Q2 / 150 · MHT CET 2025 (26 Apr Shift 1) · Chemistry

Which of the following reagents is used to convert $C \equiv C$ triple bond to $\mathrm{C}=\mathrm{C}$ double bond to give Cis isomer of alkene?

  • A
    $\mathrm{ZnCl}_2 / \mathrm{HCl}$
  • Correct answer
    B
    Pd-C/quinoline
  • C
    $\mathrm{Na} /$ liquid $\mathrm{NH}_3$
  • D
    $\mathrm{Na} / \mathrm{Hg}$ in $\mathrm{H}_2 \mathrm{O}$

Solution

The selective conversion of a carbon-carbon triple bond ($C \equiv C$) to a cis alkene requires partial hydrogenation via syn addition.

Lindlar's catalyst (Pd-C/quinoline) is specifically designed for this transformation, as the poisoned palladium surface prevents over-reduction while promoting syn addition to yield the cis isomer.

Alternative reagents are unsuitable: ZnCl$_2$/HCl distinguishes alcohols; Na/liquid NH$_3$ produces trans alkenes through anti addition; Na/Hg in H$_2$O reduces carbonyls rather than alkynes selectively.

The correct reagent is $\boxed{B}$.

Hence, the correct answer is option: 'B'.

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