Question

The reduction of 3 -hexyne with II \(_{2}\) / Lindlar's catalyst gives predominantly (1) \(\mathrm{n}\) -hexane (2) trans-3-hexene (3) cis-3-hexene (4) a mixture of cis and trans 3 -hexene

Short answer

cis-3-hexene

Step-by-step solution

Understand the Reactants

Identify the reactants: 3-hexyne and II the catalyst, Lindlar's catalyst.

Know the Nature of the Catalyst

Lindlar's catalyst is used to hydrogenate alkynes (triple bonds) to cis-alkenes (double bonds). It selectively stops the reaction at the alkene stage.

Predict the Product

Since Lindlar's catalyst produces cis-alkenes, the hydrogenation of 3-hexyne will result in cis-3-hexene.

Select the Correct Answer

Based on the steps above, the predominant product will be cis-3-hexene.

Key concepts

3-hexyne

3-hexyne is an organic molecule characterized by a triple bond between the 3rd and 4th carbon atoms in a six-carbon chain. The general structure is CH₃−CH₂−C≡C−CH₂−CH₃. It belongs to the family of alkynes, where at least one carbon-carbon triple bond is present. This triple bond makes the molecule quite reactive. Alkynes are known for their electron-rich triple bonds, which can participate in various chemical reactions.

Lindlar's Catalyst

Lindlar's catalyst is a unique and important reagent in organic chemistry used for the partial hydrogenation of alkynes to alkenes. This catalyst is typically composed of palladium deposited on calcium carbonate and poisoned with lead acetate or quinoline. This poisoning prevents the complete hydrogenation of the alkyne to an alkane. Instead, it stops the reaction at the alkene stage, ensuring the formation of a double bond. This selectivity is crucial in organic synthesis for obtaining specific products, such as cis-alkenes.

Cis-alkene formation

The formation of cis-alkenes is an important aspect when using Lindlar's catalyst for alkyne reduction. The term 'cis' refers to the arrangement of substituent groups attached to the double-bonded carbons, where these groups are on the same side. In the context of reducing 3-hexyne, Lindlar's catalyst adds hydrogen atoms on the same side of the triple bond, converting it into a double bond. This forms a cis-alkene, specifically cis-3-hexene. The reactivity of the catalyst ensures that only the double bond forms, preventing further reduction to hexane.