What is the Best Method for Conversion of Ethanol to Ethanal? | Class 12 Chemistry

📝 How to Write Answer ✅ Step-by-Step Solution

The best method for converting ethanol to ethanal is by passing ethanol vapors over heated copper at 573-578 K, which selectively oxidizes the primary alcohol to an aldehyde without further oxidation. This controlled oxidation process is fundamental in organic chemistry and appears frequently in Class 12 board examinations. In this comprehensive guide, you’ll learn the complete reaction mechanism, comparison of different oxidizing agents, step-by-step solution approach, and exam-ready answer format with proper chemical equations.

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⚡ Quick Answer

Question: What is the best method for conversion of ethanol to ethanal?

Answer: The best method is passing ethanol vapors over heated copper (Cu) at 573-578 K. This catalytic oxidation selectively converts ethanol (C₂H₅OH) to ethanal (CH₃CHO) without further oxidation to carboxylic acid. The copper acts as a catalyst and dehydrogenates the primary alcohol to form the aldehyde. This method is preferred because it provides controlled oxidation, high selectivity, and prevents over-oxidation to acetic acid.

🧪 Understanding the Conversion: Ethanol to Ethanal

Ethanol (C₂H₅OH) is a primary alcohol, meaning the hydroxyl group (-OH) is attached to a carbon atom that is bonded to only one other carbon atom. When we want to convert ethanol to ethanal (CH₃CHO), we need to perform an oxidation reaction. However, the challenge lies in controlling the oxidation so that we stop at the aldehyde stage and don’t proceed further to form a carboxylic acid (acetic acid).

The general oxidation pathway for primary alcohols is:

    Primary Alcohol → Aldehyde → Carboxylic Acid

    C2H5OH → CH3CHO → CH3COOH

    (Ethanol) → (Ethanal) → (Acetic Acid)

Our goal is to stop the reaction at the aldehyde stage (ethanal) and prevent further oxidation. This requires selecting the right oxidizing agent and reaction conditions. Let’s explore why passing ethanol vapors over heated copper is the best method.

⚙️ The Copper Catalyst Method: Why It’s the Best

🔬 Reaction Details

When ethanol vapors are passed over heated copper at 573-578 K (approximately 300-305°C), the following reaction occurs:

C₂H₅OH → CH₃CHO + H₂
(Ethanol) → (Ethanal) + (Hydrogen gas)

Conditions: Cu catalyst at 573-578 K

✨ Key Advantages of This Method

Selective Oxidation: The copper catalyst promotes dehydrogenation (removal of hydrogen) rather than addition of oxygen, which naturally stops at the aldehyde stage.
No Over-oxidation: Unlike strong oxidizing agents (K₂Cr₂O₇, KMnO₄), copper doesn’t oxidize the aldehyde further to carboxylic acid.
High Yield: The reaction produces ethanal with high efficiency and purity.
Simple Setup: The process is straightforward and doesn’t require complex reagents or harsh conditions.
Industrial Viability: This method is economically feasible and used in industrial production of aldehydes.

🔄 Mechanism of the Reaction

The copper-catalyzed oxidation proceeds through a catalytic dehydrogenation mechanism:

Adsorption: Ethanol molecules are adsorbed onto the copper surface at high temperature (573-578 K).

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Dehydrogenation: The copper catalyst facilitates the removal of two hydrogen atoms (one from -OH group and one from the adjacent carbon atom), forming a C=O double bond.

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Desorption: The ethanal molecule and hydrogen gas (H₂) are released from the copper surface, regenerating the catalyst for the next cycle.

This mechanism is fundamentally different from oxidation by chemical oxidizing agents, which add oxygen to the molecule. Here, the process is dehydrogenation (removal of hydrogen), which naturally stops at the aldehyde stage because aldehydes are much more difficult to dehydrogenate further.

