Concepts

Alkene Hydroxylation

When alkenes like ethylene undergo hydroxylation, it involves adding hydroxyl groups (\( \text{OH} \)) across the carbon-carbon double bond. This reaction is important because it changes the structure of the alkene, introducing new functional groups that alter its chemical properties. The process generally involves the use of a specific reagent – in this case, cold, dilute potassium permanganate (\( \text{KMnO}_4 \)) in an alkaline medium.

The purpose of the hydroxylation is to transform a nonpolar molecule into a more polar one, which can significantly enhance the molecule's reactivity and solubility in water. By converting the double bond into an alcohol group, the compound becomes more versatile in chemical reactions. This method is widely used in synthetic chemistry to prepare alcohols from alkenes with precision and control.

Dihydroxylation of Alkenes

Dihydroxylation is a specific type of hydroxylation where two hydroxyl groups are simultaneously added to an alkene. Cold, dilute \( \text{KMnO}_4 \) is a classic reagent for this reaction, performing what's known as syn dihydroxylation.

Understanding the reaction mechanism helps in comprehensive learning. Potassium permanganate reacts with the alkene to form a cyclic manganate ester intermediate. Subsequently, this intermediate splits, resulting in the addition of hydroxyl groups to the same side of the double bond, known as 'syn addition'.
This characteristic of syn addition is crucial as it determines the stereochemistry of the resulting product, ensuring that both hydroxyl groups are on the same side of the former double bond. This specificity is immensely useful in designing synthetic pathways for complex organic molecules.

Vicinal Diols Formation

Vicinal diols are compounds with two hydroxyl groups attached to adjacent carbon atoms. The term "vicinal" refers to these hydroxyl groups being next to each other, often resulting from the dihydroxylation of alkenes. In the process involving cold, dilute \( \text{KMnO}_4 \), ethylene reacts to form ethylene glycol, a simple example of a vicinal diol.

The importance of vicinal diols in organic chemistry cannot be overstated as they serve as key intermediates in the synthesis of various chemical compounds. They are versatile building blocks because their dual hydroxyl groups allow for multiple reaction pathways.

• These diols can undergo dehydration to form ketones or involve in oxidation reactions to give rise to carboxylic acids.
• In real-world applications, they are integral in the manufacturing of polymers, pharmaceuticals, and as antifreeze agents due to ethylene glycol's cryoprotectant properties.

Understanding the formation and properties of vicinal diols is essential for chemists looking to manipulate molecular structures for targeted functionalization.