You will be familiar with the idea that many metals react with acid to liberate hydrogen, forming a salt at the same time. There is an example in the margin. The metal cation (Mg2+ in this example) results from the loss of electrons, and these electrons reduce 2 × H+ to give H2.
The same thing happens even in very weak acids (water, alcohols...even liquid ammonia) if the metal is very reactive (sodium or potassium, say). You can think of the process here in two steps: first sodium releases an electron, then the electron is captured by a proton from NH3 to give H, which forms H2. Sodium ethoxide (NaOEt) and sodium amide (NaNH2) are made by dissolving sodium in ethanol or liquid ammonia, respectively. But what if, instead of just reducing the solvent to liberate hydrogen, we harness the electrons by giving them a more easily reduced substrate instead? A dissolving metal reduction results: note dissolving. The electrons have to be captured as the metal releases them, otherwise they will just reduce the solvent to give H2.
Dissolving metal reductions work because the electrons released as reactive metals form soluble cations that can be harnessed to do other, more useful, reductions. Electrons are the simplest possible reducing agents, and they will reduce carbonyl compounds, alkynes, or aro matic rings—in fact any functional group with a low-energy π orbital into which the electron can go.
