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

(a) acetaldehyde

Step 1: Understanding the Reaction Conditions

The problem states that ethyl alcohol (C_2H_5OH) vapors are passed over red-hot copper at 300°C. This setup typically involves a dehydrogenation reaction where an alcohol loses hydrogen atoms.

Step 2: Identify the Type of Reaction

Passing alcohol vapors over heated copper usually results in the oxidation of the alcohol to form an aldehyde. The temperatures and presence of copper facilitate the dehydrogenation process.

Step 3: Determine the Product Formed

The dehydrogenation of ethyl alcohol (C_2H_5OH) results in the formation of acetaldehyde (CH_3CHO). The reaction can be represented as:\[ C_2H_5OH ightarrow CH_3CHO + H_2 \]This shows that ethyl alcohol loses two hydrogen atoms and forms acetaldehyde.

Step 4: Evaluate Other Options

(a) Acetaldehyde: Correct, this compound is formed from the dehydrogenation of ethyl alcohol. (b) Acetic acid: This would require further oxidation beyond what is specified by the conditions. (c) Ethylene: This is not the product formed in this reaction due to the presence of copper acting as a catalyst for dehydrogenation, not dehydration. (d) None of these: Incorrect, as acetaldehyde is indeed formed.

Key Concepts

Dehydrogenation

Dehydrogenation is a chemical reaction that involves the removal of hydrogen atoms from a molecule. This process is essential in transforming alcohols into aldehydes or ketones. In our scenario, ethyl alcohol, also known as ethanol often represented as \(C_2H_5OH\), undergoes dehydrogenation. During this reaction, the hydrogen atoms are removed, ultimately transforming the ethanol into acetaldehyde. Ethanol loses two hydrogen atoms according to the reaction: \[ C_2H_5OH \rightarrow CH_3CHO + H_2 \] where the hydrogen atoms, represented as \(H_2\), are released as a gas. The absence of these hydrogen atoms converts the alcohol into an aldehyde group, which is what forms acetaldehyde. This type of reaction is crucial in organic chemistry for producing specific carbonyl compounds from alcohol precursors.

Acetaldehyde Formation

Acetaldehyde, a simple aldehyde represented as \(CH_3CHO\), is the primary product of the dehydrogenation of ethanol. Its formation is particularly notable because it involves the breaking and formation of bonds facilitated by the loss of hydrogen. Understanding how acetaldehyde is formed helps in grasping the broader concept of alcohol oxidation reactions.

• Initially, the ethanol molecule interacts with the copper catalyst.
• The interaction facilitates the removal of two hydrogen atoms from ethanol.
• This conversion at around 300°C results in the formation of acetaldehyde and molecular hydrogen (\(H_2\)).

Acetaldehyde formation is part of many industrial applications and laboratory procedures where aldehydes are desired products. In organic chemistry, this type of reaction is classified under oxidation reactions, where an alcohol is oxidized to form a corresponding aldehyde or ketone, depending on the type of alcohol. The process is efficient and can be easily controlled under laboratory conditions.

Copper Catalyst

Copper acts as a catalyst in the conversion of ethanol to acetaldehyde. A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Copper, specifically in its heated form at 300°C, effectively facilitates the dehydrogenation of ethanol. The role of the copper catalyst is:

• To accelerate the chemical reaction and provide a surface for the reaction to occur.
• To lower the energy barrier of the reaction, allowing ethanol to lose hydrogen more efficiently.
• Copper's surface aids the breaking of chemical bonds, promoting the formation of acetaldehyde from ethanol.

In general, copper’s ability to aid in alcohol oxidation reactions makes it a valuable catalyst in the chemical industry. Using copper minimizes the need for more extreme reaction conditions that could lead to undesired byproducts. Instead, copper ensures that only the desired oxidation to acetaldehyde occurs, providing a clean and efficient transformation.