Clemmensen Reduction & Birch Reduction

Session Objectives

By the end of this session, students will be able to:

• Clemmensen Reduction

• Mechanism of clemmensen reduction

• Birch Reduction

• Mechanism of birch reduction

Clemmensen reduction

• Clemmensen reduction is an organic reaction used to reduce an aldehyde or ketone to an alkane using amalgamated zinc and hydrochloric acid.

• The "Carbanionic mechanism", where the zinc attacks the protonated carbonyl directly, and the "Carbenoid mechanism" which is a radical process and the reduction happens on the surface of the zinc metal.

Mechanism of Clemmensen reduction

Birch reduction

• The Birch reduction is an organic reaction where aromatic rings undergo a 1,4-reduction to provide unconjugated cyclohexadienes.

• The reduction is conducted by sodium or lithium metal in liquid ammonia and in the presence of an alcohol.

• The mechanism begins with a single electron transfer (SET) from the metal to the aromatic ring, forming a radical anion.

• The anion then picks up a proton from the alcohol which results in a neutral radical intermediate.

• Another SET, and abstraction of a proton from the alcohol results in the final cyclohexadiene product and two equivalents of metal alkoxide salt as a byproduct.

Mechanism of Birch reduction

Summary

• Clemmensen reduction is an organic reaction used to reduce an aldehyde or ketone to an alkane using amalgamated zinc and hydrochloric acid.

• The "Carbanionic mechanism", where the zinc attacks the protonated carbonyl directly, and the "Carbenoid mechanism", which is a radical process and the reduction happens on the surface of the zinc metal.

• The Birch reduction is an organic reaction where aromatic rings undergo a 1,4-reduction to provide unconjugated cyclohexadienes.

• The reduction is conducted by sodium or lithium metal in liquid ammonia and in the presence of an alcohol.

• The mechanism begins with a single electron transfer (SET) from the metal to the aromatic ring, forming a radical anion.