Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

In the preparation of alkene from alcohol using \(\mathrm{Al}_{2} \mathrm{O}_{3}\), which is the most effective factor? (a) porosity of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (b) temperature (c) surface area of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (d) concentration

Step-by-Step Solution

Answer

The most effective factor is the surface area of \( \mathrm{Al}_2\mathrm{O}_3 \).

Step 1: Identify the reaction process

The preparation of alkenes from alcohols typically involves a dehydration reaction, where water is removed from the alcohol. In this context, \( \mathrm{Al}_2\mathrm{O}_3 \) (alumina) is often used as a catalyst to facilitate the reaction.

Step 2: Consider catalyst effectiveness

The effectiveness of a catalyst in a reaction is significantly influenced by its surface properties and the conditions under which it operates. Understand that the role of \( \mathrm{Al}_2\mathrm{O}_3 \) is to increase the reaction rate by providing a surface for the reaction to occur.

Step 3: Analyze the given factors

Review the provided factors: porosity of \( \mathrm{Al}_2\mathrm{O}_3 \), temperature, surface area of \( \mathrm{Al}_2\mathrm{O}_3 \), and concentration. Determine how each might affect the reaction. Porosity and surface area are related to the catalyst's ability to facilitate more reactions, while temperature is vital for providing the necessary energy to reach the reaction activation barrier.

Step 4: Choose the most effective factor

Although temperature is crucial in ensuring reactions occur, the surface area of the catalyst is often the most effective factor because increased surface area of \( \mathrm{Al}_2\mathrm{O}_3 \) enhances the catalyst's ability to expose more reactive sites, accelerating the dehydration of alcohols to alkenes.

Key Concepts

Catalyst Surface Area

In chemical reactions, the surface area of a catalyst like alumina ({Al}_2{O}_3) plays a crucial role. This is because the surface area determines how many molecules can interact with the catalyst at any given time. - The larger the surface area, the more molecules can be accommodated, facilitating more interactions. - This increase in interactions typically leads to a faster reaction rate. In the context of alkene preparation from alcohols, a larger surface area means that more alcohol molecules can dehydrate at once. Alumina's ability to act as an effective catalyst is significantly enhanced by maximizing these surface interactions, making the process more efficient.

Dehydration Reaction

A dehydration reaction involves the removal of a water molecule from the starting material. In the case of converting alcohol to alkene, this process is critical. - The dehydration of alcohols often requires an acidic catalyst to proceed efficiently. - During this reaction, the hydroxyl group of the alcohol is lost, together with a hydrogen atom from an adjacent carbon atom, forming the double bond characteristic of alkenes. This conversion is not only fundamental in organic chemistry but also widely applied in the industrial synthesis of alkenes. The success of this reaction often depends on efficiently managing various factors such as catalyst type, particularly when aiming for high yield.

Alumina Catalyst

Alumina, or {Al}_2{O}_3, is frequently used in organic chemistry as a catalyst due to its beneficial properties. - It provides a robust surface for reactions to occur, enhancing the overall reaction speed. - Alumina is both chemically inert and thermally stable, making it suitable for high-temperature reactions. In the preparation of alkenes via dehydration, alumina serves as a solid acid, promoting the essential steps that lead to water elimination from the alcohols. The universality and efficiency of alumina make it a popular choice in laboratory and industrial settings alike.

Reaction Temperature

Temperature is a critical factor in chemical reactions, including the dehydration of alcohols to form alkenes. - Higher temperatures generally increase the kinetic energy of the molecules involved, which can lead to more effective collisions. - This added energy helps to overcome activation energy barriers, making it simpler for reactions to proceed. Despite the importance of temperature, it must be carefully controlled. Excessively high temperatures may lead to unwanted by-products, whereas too low temperatures might not provide the necessary energy for the dehydration to occur efficiently. Thus, maintaining an optimal temperature is crucial for favorable reaction outcomes.