Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions does not hold true when carried out in the presence of heated copper at \(300^{\circ} \mathrm{C} ?\) (a) phenol \(\longrightarrow\) benzyl alcohol (b) primary alcohol \(\longrightarrow\) aldehyde (c) secondary alcohol \(\longrightarrow\) ketone (d) tertiary alcohol \(\longrightarrow\) olefin

Step-by-Step Solution

Answer

Option (a) phenol  benzyl alcohol is incorrect.

Step 1: Understand the Role of Copper and Temperature

When reactions are carried out in the presence of heated copper at \(300^{\circ} \mathrm{C}\), copper acts as a catalyst for dehydrogenation reactions. Such reactions typically involve the removal of hydrogen atoms from the alcohols to form aldehydes, ketones, or olefins, depending on the type of alcohol.

Step 2: Analyze Each Option

Let's consider each option: - Option (a) suggests phenol to benzyl alcohol conversion. Phenol does not convert to benzyl alcohol as it is typically oxidized or remains unchanged. - Option (b) involves a primary alcohol to aldehyde conversion. This is a standard dehydrogenation pathway in the presence of copper. - Option (c) describes a secondary alcohol converting to a ketone. This also aligns with typical dehydrogenation reactions. - Option (d) involves tertiary alcohols forming olefins. This is characteristic of dehydration reactions often facilitated by copper.

Step 3: Evaluate Option (a)

Phenol, being an aromatic alcohol, does not undergo typical dehydrogenation to form benzyl alcohol. Instead, under these conditions, it would either remain unchanged or potentially undergo different oxidation reactions. Since copper at high temperatures does not facilitate phenol to benzyl alcohol conversion, Option (a) is incorrect.

Step 4: Conclusion

Based on the evaluation, we find that all options except (a) are consistent with expected chemical reactions involving heated copper as a catalyst. Therefore, Option (a) does not hold true under the given conditions.

Key Concepts

Dehydrogenation

Dehydrogenation is an important chemical reaction where hydrogen atoms are removed from a compound. This process is often key in converting alcohols into their corresponding aldehydes, ketones, or olefins. Dehydrogenation is facilitated by the presence of a catalyst, typically metals like copper, which can efficiently conduct this reaction at high temperatures. For example:

• Primary alcohols through dehydrogenation can yield aldehydes.
• Secondary alcohols can transform into ketones.
• Tertiary alcohols may lose water (dehydration) to become olefins.

The key to these reactions is the removal of hydrogen, which changes the chemical structure significantly, allowing for the formation of different functional groups.

Copper Catalyst

The role of copper as a catalyst is crucial in facilitating dehydrogenation reactions, particularly at elevated temperatures such as 300°C. As a catalyst, copper doesn't alter the equilibrium of the reaction but significantly increases the rate without being consumed by it. Copper's effectiveness in catalysis is due to:

• Its excellent heat conductivity, which helps maintain the necessary high temperature for reactions.
• Its ability to lower the energy barrier for hydrogen removal from alcohols.

This is why in reactions involving alcohols at 300°C, copper can efficiently assist in transforming alcohols into aldehydes, ketones, and alkenes, contingent upon the type of alcohol involved.

Phenol Reaction

Phenol is a unique compound due to its aromatic structure, which largely influences its chemical reactions. Unlike other alcohols, phenol does not typically undergo dehydrogenation to form benzyl alcohol. Under the presence of heated copper, phenol might remain unchanged or undergo different types of reactions such as:

• Becoming oxidized through other pathways, perhaps converting into quinones or remaining as phenol.
• Participating in reactions that involve the aromatic ring rather than the hydroxyl group.

Hence, phenol's behavior under such conditions varies significantly compared to aliphatic alcohols, making their dehydrogenation into benzyl alcohol improbable.

Alcohol Oxidation

Alcohol oxidation is a widely used chemical reaction which involves the transformation of an alcohol into a carbonyl group containing compound. This transformation is highly dependent on the type of alcohol:

• Primary Alcohols: Oxidized to aldehydes, and in some cases further to carboxylic acids.
• Secondary Alcohols: Convert to ketones as they can only be partially oxidized.
• Tertiary Alcohols: Often do not oxidize directly because there is no hydrogen atom attached to the carbon with the hydroxyl group, leading instead to dehydration reactions.

Using copper at high temperatures plays a crucial role, particularly in ensuring that the reaction proceeds effectively via dehydrogenation pathways. However, aromatic alcohols like phenol behave differently, showcasing a variance from typical alcohol oxidation pathways.