Question

Ethene on oxidation with cold alkaline \(\mathrm{KMnO}_{4}\) gives ethene glycol.

Short answer

Question: Describe the reaction of ethene when it gets oxidized with cold alkaline potassium permanganate (KMnO4). Answer: When ethene reacts with cold alkaline potassium permanganate (KMnO4), a nucleophilic addition reaction occurs, leading to the formation of an intermediate cyclic ester. This intermediate then undergoes hydrolysis to produce ethylene glycol (HOCH2CH2OH), potassium hydroxide (KOH), and manganese dioxide (MnO2). The cold alkaline conditions ensure selective oxidation, preventing further oxidative cleavage.

Step-by-step solution

Write the balanced chemical reaction

First, we must write the balanced chemical equation for the reaction of ethene with cold alkaline KMnO4. The equation is: $$\mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{KMnO}_{4} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{HOCH}_{2}\mathrm{CH}_{2}\mathrm{OH} + \mathrm{KOH} + \mathrm{MnO}_{2}$$ This reaction includes ethene (C2H4), potassium permanganate (KMnO4), water (H2O) as reactants, and ethylene glycol (HOCH2CH2OH), potassium hydroxide (KOH), and manganese dioxide (MnO2) as products.

Explain the oxidation of ethene by cold alkaline KMnO4

When ethene reacts with cold alkaline KMnO4, a nucleophilic addition reaction takes place. The permanganate ion (MnO4-) acts as a nucleophile which attacks the double bond in ethene, breaking the double bond and creating new single bonds between carbon atoms and the oxygen from the permanganate ion. This forms an intermediate called a cyclic ester, which then undergoes hydrolysis in the presence of water to form ethylene glycol. The use of cold alkaline KMnO4 conditions ensures only one equivalent of the oxidizing agent is consumed and the reaction proceeds with selective oxidation, leading to the desired product, ethylene glycol, without further oxidative cleavage.

Show evidence of the formation of ethylene glycol as the product

As a result of the oxidation of ethene by cold alkaline KMnO4, ethylene glycol (HOCH2CH2OH) is formed. In this process, the oxidizing agent (KMnO4) has been reduced to manganese dioxide (MnO2), and the acidic hydrogen of the formed ester is deprotonated by the hydroxide ion (OH-) giving potassium hydroxide (KOH) as a by-product. The formation of ethylene glycol as the product can be confirmed by several chemical techniques, such as infrared spectroscopy, which allows for the identification of the hydroxyl (O-H) groups present, or nuclear magnetic resonance (NMR), which can detect unique proton environments in the ethylene glycol molecule. In conclusion, when ethene is oxidized by cold alkaline KMnO4, it results in the formation of ethylene glycol through a selective oxidation process involving nucleophilic addition and hydrolysis of the intermediate cyclic ester.

Key concepts

Nucleophilic Addition Reaction

Understanding the basics of a nucleophilic addition reaction is crucial when studying organic chemistry, as it is a common mechanism for many transformations. In this type of reaction, a nucleophile, which is an electron-rich species, donates a pair of electrons to an electron-deficient carbon atom, typically one that is part of a multiple bond such as a double bond or a triple bond.

Take ethene, for example. It consists of a planar molecule with a carbon-carbon double bond, which is an area of high electron density. This characteristic makes it susceptible to attack by nucleophiles. In the oxidation of ethene, the electron-deficient component is the permanganate ion from potassium permanganate (KMnO4). The permanganate ion acts as a nucleophile despite being associated with oxidation reactions. During this reaction, the permanganate ion attacks the double bond of ethene, leading to the eventual formation of ethylene glycol.

Potassium Permanganate

Potassium permanganate, chemically notated as KMnO4, is a versatile oxidizing agent used in various chemical processes, including the oxidation of ethene to form ethylene glycol. It is highly valued for its potent oxidative properties, particularly in its cold, alkaline form, which ensures a controlled and selective oxidation.

In this colder, more controlled environment, the reactivity of potassium permanganate is moderated. This is essential in the chemical synthesis of sensitive compounds where over-oxidation could result in the degradation of the desired product. The unique feature of KMnO4 in these reactions is not only its oxidative power but also its selectivity, allowing for precise transformations such as the conversion of ethene to ethylene glycol without further degradation.

Ethylene Glycol Synthesis

Ethylene glycol, also known as HOCH2CH2OH or 1,2-ethanediol, is a valuable organic compound widely used as antifreeze in cooling and heating systems and in the manufacture of polyester fibers. The synthesis of ethylene glycol involves the oxidation of ethene in the presence of an oxidizing agent like potassium permanganate (KMnO4).

The specific conditions of cold and alkaline are used to facilitate the formation of ethylene glycol. The cold conditions help limit the reactivity of KMnO4, and the alkaline conditions are provided by the presence of hydroxide ions. Following the correct procedures in this synthesis ensures the production of high-purity ethylene glycol and limits the creation of unwanted by-products.

Chemical Reaction Balancing

The principle of conservation of mass requires that a chemical equation must be balanced—meaning the number of atoms of each element should remain the same before and after the reaction. Balancing a chemical reaction is fundamental to understanding the stoichiometry of a reaction, which refers to the quantitative relationships between the amounts of reactants and products.

When balancing the reaction for the oxidation of ethene to ethylene glycol, care must be taken to ensure that the atoms of carbon, hydrogen, oxygen, potassium, and manganese are present in equal numbers on both sides of the equation. By achieving a balanced equation, it becomes possible to deduce ratios and predict the quantities of reactants needed and products formed, allowing chemists to scale reactions accurately for industrial or laboratory synthesis.