Chapter 19, Problem 19.26P

Interpretation Introduction

Interpretation:

It is to be explained why the double bonds in the starting material are unaffected by hydrogenation in the given reaction.

Concept introduction:

The reduction of an alkyne to the corresponding alkene via catalytic hydrogenation leads to form the cis isomer over the trans isomer. For this transformation, a Lindlar catalyst is commonly used. Palladium adsorbed on calcium carbonate (${\text{CaCO}}_{\text{3}}$) or barium sulfate (${\text{BaSO}}_{\text{4}}$) and treated with a small amount of quinoline and a lead salt $\text{Pb}{\left(\text{OAc}\right)}_{\text{2}}$ is called the Lindlar catalyst. In a benzene ring, double bonds are a part of an aromatic π system, which is exceptionally stable under normal conditions. A poisoned catalyst is used to reduce an alkyne to an alkene. The catalytic activity of the metal is decreased by treating it with calcium carbonate (${\text{CaCO}}_{\text{3}}$) or barium sulfate (${\text{BaSO}}_{\text{4}}$), simply called a poisoned catalyst.