Question

$\text{A and B }$ are geometrical isomers ( $R-CH=CH-R$ ).

• A. A is trans, B is cis
• B. A and B both are cis
• C. A and B both are trans
• D. A is cis, B is trans

Answer 1

Answer: (A)

Action of an alkali metal in liquid ammonia on a symmetrical alkyne gives the trans-alkene. It is an example of stereoselective reaction as the product is selective between the two possible stereoisomers (cis and trans).
Action of Lindlar's catalyst on a symmetrical alkyne gives the cis-alkene. It is an example of stereoselective reaction as the product is selective between the two possible stereoisomers (cis and trans).
Lindlar's catalyst is $Pd$ supported over $BaS{O}_4$ with catalytic poison sulphur or quinoline.
In the cis form, two same groups are present on the same side but in the trans form two same groups are present opposite sides of $C=C$ .