Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic compound on heating with \(\mathrm{CuO}\) produces \(\mathrm{CO}_{2}\) but not water. It may be (a) \(\mathrm{CH}_{4}\) (b) \(\mathrm{CCl}_{4}\) (c) \(\mathrm{CHCl}_{3}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I}\)

Step-by-Step Solution

Answer

The compound is \( \mathrm{CCl}_{4} \).

Step 1: Analyzing the reaction

When an organic compound is heated with \( \mathrm{CuO} \), it will produce \( \mathrm{CO}_{2} \) and \( \mathrm{H}_{2} \mathrm{O} \) if hydrogen is present in the compound. The production of \( \mathrm{CO}_{2} \) but not water indicates that the compound does not contain hydrogen.

Step 2: Identifying the compounds

Let's examine the given options to see if they contain hydrogen: - (a) \( \mathrm{CH}_{4} \) contains hydrogen.- (b) \( \mathrm{CCl}_{4} \) does not contain hydrogen.- (c) \( \mathrm{CHCl}_{3} \) contains hydrogen.- (d) \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I} \) contains hydrogen.

Step 3: Choosing the correct compound

Since we are looking for a compound that does not produce water (i.e., does not contain hydrogen), \( \mathrm{CCl}_{4} \) is the only option that fits this criterion. It will produce \( \mathrm{CO}_{2} \) only when reacted with \( \mathrm{CuO} \) since it has no hydrogen to form water.

Key Concepts

Combustion Reactions

Combustion reactions are fundamental in chemistry, particularly in organic chemistry. They occur when a substance, usually an organic compound, reacts with oxygen. The most common products are carbon dioxide (\(\mathrm{CO}_2\)) and water (\(\mathrm{H}_2\mathrm{O}\)), provided the compound contains carbon and hydrogen.

In the context of combustion, organic compounds typically burn in the presence of oxygen to release energy. This reaction can be expressed generally as:

• \[\text{Compound} + \text{Oxygen} \rightarrow \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} + \text{Energy} \]

In this equation, energy is often released in the form of heat and light. The key point is that water is a byproduct only if hydrogen is present in the compound. In our particular exercise, the absence of water indicates the absence of hydrogen, leading us to question the structure of the compound involved.

Absence of Hydrogen

The absence of hydrogen in a compound has significant implications for its reactions, especially in combustion. If hydrogen is not present, water (\(\mathrm{H}_2\mathrm{O}\)) cannot be produced as a product of the reaction.

In organic chemistry, hydrogen is commonly found in hydrocarbons, the simplest being methane (\(\mathrm{CH}_4\)). Most organic molecules contain some combination of hydrogen and carbon. However, certain compounds, like carbon tetrachloride (\(\mathrm{CCl}_4\)), contain no hydrogen. This absence is crucial in determining the products of reactions.

When a compound like \(\mathrm{CCl}_4\) undergoes combustion in the presence of a strong oxidizing agent such as copper(II) oxide (\(\mathrm{CuO}\)), it will only produce \(\mathrm{CO}_2\), as there are no hydrogen atoms to form water. Understanding this helps predict the behavior and results of chemical reactions.

Copper Oxide Reaction

Copper(II) oxide (\(\mathrm{CuO}\)) acts as an oxidizing agent in various chemical reactions. Its role is to facilitate the oxidation of other substances, commonly organic compounds. In doing so, \(\mathrm{CuO}\) itself is reduced, typically forming copper metal or other stable compounds depending on the reaction conditions.

In the original exercise, \(\mathrm{CuO}\) is used to oxidize an organic compound that does not contain hydrogen. The resulting products are limited to carbon dioxide (\(\mathrm{CO}_2\)) since there is no hydrogen present to form water.

This specific reaction is essential for distinguishing between different types of organic compounds. In the absence of water production, it confirms that the compound in question lacks hydrogen. This is a critical insight when identifying compounds like carbon tetrachloride, as its reaction with \(\mathrm{CuO}\) confirms the absence of hydrogen and predicts the formation of \(\mathrm{CO}_2\) only. Understanding this oxidation process enhances our comprehension of different reaction pathways in organic chemistry.