Question

Which of the following can be used to prepare pure cis (Z)-2-butene from 2-butyne? (1) Nickel boridc (p-2) catalyst (2) Lindlar's catalyst (3) IIydroboration followed by protolysis (4) All of the above

Short answer

All of the above

Step-by-step solution

- Understanding the Question

The problem asks for a method to prepare pure cis (Z)-2-butene from 2-butyne. Consider the possible catalysts and reactions given in the choices.

- Assess Nickel Boride (p-2) Catalyst

Nickel boride (p-2) catalyst is known to selectively hydrogenate alkynes to cis-alkenes. Therefore, it can be used to prepare cis (Z)-2-butene from 2-butyne.

- Assess Lindlar's Catalyst

Lindlar's catalyst is a well-known catalyst for the partial hydrogenation of alkynes to cis-alkenes. It is specifically used to selectively produce cis (Z)-olefins, such as cis-2-butene from 2-butyne.

- Assess Hydroboration Followed by Protolysis

In hydroboration followed by protolysis, an alkyne is first converted to an alkene by addition of borane, followed by protonation. This method also tends to produce cis-alkenes.

- Evaluate All Options

Since all the listed methods can selectively hydrogenate or convert an alkyne (2-butyne) to a cis-alkene (cis-2-butene), all the options are correct.

Key concepts

alkyne hydrogenation

Alkyne hydrogenation is a chemical reaction in which an alkyne is converted into an alkene or alkane by the addition of hydrogen molecules. This reaction is typically catalyzed by metals such as palladium, platinum, or nickel.
For selective conversion of an alkyne to a cis-alkene, specific catalysts are used to control the reaction. The goal is to add hydrogen atoms to the triple bond without further hydrogenation to an alkane. This selective hydrogenation is crucial when preparing cis (Z)-2-butene from 2-butyne.
In this context, the use of suitable catalysts ensures that the conversion stops at the alkenic stage, producing the desired cis configuration. This method is widely employed in organic synthesis to achieve precise product formation.

Nickel boride catalyst

The Nickel boride (p-2) catalyst is a well-known catalyst used for the selective hydrogenation of alkynes to cis-alkenes. It is highly effective because it allows the partial addition of hydrogen to the triple bond, resulting in a cis-alkene rather than a fully saturated alkane.
This selectivity is due to the catalyst’s ability to adsorb hydrogen on its surface and facilitate the addition to the alkyne. In the case of preparing cis (Z)-2-butene from 2-butyne, the Nickel boride (p-2) catalyst performs this conversion efficiently with high yield and selectivity towards the cis form.
Hence, this catalyst is a preferred choice in organic chemistry labs when there is a need to control the hydrogenation process to obtain the specific cis product without further hydrogenation to the trans or alkane forms.

Lindlar's catalyst

Lindlar's catalyst is another popular catalyst specifically used for the partial hydrogenation of alkynes to cis-alkenes. It typically consists of palladium deposited on calcium carbonate and poisoned with lead acetate and quinoline to reduce its activity.
This poisoning is essential as it prevents the catalyst from fully hydrogenating the alkyne to an alkane, thereby stopping the reaction at the cis-alkene stage. By selectively introducing hydrogen molecules, Lindlar's catalyst facilitates the conversion of 2-butyne to cis (Z)-2-butene without over-hydrogenation.
This precise control is valuable in synthetic organic chemistry where the cis configuration is needed. Lindlar’s catalyst is thus widely used for applications requiring stereospecificity in hydrocarbon transformations.

hydroboration-protolysis

Hydroboration-protolysis is a two-step process used to convert alkynes to cis-alkenes. In the first step, hydroboration, a borane (BH₃) molecule adds across the triple bond of the alkyne, leading to the formation of an alkene with the boron atom attached.
This addition typically occurs in a syn manner, meaning both hydrogen and boron add to the same side of the alkyne, resulting in a cis configuration. In the second step, protolysis, the boron atom is replaced by a hydrogen atom through protonation, giving the final cis-alkene product.
This method is particularly useful as it tends to produce high yields of cis-alkenes. When applied to 2-butyne, hydroboration followed by protolysis produces cis (Z)-2-butene selectively and efficiently.