Addition of H_2 to 2-butyne in presence of Lindlar's catalyst . Gives cis-2-butene while with Na/in liq. NH_3 gives trans-2-butene.Why so ? Explain.

Addition of H_2 to 2-butyne in presence of Lindlar's catalyst . Gives

1.	Addition of H_2 to 2-butyne in presence of Lindlar's catalyst . Gives cis-2-butene while with Na/in liq. NH_3 gives trans-2-butene.Why so ? Explain.
Answer» Solution :In presence of Lindlar's catalyst (Pd deposited over `BaSO_4` and partially POISONED by addition of S or quinoline), `H_2` gets first absorbed over the catalyst as H atoms and then transferred to the same face of 2-butyne giving cis-2-butene. HOWEVER, in CASE of Na in liquid `NH_3`, (Birch REDUCTION), the reduction OCCURS through two electron transfers and two proton transfer giving trans-2-butene .