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.

مواضيع ذات صلة

The Boc protecting group—gastrin and aspartame

The Cbz protecting group—oxytocin

Peptide synthesis

A survey of protecting groups

How to react the less reactive group (II): protecting groups

Chemoselectivity in the reactions of dianions

How to react the less reactive group (I): react both then ‘unreact’ one

Competing reactivity: choosing which group reacts

How to oxidize primary alcohols to aldehydes

How to oxidize secondary alcohols to ketones

Birch reduction of arenes

Getting rid of functional groups