Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

When the vapours of ethyl alcohol are passed over red hot copper \(\left(300^{\circ} \mathrm{C}\right)\), the product formed is (a) acetaldehyde (b) acetic acid (c) ethylene (d) none of these

Step-by-Step Solution

Answer

The correct product is acetaldehyde (a).

Step 1: Understanding the Reaction Conditions

When ethyl alcohol (ethanol) is passed over red hot copper, which is at a temperature of about 300°C, it undergoes a chemical reaction. Understanding the conditions is crucial as the heat and catalytic surface often result in a dehydrogenation process.

Step 2: Identifying the Reactant

The reactant in this chemical process is ethyl alcohol, also known as ethanol, with the chemical formula C₂H₅OH. Understanding this helps in foreseeing the changes it undergoes during the reaction.

Step 3: Predicting the Reaction Outcome

At high temperatures and in the presence of copper, ethanol commonly undergoes dehydrogenation, where it loses hydrogen atoms. This reaction leads to the formation of acetaldehyde (CH₃CHO), as ethanol loses two hydrogen atoms.

Key Concepts

Acetaldehyde formation

When ethanol undergoes dehydrogenation, it loses hydrogen atoms, transforming into a different compound. The process yields acetaldehyde, a significant organic compound marked by the chemical formula \( \text{CH}_3\text{CHO} \). This conversion is a convenient method for producing acetaldehyde because it is simple and efficient.
The process happens as ethanol loses two hydrogen atoms. This changes ethanol into acetaldehyde and releases hydrogen gas as a byproduct. The reaction can be shown as follows:

• \( \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{CHO} + \text{H}_2 \)

This conversion plays a vital role in industrial chemistry, helping generate essential chemicals. Acetaldehyde itself is widely used in producing various compounds, including acetic acid and perfumes. Understanding this transformation sheds light on how simple modifications at the atomic level lead to new materials and products. Acetaldehyde is valued due to its versatility and reactivity with other chemicals, further expanding its usage in producing adhesives, plastics, and synthetic rubbers. Learning about this process provides a foundation for engaging with broader chemical engineering and organic synthesis topics.

Copper as catalyst

The choice of copper in this reaction is no coincidence. Copper acts as a catalyst in the dehydrogenation of ethanol. Catalysts are substances that speed up chemical reactions without being consumed themselves. This means copper is not used up in the process, making it cost-effective for repeated reactions.
Copper's ability to serve as a catalyst lies in its physical and chemical properties, particularly its capacity to undergo oxidation and reduction reactions. In this case, copper facilitates the removal of hydrogen atoms from ethanol, effectively allowing the conversion to acetaldehyde.
The use of copper also offers several advantages:

• It lowers the activation energy required for the reaction, enabling it to proceed at a lower temperature.
• Copper's high thermal conductivity helps distribute heat evenly, promoting consistent reaction conditions.
• The metal's surface provides an active site where ethanol molecules can interact and transform more efficiently.

Understanding catalysts like copper highlights their critical role in industrial and laboratory settings. They make chemical processes more efficient and economically viable, which is essential for large-scale production.

Chemical reaction temperature

The temperature at which a chemical reaction occurs can significantly influence the reaction's outcome. In the dehydrogenation of ethanol using a copper catalyst, maintaining the correct temperature is crucial. The reaction typically occurs at approximately 300°C.
This temperature is necessary to ensure that ethanol molecules have enough energy to interact with the copper surface effectively. At this high temperature, ethanol gains the kinetic energy needed to break its C-H bonds, allowing hydrogen atoms to detach and the formation of acetaldehyde.
High temperature also affects:

• The rate of reaction: Higher temperatures generally increase reaction rates by providing reactants with the necessary energy to overcome activation barriers more quickly.
• The stability of reactants and catalysts: The temperature must be high enough to drive the reaction but not so high that it causes undesirable side reactions or degrades the catalyst.

Understanding how temperature impacts chemical reactions forms the basis for controlling and optimizing industrial processes. Properly managing reaction conditions ensures the desired product is obtained efficiently and safely.