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Question

The reduction of 4-octyne with H2 in the presence of Pd/BaSO4 and quinoline gives:

A
trans-4-octene
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B
cis-4-octene
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C
a mixture of cis and trans-4-octene
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D
a completely reduced product C8H18
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Solution

The correct option is B cis-4-octene
The complete reduction of alkynes leads to alkane so while adding H2 to alkyne, if the reduction is to be stopped at the alkene stage, the catalyst needs to be poisoned.
A palladium catalyst (Pd/BaSO4) is deliberately poisoned with lead/sulphur compounds/Quinoline.
This reagent is known as Lindlar's catalyst. They give only cis product.
Reduction of 4-octyne with H2 and Pd/BaSO4 with quinoline gives cis-4-octene. The reaction is shown below.


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