⚖️ Comparison with Other Oxidizing Agents

Let’s compare the copper catalyst method with other common oxidizing agents to understand why it’s the best choice for this specific conversion:

Oxidizing Agent/Method	Product Formed	Reason/Mechanism	Suitable?
Cu at 573-578 K (Copper catalyst)	Ethanal (CH₃CHO) ✓	Catalytic dehydrogenation; removes H₂ without adding oxygen; stops at aldehyde stage naturally	✅
K₂Cr₂O₇/H₂SO₄ (Potassium dichromate)	Acetic Acid (CH₃COOH) ✗	Strong oxidizing agent; oxidizes primary alcohol all the way to carboxylic acid; cannot stop at aldehyde	❌
KMnO₄ (Potassium permanganate)	Acetic Acid (CH₃COOH) ✗	Very strong oxidizing agent; rapidly oxidizes to carboxylic acid; impossible to control at aldehyde stage	❌
Acidified MnO₂ (Manganese dioxide)	Ethanal (possible) ⚠️	Milder oxidizing agent; can produce aldehyde but reaction is less selective and harder to control than Cu method	⚠️
PCC/PDC (Pyridinium chlorochromate)	Ethanal (CH₃CHO) ✓	Mild oxidizing agent; stops at aldehyde; but expensive and not commonly used in basic laboratory/industry	✓

💡 Key Insight

The copper catalyst method is preferred because it’s a dehydrogenation reaction rather than an oxidation by oxygen addition. This fundamental difference in mechanism ensures that the reaction naturally stops at the aldehyde stage, making it the most selective and reliable method for converting ethanol to ethanal.

🔑 Key Points to Remember

Best Method: Passing ethanol vapors over heated copper (Cu) at 573-578 K is the most selective method for converting ethanol to ethanal.
Mechanism: The reaction proceeds via catalytic dehydrogenation (removal of H₂), not oxidation by oxygen addition.
Product: Ethanal (CH₃CHO) is formed along with hydrogen gas (H₂) as a byproduct.
Why Not Strong Oxidizers: K₂Cr₂O₇ and KMnO₄ are too strong and oxidize ethanol all the way to acetic acid (carboxylic acid).
Selectivity: Copper catalyst naturally stops at the aldehyde stage because aldehydes are resistant to further dehydrogenation.
Industrial Use: This method is economically viable and widely used in industrial production of aldehydes from primary alcohols.
Exam Tip: Always mention the temperature (573-578 K) and the byproduct (H₂) in your answer for full marks.

📝 How to Write the Perfect Exam Answer (3 Marks)

✍️ Marking Scheme Breakdown

1 Mark: Identifying the method (Cu catalyst at 573-578 K)
1 Mark: Writing the correct chemical equation with proper formulas
1 Mark: Explaining why this method is best (selective oxidation/dehydrogenation, no over-oxidation)

📋 Model Answer (Write This in Your Exam)

Question: What is the best method for the conversion of ethanol to ethanal? Explain.

Answer:

The best method for converting ethanol to ethanal is by passing ethanol vapors over heated copper at 573-578 K.

Chemical Equation:

C₂H₅OH → CH₃CHO + H₂
Copper catalyst, 573-578 K

Explanation: This method is preferred because copper acts as a catalyst for dehydrogenation (removal of hydrogen) rather than oxidation by oxygen addition. The reaction selectively produces ethanal (aldehyde) without further oxidation to acetic acid (carboxylic acid). Other strong oxidizing agents like K₂Cr₂O₇ or KMnO₄ would oxidize ethanol completely to acetic acid, making them unsuitable for this conversion.

💡 Exam Tip: Always write the temperature (573-578 K) and mention that H₂ gas is produced as a byproduct. Also, briefly explain why other oxidizing agents are not suitable. This shows complete understanding and helps you score full marks.

✅ Step-by-Step Solution Approach

When solving questions about converting ethanol to ethanal, follow this systematic approach:

Identify the Functional Groups

Recognize that ethanol (C₂H₅OH) is a primary alcohol and ethanal (CH₃CHO) is an aldehyde. This tells you that an oxidation reaction is needed.

Understand the Oxidation Pathway

Remember: Primary Alcohol → Aldehyde → Carboxylic Acid. Your goal is to stop at the aldehyde stage, so you need a selective oxidizing agent.

Evaluate Different Methods

Consider various oxidizing agents: Strong oxidizers (K₂Cr₂O₇, KMnO₄) will over-oxidize to carboxylic acid. Copper catalyst provides controlled dehydrogenation.

Select the Best Method

Choose copper catalyst at 573-578 K because it selectively produces ethanal through dehydrogenation without further oxidation.

Write the Chemical Equation

C₂H₅OH → CH₃CHO + H₂ (with Cu catalyst at 573-578 K). Don’t forget to mention the byproduct H₂!

Explain Why It’s the Best

State that this method is selective, prevents over-oxidation, and is economically viable. Mention why other methods (strong oxidizers) are unsuitable.

