JEE Main 2021 · 16 March · morning · Single correct · +4 / −1
Which of the following is Lindlar catalyst?
- (a) Partially deactivated palladised charcoal
- (b) Zinc chloride and HCl
- (c) Sodium and Liquid NH₃
- (d) Cold dilute solution of KMnO₄
Answer and solution
Answer (a) Partially deactivated palladised charcoal
Solution
Chemistry ma’am · 4 steps
Lindlar is palladium made deliberately lazy, so hydrogen stops at the alkene and goes no further.
- What it is. Lindlar catalyst is palladium on charcoal (or on BaSO₄/CaCO₃) that has been partially poisoned with lead acetate or quinoline. The poison lowers its activity on purpose.
- Why deactivate it. Ordinary Pd/C with H₂ takes an alkyne all the way to the alkane. The weakened catalyst adds only one mole of H₂, so the reaction stops at the alkene.
R-C≡C-R + H₂ →(Pd/C, quinoline) cis-RCH=CHR
- Rule out the rest. Sodium in liquid NH₃ is the Birch-type reduction that gives the trans alkene. Cold dilute KMnO₄ is Baeyer's reagent for detecting unsaturation. ZnCl₂ with HCl is the Lucas reagent for alcohols.
- So. Lindlar catalyst is partially deactivated palladised charcoal, option (a). Both hydrogens add from the same face, so the product is the cis alkene.
The idea to keep. Alkyne to alkene: Lindlar (H₂, poisoned Pd) gives cis, Na in liquid NH₃ gives trans. Remember the pair together and you will not mix them up.
Answer from the NTA final answer key. Solution by PrepWiser.