BLindral/H2←−−−−−−−R−C≡C−RNa/NH3−−−−−→ A and B are geometrical isomers (R - CH = CH - R):
A is trans, B is cis
Controlled hydrogenation of alkyne with lindlar catalyst follows syn addition giving rise to cis alkene; and with Na/NH3 follows anti addition giving rise to trans alkene.