❓ Frequently Asked Questions (FAQs)

❓ Why can’t we use K₂Cr₂O₇ to convert ethanol to ethanal?

Potassium dichromate (K₂Cr₂O₇) is a strong oxidizing agent that cannot be controlled to stop at the aldehyde stage. When ethanol is treated with acidified K₂Cr₂O₇, it rapidly oxidizes the primary alcohol first to ethanal and then immediately further to acetic acid (CH₃COOH). The reaction is so vigorous that it’s practically impossible to isolate ethanal as the product. Therefore, K₂Cr₂O₇ is suitable for preparing carboxylic acids from primary alcohols, but not aldehydes.

❓ What is the role of copper in this reaction? Is it consumed?

Copper acts as a catalyst in this reaction, which means it speeds up the reaction without being consumed itself. The copper surface facilitates the dehydrogenation process by providing a site where ethanol molecules can adsorb, lose hydrogen atoms, and then desorb as ethanal. After the reaction, the copper catalyst remains unchanged and can be used repeatedly. This is why the method is economically viable for industrial production—the copper doesn’t need to be replaced after each reaction cycle.

❓ Why is the temperature 573-578 K specifically mentioned?

The temperature range of 573-578 K (300-305°C) is crucial for this reaction because it provides the optimal conditions for catalytic dehydrogenation. At this temperature, the copper catalyst is sufficiently active to facilitate hydrogen removal from ethanol, and the ethanol vapors have enough kinetic energy to undergo the reaction. If the temperature is too low, the reaction rate will be very slow. If it’s too high, unwanted side reactions may occur, or the selectivity toward ethanal may decrease. This specific temperature range ensures maximum yield and selectivity for ethanal production.

❓ Can this method be used for converting other alcohols to aldehydes?

Yes! This copper-catalyzed dehydrogenation method can be used to convert any primary alcohol to its corresponding aldehyde. For example, methanol (CH₃OH) can be converted to formaldehyde (HCHO), and propanol (C₃H₇OH) can be converted to propanal (C₂H₅CHO). However, this method does not work for secondary alcohols (which form ketones) or tertiary alcohols (which resist oxidation). The selectivity and efficiency make it the preferred industrial method for producing aldehydes from primary alcohols.

❓ What happens to the hydrogen gas (H₂) produced in this reaction?

The hydrogen gas (H₂) produced during the dehydrogenation reaction is released as a gaseous byproduct. In industrial settings, this hydrogen gas is often collected and used for other chemical processes, making the reaction even more economically valuable. In laboratory settings, the hydrogen gas is typically allowed to escape or can be collected over water for experimental purposes. The production of H₂ gas is an important indicator that dehydrogenation (not oxidation) is occurring.

❓ Is PCC (Pyridinium Chlorochromate) better than copper catalyst?

While PCC is an excellent mild oxidizing agent that also selectively converts primary alcohols to aldehydes, the copper catalyst method is generally considered better for several reasons: (1) Copper is much cheaper and more readily available than PCC, (2) The copper method is suitable for large-scale industrial production, (3) PCC requires anhydrous conditions and organic solvents (like dichloromethane), making it more complex, (4) Copper catalyst can be reused indefinitely, while PCC is consumed in the reaction. However, PCC is preferred in research laboratories when working with sensitive or complex molecules where precise control is needed.

📚 Related Concepts You Should Know

🔬 Primary vs Secondary vs Tertiary Alcohols

Understanding the classification of alcohols is crucial. Primary alcohols (like ethanol) oxidize to aldehydes then carboxylic acids. Secondary alcohols oxidize to ketones. Tertiary alcohols resist oxidation under normal conditions.

⚗️ Oxidation vs Dehydrogenation

Oxidation involves adding oxygen or removing hydrogen. Dehydrogenation specifically removes hydrogen atoms. The copper catalyst method is technically dehydrogenation, which is a type of oxidation reaction.

🧪 Functional Group Interconversion

This reaction is an example of functional group interconversion—changing one functional group (alcohol) into another (aldehyde). This is a fundamental concept in organic synthesis and transformation reactions.

⚙️ Catalysis in Organic Chemistry

Copper acts as a heterogeneous catalyst (solid catalyst with gaseous reactants). Understanding catalysis mechanisms helps explain why certain reactions are selective and efficient.

🎯 Common Mistakes to Avoid in Exams

❌ Mistake #1: Forgetting to Mention Temperature

Wrong: “Pass ethanol vapors over heated copper.”
Correct: “Pass ethanol vapors over heated copper at 573-578 K.”

