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.