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 \(\Rightarrow\) olefin

Step-by-Step Solution

Answer

The reaction phenol to benzyl alcohol does not hold true.

Step 1: Understanding the Reactions

We need to examine each reaction to determine its validity in the presence of heated copper at \(300^{\circ} \mathrm{C}\). Each option represents a potential chemical transformation that could occur under these conditions.

Step 2: Evaluate Phenol to Benzyl Alcohol Reaction (Option a)

Phenol cannot be converted to benzyl alcohol under the given conditions. The copper at \(300^{\circ} \mathrm{C}\) does not facilitate this transformation because phenol lacks the necessary chemical structure to form benzyl alcohol directly. This reaction is not standard and therefore not feasible.

Step 3: Evaluate Primary Alcohol to Aldehyde Reaction (Option b)

The conversion of a primary alcohol to an aldehyde is a well-known oxidation reaction and can occur in the presence of heated copper. Hence, this reaction holds true under the given conditions.

Step 4: Evaluate Secondary Alcohol to Ketone Reaction (Option c)

This reaction involves the oxidation of a secondary alcohol to form a ketone, which is feasible and commonly catalyzed by copper at high temperatures. Hence, this reaction is legitimate under the stated conditions.

Step 5: Evaluate Tertiary Alcohol to Olefin Reaction (Option d)

Tertiary alcohols can dehydrate to form olefins in the presence of heat and a catalyst like copper. This elimination reaction aligns with the conditions provided and is feasible.

Step 6: Conclusion of the Analysis

Upon evaluating all options, the reaction phenol \(\longrightarrow\) benzyl alcohol (option a) does not hold true as it is not feasible under the given conditions. It involves a transformation that copper at \(300^{\circ} \mathrm{C}\) does not facilitate.

Key Concepts

Chemical Transformations

Chemical transformations refer to the processes where substances experience changes in their chemical structure, resulting in the formation of new products. In these transformations, the breaking and forming of chemical bonds occur, converting one or more reactants into different substances. This concept can encompass a variety of reactions, such as oxidation, reduction, dehydration, and more.

• Oxidation refers to the increase in oxidation state of a molecule, atom, or ion, with the most common involving the loss of electrons.


• Dehydration involves the removal of water (H₂O) from a compound, usually resulting in the formation of double bonds or new cyclic structures.

In the context of the original exercise, the chemical transformations being analyzed involve the conversion of alcohols under various conditions facilitated by heated copper. Each alcohol type—primary, secondary, and tertiary—undergoes specific reactions, either by oxidizing or through dehydration. Understanding these transformations provides key insights into the nature of chemical reactions in organic chemistry.

Oxidation Reactions

Oxidation reactions involve the loss of hydrogen or the gain of oxygen, and they play a significant role in the transformation of alcohols.

• Primary Alcohols: When oxidized, primary alcohols transform into aldehydes. In the exercise, it is mentioned that this transformation can occur in the presence of heated copper.


• Secondary Alcohols: These are oxidized to form ketones, another reaction efficiently catalyzed by copper at high temperatures.

The efficiency of copper as a catalyst in these reactions is due to its ability to remove hydrogen effectively, thereby facilitating the oxidation process. This transformation is integral in both laboratory and industrial settings to produce various aldehydes and ketones from alcohol precursors. In summary, oxidation reactions are crucial for changing the oxidation states of organic molecules, particularly alcohols, allowing them to be converted into new compounds used in diverse chemical applications.

Dehydration Reactions

Dehydration reactions are chemical processes where water is removed from a molecule. This type of reaction is particularly relevant when dealing with alcohols that can undergo dehydration, especially under hot conditions with the presence of a catalyst like copper.

• Tertiary Alcohols: These undergo dehydration reactions to form olefins. This happens because the structure of tertiary alcohols allows them to easily lose a water molecule, forming a carbon-carbon double bond in the process. This is known as an elimination reaction.

This reaction is responsible for transforming tertiary alcohols into alkenes or olefins, which are significant as starting materials in the synthesis of many plastics and chemicals. Dehydration of alcohols exemplifies how conditions such as heat and catalysts—like copper at high temperatures—can drive specific transformations, offering insights into understanding both the mechanisms and practical applications of these chemical processes.