💡 The temperature is crucial and often carries marks. Always include it!

❌ Mistake #2: Not Mentioning the Byproduct

Wrong: C₂H₅OH → CH₃CHO
Correct: C₂H₅OH → CH₃CHO + H₂

💡 The hydrogen gas (H₂) is an important product. Don’t forget to include it in the equation!

❌ Mistake #3: Suggesting Strong Oxidizing Agents

Wrong: “Use K₂Cr₂O₇ to convert ethanol to ethanal.”
Correct: “K₂Cr₂O₇ will over-oxidize to acetic acid. Use Cu catalyst instead.”

💡 Understanding why certain methods don’t work is as important as knowing the correct method!

❌ Mistake #4: Incorrect Chemical Formulas

Wrong: C₂H₆O → C₂H₄O
Correct: C₂H₅OH → CH₃CHO (or C₂H₅OH → C₂H₄O)

💡 Use structural formulas (C₂H₅OH, CH₃CHO) to show functional groups clearly!

❌ Mistake #5: Not Explaining “Why”

Incomplete: “Copper catalyst is the best method.”
Complete: “Copper catalyst is best because it causes selective dehydrogenation, stopping at aldehyde without over-oxidation to carboxylic acid.”

💡 Always provide reasoning to demonstrate complete understanding and score full marks!

💼 Industrial Applications

The conversion of ethanol to ethanal using copper catalyst isn’t just a theoretical concept—it has significant real-world applications:

🏭 Industrial Production of Acetaldehyde

Ethanal (acetaldehyde) is an important industrial chemical used in the production of acetic acid, perfumes, flavors, plastics, and synthetic resins. The copper-catalyzed dehydrogenation of ethanol is one of the primary industrial methods for producing acetaldehyde on a large scale.

🌾 Bioethanol Conversion

With the growing emphasis on renewable resources, bioethanol (produced from fermentation of agricultural products) can be converted to valuable aldehydes using this copper catalyst method, creating a sustainable chemical production pathway.

♻️ Green Chemistry Advantage

This method aligns with green chemistry principles: it uses a reusable catalyst (copper), produces only hydrogen gas as a byproduct (which can be collected and used), requires no harsh chemicals or solvents, and operates at moderate temperatures. This makes it environmentally friendly compared to traditional oxidation methods.

📖 Summary & Quick Revision Points

🎯 One-Minute Revision

✅ BEST METHOD

Cu catalyst at 573-578 K

✅ EQUATION

C₂H₅OH → CH₃CHO + H₂

✅ MECHANISM

Catalytic dehydrogenation

✅ WHY BEST?

Selective, no over-oxidation

❌ DON’T USE

K₂Cr₂O₇, KMnO₄

❌ WHY NOT?

Over-oxidize to COOH

🏆 Exam Success Formula

Step 1: State the method → “Pass ethanol vapors over heated copper at 573-578 K”
Step 2: Write the equation → C₂H₅OH → CH₃CHO + H₂
Step 3: Explain why → “Selective dehydrogenation, no over-oxidation”
Step 4: Mention alternatives → “K₂Cr₂O₇ unsuitable (over-oxidizes to COOH)”

👨‍🏫 About the Author

👨‍🔬

Dr. Irfan Mansuri

Qualification: M.Sc., Ph.D. in Chemistry
Experience: 25+ years teaching Science, Physics & Chemistry
Specialization: Organic Chemistry, Physical Chemistry, NCERT Solutions

Dr. Mansuri has helped thousands of students excel in board examinations through his clear explanations and exam-focused teaching methodology. He specializes in breaking down complex chemistry concepts into easy-to-understand formats.

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⚠️ Important Exam Note

This question frequently appears in CBSE Class 12 Chemistry board exams and competitive examinations like JEE and NEET. Make sure you understand not just the method, but also why this method is preferred over others. Questions may ask you to compare different oxidizing agents or explain the mechanism—be prepared for all variations!

🔗 Related Topics You Should Study

Oxidation of primary, secondary, and tertiary alcohols
Preparation methods of aldehydes and ketones
Distinction between aldehydes and ketones
Chemical properties of alcohols
Catalytic reactions in organic chemistry
Functional group transformations
Industrial applications of aldehydes

📚 Study Smart, Score Better!
Remember: Understanding the “why” behind reactions is more important than memorizing them.
Good luck with your exams! 🎯

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