Question

Which acts as poison for Pd-charcoal in Lindlar’s catalyst
A) $BaS{O_4}$
B) Quinoline
C) Both A and B
D) None of the above

Answer 1

Hint:
Lindlar’s catalyst is used for the selective reduction of alkynes into alkenes. It is a heterogeneous catalyst consisting of palladium deposited over the surface of barium sulphate with Sulphur or quinoline added to it as well.

Complete step by step solution:
Hydrogen gas over palladised carbon in the presence of barium sulphate can be used to reduce alkyne or alkenes into alkanes. But it is treated with Sulphur that reduces the efficiency of the catalyst and selective reduction is possible. As a result, catalytic reduction in the presence of Lindlar’s catalyst yields alkenes and not alkanes.
The substances that reduce the efficiency of a catalyst are known as poison. In this case quinoline acts as the poison for the catalyst palladium. The Sulphur poison reduces the absorption of hydrogen gas on the surface of palladium. Lead acetate or lead oxide can also be used as poison. Quinoline helps to avoid the formation of unwanted by- products thereby acts as a promoter for the reaction.

Thus, the correct answer is D.

Additional information:
The hydrogen gas is adsorbed on the surface of palladium. When the alkyne is passed over the catalyst, the pi- bond electron density on the alkyne is attracted by the vacant d- orbitals of the palladium atoms and the hydrogen attaches on the both sides of the double bond. The hydrogen atoms are attached from the same side and the addition is known as syn addition.

Note:
Lindlar’s catalyst is also used in Rosenmund’s reduction. It is the process of conversion of acid chlorides into aldehydes by catalytic reduction in the presence of palladised carbon and $BaS{O_4}$ in Sulphur or quinolone.