Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Which substance chars when heated with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) ? (a) protein (b) hydrocarbon (c) fat (d) carbohydrate

Step-by-Step Solution

Answer

Carbohydrate chars when heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\).

Step 1: Identify the Reactants and Conditions

We are asked to identify which substance will char when heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). The reactants, in this case, are \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and the four given substances: protein, hydrocarbon, fat, and carbohydrate.

Step 2: Understand the Chemistry Involved

Concentrated sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) acts as a strong dehydrating agent. When heated with organic compounds, it can remove water molecules from them, leading to charring, especially if the compound contains a high proportion of oxygen and hydrogen as in carbohydrates.

Step 3: Assess Substance Compatibility

Protein, hydrocarbon, and fat contain carbon but differ in their susceptibility to dehydration. Carbohydrates have the general formula \(C_n(H_2O)_m\), which means they have many hydroxyl groups (\(-OH)\) attached, making them good targets for dehydration by \(\mathrm{H}_{2} \mathrm{SO}_{4}\). This dehydration leads to charring, forming carbon and release of water vapor.

Step 4: Conclude with the Most Likely Outcome

Given that carbohydrates have a higher tendency to lose water and char when exposed to \(\mathrm{H}_{2} \mathrm{SO}_{4}\) due to their structure, they are more likely to char than proteins, hydrocarbons, or fats.

Key Concepts

Dehydrating Agents

Dehydrating agents are substances used to remove water molecules from other compounds. They play a crucial role in various chemical reactions. One of the most common dehydrating agents is concentrated sulfuric acid (
) ( H_2SO_4 ). It is known for its strong ability to absorb water rapidly. Dehydrating agents achieve this by creating conditions where water is drawn out of compounds, often leading to interesting chemical transformations. This is particularly true with organic compounds.

• **Charring of substances:** When dehydrating agents like sulfuric acid come into contact with certain organic compounds, the rapid loss of water can lead to charring. This process forms carbon as a residue.
• **Reactions with carbohydrates:** Carbohydrates, due to their structure, are especially prone to dehydration. This is because they contain a significant number of hydroxyl groups ( -OH).

Through the action of dehydrating agents, organic compounds can undergo transformations that wouldn't be possible otherwise. It's an essential concept in understanding how chemical reactions like charring occur.

Organic Compounds

Organic compounds are molecules that contain carbon, often bonded with elements like hydrogen, oxygen, nitrogen, and more. This broad category includes proteins, hydrocarbons, fats, and carbohydrates. Each organic compound type has different properties, and their reactions with chemicals like dehydrating agents can differ significantly.

• **Proteins:** These are made of amino acids and contain nitrogen. While they have carbon, they aren't highly prone to charring compared to carbohydrates.
• **Hydrocarbons:** Composed entirely of carbon and hydrogen, hydrocarbons have little oxygen. They don't readily undergo dehydration in the presence of sulfuric acid.
• **Fats:** Fats are esters of fatty acids and glycerol. Similar to hydrocarbons, they aren't the best candidates for dehydration with strong acids.
• **Carbohydrates:** These molecules have the formula Cₖ(H₂O)ₙ , meaning they have water molecules in their structure. They're more likely to lose water, leading to charring with sulfuric acid.

Understanding the types of organic compounds and their composition can help predict their behavior in chemical reactions. Carbohydrates stand out in this context because their structure makes them particularly suitable for dehydration, resulting in charring.

Concentrated Sulfuric Acid

Concentrated sulfuric acid is a highly corrosive and dense liquid, known for its strong dehydrating properties. It is widely used in laboratory and industrial settings to cause or speed up chemical reactions.
**Properties of Concentrated Sulfuric Acid:**

• **High boiling point and density:** These properties make it particularly effective for removing water rapidly from organic materials.
• **Corrosive nature:** Contact with concentrated H₂SO₄ can cause severe burns. It's essential to handle it with care and appropriate safety gear.
• **Dehydrating ability:** This is one of its standout characteristics. By removing water from compounds, it can cause changes like carbonization.

When concentrated sulfuric acid interacts with organic compounds, it often catalyzes reactions by stripping away water. For instance, when applied to carbohydrates like sugar, it catalyzes dehydration, resulting in a black, charred mass that's primarily carbon. This is an excellent example of how potent concentrated sulfuric acid can be in driving chemical changes.