Problem 42 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An organic liquid (A), containing \(\mathrm{C}, \mathrm{H}\) and \(\mathrm{O}\) with boiling point: \(78^{\circ} \mathrm{C}\), and possessing a rather pleasant odour, on heating with concentrated sulphuric acid gives a gaseous product (B) - with the empirical formula, \(\mathrm{CH}_{2} .\) 'B' decolourises bromine water as well as alkaline \(\mathrm{KMnO}_{4}\) solution and takes up one mole of \(\mathrm{H}_{2}\) (per mole of 'B') in the presence of finely divided nickel at high temperature. Identify the substances 'A' and 'B'.

Step-by-Step Solution

Answer

A is ethanol, B is ethene.

Step 1: Understanding the Problem

We have a liquid (A) that transforms into a gaseous product (B) when heated with concentrated sulfuric acid. We know the empirical formula of B is \(\text{CH}_2\). B has specific reactions: it decolorizes bromine water and alkaline \(\text{KMnO}_4\), and it reacts with one mole of hydrogen in the presence of nickel catalyst.

Step 2: Boiling Point and Odor Clue

The boiling point of 78°C and the pleasant odor suggests that substance A is likely ethanol \((\text{C}_2\text{H}_5\text{OH})\). Ethanol is known for its pleasant odor and has a boiling point close to 78°C.

Step 3: Reaction with Sulfuric Acid

When ethanol \((\text{C}_2\text{H}_5\text{OH})\) is heated with concentrated sulfuric acid, it undergoes dehydration to form ethene \((\text{C}_2\text{H}_4})\). The dehydration reaction removes water from ethanol, producing a gas with an empirical formula \(\text{CH}_2\).

Step 4: Decolorizing Bromine Water

The fact that B decolorizes bromine water indicates that it contains a carbon-carbon double bond, as alkenes typically react with bromine. Ethene \((\text{C}_2\text{H}_4})\), formed in the dehydration process, is an alkene, thus matching this characteristic.

Step 5: Reaction with Alkaline KMnO4

Ethene discharges the purple color of alkaline \(\text{KMnO}_4\), indicating it is capable of oxidation, another key behavior of alkenes. This supports the hypothesis that B is ethene.

Step 6: Hydrogenation Reaction

Ethene reacts with one mole of \(\text{H}_2\) in the presence of a nickel catalyst to form ethane, confirming the presence of a double bond. This hydrogenation reaction further confirms that B is indeed ethene.

Key Concepts

Dehydration Reaction

A dehydration reaction is a type of chemical reaction where a molecule of water is removed from a compound. In the context of organic chemistry, dehydration typically refers to the process where an alcohol loses a water molecule to form an alkene. This reaction is usually facilitated by the presence of a strong acid, such as concentrated sulfuric acid.

In the example of ethanol, when it is heated with concentrated sulfuric acid, the process results in the formation of ethene. Here, the acid acts as a catalyst, enabling ethanol (\(\mathrm{C}_2\mathrm{H}_5\mathrm{OH}\)) to lose a water molecule (\(\mathrm{H}_2\mathrm{O}\)), resulting in the formation of ethene (\(\mathrm{C}_2\mathrm{H}_4\)).

• The acid catalyst helps in protonating the alcohol group, making it a better leaving group.
• Once the water molecule is eliminated, a double bond forms between carbon atoms, producing an alkene.

This type of reaction is significant as it is a classic method for synthesizing alkenes from alcohols.

Alkenes

Alkenes are a class of hydrocarbons known for containing at least one carbon-carbon double bond. This double bond is a key functional group, giving alkenes their unique reactive properties. Because of the double bond, alkenes are considered unsaturated hydrocarbons.

Ethene (\(\mathrm{C}_2\mathrm{H}_4\)) is the simplest alkene, formed through the dehydration of ethanol. Due to the presence of the double bond, alkenes like ethene can undergo specific reactions:

• Decolorization of Bromine Water: Alkenes react with bromine, breaking the double bond to add bromine atoms. This results in the decolorization of the reddish-brown bromine water, a classic test for unsaturation.
• Oxidation: Alkenes react with oxidizing agents such as alkaline \(\mathrm{KMnO}_4\), which can break the double bond and form glycol-type compounds.
• Hydrogenation: When exposed to hydrogen gas in the presence of a catalyst like nickel, alkenes can be converted to alkanes by adding hydrogen across the double bond.

These reactions highlight the significance of the double bond in alkenes, offering a variety of pathways for chemical transformations.

Empirical Formula Determination

The empirical formula of a compound is the simplest representation of its composition, showing the ratio of different atoms present in the compound. It is important for understanding the basic makeup of a chemical species, particularly in organic chemistry experiments and identification.

In the exercise, the compound 'B' was determined to have the empirical formula \(\mathrm{CH}_2\). This provides a clue about its composition and structure. Since empirical formulas represent the simplest whole-number ratio, \(\mathrm{CH}_2\) suggests a structure with twice as many hydrogen atoms as carbon, characteristic of alkenes, where a general formula is \(\mathrm{C}_n\mathrm{H}_{2n}\).

Determining an empirical formula often involves analyzing the mass percentage of each element present in a compound. Here's a simple approach:

• Determine the mass of each element in a sample.
• Convert these masses to moles.
• Find the simplest whole-number ratio of these moles to determine the empirical formula.

For ethene, the empirical formula \(\mathrm{CH}_2\) precisely matches its chemical behavior as inferred from experimental reactions.

Boiling Point Significance

Boiling points of substances serve as important identifiers, especially in organic chemistry, where they offer clues to the molecular structure and intermolecular forces present in a compound.

In the given exercise, the substance 'A' has a boiling point of \(78^{\circ}\mathrm{C}\), suggesting a specific molecular identity. Ethanol is a common organic compound known to have a boiling point around this temperature. The boiling point of ethanol is due to hydrogen bonding between its molecules, a characteristic of alcohols causing increased boiling points compared to ethers or hydrocarbons of similar molar mass.

The boiling point is significant because:

• It can indicate the presence of intermolecular forces, such as hydrogen bonds, dipole-dipole interactions, or Van der Waals forces.
• It helps in identifying the purity of a compound, as impurities often alter boiling points.
• In organic synthesis, boiling points can guide purification processes like distillation where substances are separated based on their boiling points.

Understanding the boiling point can thus aid in identifying unknown compounds, elucidating their structures, and assessing their purity.