The alkyne shown is treated with sodium metal in liquid ammonia. Which structure is the major product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The compound is but-2-yne. Na/liquid

NH X_{3}

performs anti addition of hydrogen through a radical anion and a trans-vinyl radical, giving (E)-but-2-ene. The Z isomer comes from Lindlar hydrogenation and butane from excess

H X_{2}

/Pd.

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