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

Question

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

Step-by-Step Solution

Answer

The reaction phenol to benzyl alcohol does not occur with heated copper.

Step 1: Understand the Reaction Conditions

All the given reactions involve the dehydration or oxidation of alcohols using heated copper as the catalyst. Copper at high temperatures typically facilitates dehydrogenation or dehydration of alcohols depending on their classification. To answer the question, we will analyze which reactions are feasible under these conditions.

Step 2: Analyze Phenol to Benzyl Alcohol Conversion

Phenol does not undergo typical alcohol reactions since it lacks a hydrogen atom on the hydroxyl-bearing carbon. Therefore, phenol cannot be converted into benzyl alcohol using copper under heat, as this conversion requires a different set of reaction conditions or catalysts.

Step 3: Examine Primary Alcohol to Aldehyde Conversion

Primary alcohols can be oxidized to aldehydes using heated copper. Copper acts as a catalyst to remove hydrogen, effectively dehydrogenating primary alcohols to form aldehydes.

Step 4: Evaluate Secondary Alcohol to Ketone Conversion

Secondary alcohols can be oxidized to ketones in the presence of heated copper. The process involves the removal of hydrogen (dehydrogenation) and forms a ketone group.

Step 5: Assess Tertiary Alcohol to Olefin Formation

Tertiary alcohols dehydrate to form olefins (alkenes) when heated with copper, as the high temperature facilitates the elimination of a water molecule from the alcohol to form an alkene.

Step 6: Conclusion Step: Determine Which Reaction Does Not Proceed

Based on the above analysis, phenol to benzyl alcohol is not a feasible reaction under the specified conditions of heated copper at 300°C, as phenol does not have the same reactivity as other alcohols.

Key Concepts

Dehydrogenation

Dehydrogenation is a crucial reaction in organic chemistry, particularly in the oxidation of alcohols. It involves the removal of hydrogen atoms from a molecule. In the context of alcohols and heated copper, dehydrogenation is the principal mechanism by which primary and secondary alcohols are oxidized.

This process transforms primary alcohols into aldehydes and secondary alcohols into ketones. The heated copper acts as a catalyst, facilitating the removal of the hydrogen atoms from the alcohol group, allowing for this transformation.

• In primary alcohols, the hydrogen is removed from both the hydroxyl group and the adjacent carbon atom, forming an aldehyde.
• In secondary alcohols, the adjacent carbon is bonded to two other carbon atoms, and dehydrogenation results in the formation of a ketone.

Understanding dehydrogenation helps in grasping why certain alcohols oxidize under specific conditions.

Copper Catalyst

Copper is a commonly used catalyst in oxidation reactions, particularly in the oxidation of alcohols. When heated to high temperatures, such as 300°C, copper facilitates both dehydrogenation and dehydration reactions.

Copper's role as a catalyst is to lower the activation energy needed for the reaction, thereby accelerating the reaction rate. In these reactions, copper is not consumed but merely provides the necessary surface and energy to promote the transformations.

• For primary alcohols, copper helps in the dehydrogenation process, converting them to aldehydes.
• For secondary alcohols, it assists in forming ketones through dehydrogenation.
• In tertiary alcohols, copper helps in dehydration, facilitating the formation of olefins (alkenes).

Know that not all substances will react under these conditions, as seen with phenol, which does not convert to benzyl alcohol with copper.

Hydrated Conditions

Hydrated conditions generally refer to the presence of water in a reaction environment. In alcohol reactions involving copper at high temperatures, the presence or absence of water can significantly affect the reaction pathway.

For dehydrogenation reactions, like those of primary and secondary alcohols, the presence of hydrated conditions may have less influence as these reactions primarily involve the removal of hydrogen rather than the involvement of water.

However, in the dehydration reactions of tertiary alcohols to form alkenes, hydrated conditions must be carefully managed as too much water can oppose the dehydration process.

Understanding hydrated conditions helps in determining the feasibility and the direction of the reaction, as water can either be a product or a reactant depending on the conditions and the reaction type.

Dehydration of Alcohols

Dehydration of alcohols is a chemical reaction that involves the removal of a water molecule from the alcohol structure. This typically occurs in tertiary alcohols when heated with a catalyst such as copper.

In a dehydration reaction, the alcohol loses a hydroxyl group (OH) and a hydrogen atom, which combine to form water, leaving behind an olefin (alkene).

• Tertiary alcohols are more prone to dehydration to form alkenes, making it a favored reaction in certain industrial applications.
• The copper catalyst aids in lowering the energy threshold needed for the dehydration to occur, particularly under high temperatures like 300°C.

These dehydration reactions are distinct from dehydrogenation, which involves the removal of hydrogen atoms and not water. Understanding both processes is essential in predicting the outcome of reactions involving alcohols under specific conditions.