Question

Statement - 1: When alkyne react with \(\mathrm{H}_{2} / \mathrm{Pd} / \mathrm{CaCO}_{3}-\mathrm{PbO}_{2}\) it will form alkene as a product. Statement - 2: \(\mathrm{CaCO}_{3}\) or \(\mathrm{BaSO}_{4}\) will act as a poison for \(\mathrm{Pd}\). (A) Both statements are correct and statement- 2 is correct explanation of statement-1. (B) Both statements are correct and statement- 2 is not correct explanation of statement-1. (C) Statement- 1 is correct but statement- 2 is a wrong statement. (D) Statement- 1 is wrong but statement- 2 is a correct statement.

Short answer

The correct answer is (A) Both statements are correct and statement- 2 is the correct explanation of statement-1.

Step-by-step solution

Understand Statement 1

Statement 1 states that when alkyne reacts with H2 / Pd / CaCO3-PbO2, alkene is formed as a product. In other words, this reaction is describing the hydrogenation of an alkyne, a process that results in the conversion of an alkyne to an alkene.

Assess the correctness of Statement 1

In the presence of a palladium catalyst (Pd), alkyne undergoes partial hydrogenation, forming an alkene as a product. The partial hydrogenation of alkynes using a poisoned Pd catalyst, such as Pd/CaCO3-PbO2, is known as the Lindlar catalyst, which helps to prevent further hydrogenation of the alkene to an alkane. As a result, Statement 1 is correct.

Understand Statement 2

Statement 2 states that CaCO3 or BaSO4 act as poisons for Pd. Poisoning a catalyst means that it reduces the catalyst's activity or selectivity towards the desired reaction.

Assess the correctness of Statement 2

In the case of hydrogenation reactions, additives like CaCO3 (in the Lindlar catalyst) or BaSO4 help to control the degree of hydrogenation. These additives serve to "poison" the Pd catalyst, making it less efficient in hydrogenating the alkene product, preventing over-hydrogenation to form alkanes. Hence, Statement 2 is correct.

Determine if Statement 2 is an explanation of Statement 1

Since Statement 2 provides a reason for why the alkene is formed instead of further hydrogenation to alkane, it can be considered to be an explanation for Statement 1. Now that we have analyzed both statements and their relationship, we can choose the correct answer: (A) Both statements are correct and statement- 2 is correct explanation of statement-1.

Key concepts

Lindlar Catalyst

The Lindlar catalyst is an invaluable tool in the realm of organic chemistry, particularly in the hydrogenation of alkynes to alkenes. This specialized catalyst consists of finely powdered palladium deposited on calcium carbonate and then modified with various additives, like lead acetate. The lead serves to 'poison' the catalyst's surface in a beneficial way, which is critical to its function.

In essence, the Lindlar catalyst enables a controlled hydrogenation reaction. Without such control, an alkyne might be fully hydrogenated to an alkane, the next step in hydrogenation, due to the typical robustness of catalysts like palladium. However, when 'poisoned,' the Lindlar catalyst becomes selective for the alkene formation, stopping the process at the desired stage and conserving the double bond that distinguishes alkenes from alkanes.

In practical use, chemists treat an alkyne with hydrogen gas in the presence of the Lindlar catalyst to obtain the corresponding cis-alkene. This degree of specificity allows for the introduction of double bonds at targeted positions in complex molecules without overreducing them into single bonds.

Poisoned Catalyst

The concept of a 'poisoned' catalyst might seem counterintuitive, as we typically think of catalysts as materials designed to enhance a reaction. However, in catalysis, 'poisoning' refers to the deliberate addition of a substance that partially deactivates the catalyst. This strategy is used to decrease the activity of a catalyst and make a reaction more selective.

The mechanism behind a poisoned catalyst involves the adsorption of poisoning substances onto the surface of a catalyst, leading to fewer active sites remaining for the reaction. In the case of alkyne hydrogenation with a Lindlar catalyst, compounds such as lead acetate are used to 'poison' the palladium. This deliberate decrease in catalyst efficiency is crucial when we want to prevent the complete hydrogenation of an alkyne to an alkane and instead aim for the selective formation of an alkene.

The poisons used in these catalysts are generally substances that strongly adsorb onto the catalyst surface but do not react with the reactants or products. Consequently, these 'poisoned' catalysts are immensely useful for producing specific products in various chemical syntheses.

Alkene Formation

Alkenes are hydrocarbons containing at least one carbon-carbon double bond, and their formation is a fundamental reaction in organic synthesis. Alkenes are versatile molecules that can be transformed into a wide range of products, from polymers to pharmaceuticals. One of the most common methods of alkene formation is through the partial hydrogenation of alkynes.

Hydrogenation is a reaction that adds hydrogen (H2) across the carbon-carbon multiple bonds. Without a suitable catalyst, an alkyne would typically be fully hydrogenated to an alkane. However, when we use a palladium catalyst that has been 'poisoned,' such as the Lindlar catalyst, the reaction stops once the alkene has been formed.

It is important to know that the stereochemistry of the resulting alkene is often cis due to the nature of the catalyst. This control over the degree of hydrogenation and the stereochemical outcome is crucial when designing pathways to synthesize complex organic molecules. As such, the selection of the appropriate catalyst and reaction conditions are key considerations for any chemist looking to efficiently create alkenes from alkynes.