The reduction of an alkyne to alkene using Lindlar’s catalyst result in

cis addition of hydrogen atoms

The correct answer is option 1. cis addition of hydrogen atoms.

Lindlar's catalyst is a palladium catalyst poisoned with quinoline or lead acetate, used for the partial hydrogenation of alkynes to alkenes under mild conditions. The product of this reaction typically results in **cis addition of hydrogen atoms** to the triple bond. Therefore, the correct answer is: (1) cis addition of hydrogen atoms

Lindlar's Catalyst and Alkyne Reduction:

Selective Reduction:

Lindlar's catalyst selectively reduces alkynes to alkenes, stopping at the cis-alkene stage. The poison (quinoline or lead acetate) on the palladium catalyst prevents it from fully hydrogenating the alkyne to an alkane.

Cis Addition:

The mechanism involving Lindlar's catalyst results in cis addition of hydrogen atoms across the triple bond of the alkyne. This means that both hydrogen atoms add to the same side of the double bond formed after reduction, resulting in a cis-alkene.

Why the Other Options are Not Correct:

Option (2):Trans addition of hydrogen atoms would be characteristic of a different catalyst or reaction conditions, not Lindlar's catalyst.

Option (3): Lindlar's catalyst does not produce a mixture of cis and trans alkenes in equilibrium. It specifically favors the formation of cis alkenes due to the nature of the catalyst and reaction conditions.

Option (4): Similarly, Lindlar's catalyst does not produce a mixture of cis and trans alkenes that are not in equilibrium with each other. It selectively produces cis alkenes due to the catalyst's poisoning and reaction mechanism.

Thus, Lindlar's catalyst is known for its ability to selectively hydrogenate alkynes to cis alkenes, making option (1) the correct answer: cis addition of hydrogen atoms.