Question

Lindlar catalyst is (A) \(\mathrm{Pd}-\mathrm{BaSO}_{4}\) (B) \(\mathrm{Ni}-\mathrm{BaSO}_{4}\) (C) Li (D) \(\mathrm{LiNH}_{2}\)

Short answer

The Lindlar catalyst is (A) \(\mathrm{Pd}-\mathrm{BaSO}_{4}\), which consists of palladium metal supported on an inert carrier like barium sulfate.

Step-by-step solution

Identify the components of the Lindlar catalyst

A Lindlar catalyst is a heterogeneous catalyst used to selectively hydrogenate alkynes to cis-alkenes. It consists of palladium metal supported on an inert carrier, typically barium sulfate, and is poisoned with additives like lead or quinoline to control the catalyst's reactivity and selectivity.

Find the matching option

Now that we know the composition of the Lindlar catalyst, we can examine the given options: (A) \(\mathrm{Pd}-\mathrm{BaSO}_{4}\) (B) \(\mathrm{Ni}-\mathrm{BaSO}_{4}\) (C) Li (D) \(\mathrm{LiNH}_{2}\) Option (A) consists of palladium (\(\mathrm{Pd}\)) and barium sulfate (\(\mathrm{BaSO}_{4}\)), which matches the composition we determined for the Lindlar catalyst in Step 1. The other options do not match the correct composition.

Confirm the answer

Since option (A) matches the components of the Lindlar catalyst, it is the correct answer. Therefore, the Lindlar catalyst is (A) \(\mathrm{Pd}-\mathrm{BaSO}_{4}\).

Key concepts

Hydrogenation of Alkynes

Hydrogenation is a chemical reaction between hydrogen (H2) and another compound or element, typically in the presence of a catalyst. In the context of alkynes, hydrogenation is the process of adding hydrogen atoms to the carbon-carbon triple bond, transforming the molecule into an alkane if the reaction is carried out to completion.

However, the unique aspect of using a controlled environment with a Lindlar catalyst is that this highly exothermic reaction can be stopped midway, leading to the formation of an alkene rather than an alkane. This selectivity is essential when the goal is to synthesize specific compounds in organic chemistry.

In the case of alkynes, the hydrogenation process is often done in stages because the triple bond is more reactive than a double bond. Therefore, a specialized catalyst is required to selectively hydrogenate the alkyne to a cis-alkene without further reduction to an alkane.

Cis-Alkenes Synthesis

The synthesis of cis-alkenes from alkynes is a critical process in the field of organic synthesis. The 'cis' configuration refers to the geometric orientation of the substituent groups in an alkene where both are on the same side of the double bond.

The Lindlar catalyst is especially designed for this purpose. It consists of palladium deposited on calcium carbonate and then poisoned with various compounds such as lead acetate or quinoline. This poisoned catalyst is less reactive than pure palladium, allowing for a partial hydrogenation that stops at the alkene stage. Moreover, it ensures that the alkenes formed retain the 'cis' configuration, which is often more challenging to achieve as opposed to the 'trans' configuration that can be formed if the hydrogenation is not appropriately controlled.

The ability to create 'cis' alkenes has large implications in medicinal chemistry and the development of various organic products where the shape and orientation of a molecule can vastly influence its properties and reactivity.

Heterogeneous Catalysis

Heterogeneous catalysis occurs when the catalyst is in a different phase than the reactants. This is the case with Lindlar catalyst in the hydrogenation of alkynes, where the solid catalyst facilitates the reaction between gas-phase hydrogen molecules and liquid or solid alkynes.

In heterogeneous catalysis, the reactants adsorb onto the surface of the solid catalyst. This interaction weakens bonds within the reactants, lowering the activation energy needed for the reaction and allowing the chemical process to proceed under milder conditions than would otherwise be necessary.

Lindlar catalyst is a prime example of a heterogeneous catalyst. It works by providing a surface for the alkyne and hydrogen to come together. The specificity of this catalyst, alongside its ability to be poisoned (a process that deliberately reduces its reactivity), highlights the complexity and precision that can be achieved with heterogeneous catalysis.

Since the catalyst itself does not undergo a permanent change, it can be used repeatedly, making it an efficient and sustainable option for industrial applications. This characteristic is crucial for the development of cost-effective and environmentally friendly manufacturing processes.