Birch reduction

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The Birch reduction is an organic reduction reaction, which is very useful in synthetic organic chemistry.

Mechanism

A solution of sodium in liquid ammonia consists of the electride salt [Na(NH3)x]+ e, which has an intense blue color. The solvated electrons add to the aromatic ring to give a radical anion. The added alcohol supplies a proton to the radical anion and also to the penultimate carbanion, as for most substrates ammonia is not acidic enough.

Procedure

In a reaction flask cooled to very low temperature using dry ice and acetone or EtOH and liq. N2, add anhydrous liquid ammonia and a stir bar. Prepare an anhydrous solution of the aromatic compound in a very anhydrous alcohol, like benzene or naphthalene in ethanol, tert-butanol or tert-amyl alcohol. The solution will become cloudy, as not all the aromatic alcoholic solution will dissolve. While any alkali metal can be used, lithium gives the best yield. Now add the alkali metal in small portions, while stirring the suspension continuously. The solution will turn blue, due to solvated electrons, which will turn back white. The aromatic compound is reduced to its respective compound.

NileRed has made a good video on Birch reduction.[1]

Uses

  • Reduction of aromatic compounds
  • Make tetralin

Safety

This reaction is very sensitive to water and moisture, as it uses alkali metals. Liquefied ammonia is extremely toxic and since it's very cold, contact with bare skin may cause frostbits.

References

  1. https://www.youtube.com/watch?v=gNLvgGY-D9o

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