Chapter 25, Problem 25.59P

Interpretation Introduction

Interpretation:

As we saw in this chapter, dissolving metal reductions reduce alkynes to trans alkenes. Why dissolving metal reductions fail with a terminal alkyne is to be explained. Hint: What species appear in the mechanism.

Concept introduction:

In a dissolving metal reduction, solvated electrons behave like the free radical species. When alkyne is treated with sodium metal dissolved in liquid ammonia ($\text{-78 °C}$), the triple bond is reduced to a double bond. But fail with a terminal alkyne. The reason is that a very strong basic ${\text{R}}^{\text{-}}$ species appears in the mechanism, which will irreversibly deprotonate a terminal alkyne. Reduction of such a deprotonated terminal alkyne would not be reasonable because it would proceed through an intermediate with adjacent negatively charged $\text{C}$ atoms.