Question

In the reactions
$\left(B\right)\underset{H_2}{\overset{\text{Lindlar catalyst}}{\leftarrow }}R-C\equiv C-R\stackrel{Na/N{H}_3}{\to }\left(A\right)$
$\left(A\right)$ and $\left(B\right)$ are geometrical isomers. Then:

• A. $A\text{is cis and}B\text{is trans}$
• B. $A\text{is trans and}B\text{is cis}$
• C. $A\text{and}B\text{are cis}$
• D. $A\text{and}B\text{are trans}$

Answer 1

The correct option is B $A\text{is trans and}B\text{is cis}$

Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes. Hence, $A$ will be trans and $B$ will be cis.