Chemical methods of reduction can take place by addition of electrons to the unsaturated compound followed by transfer of protons or can take place by addition of hydride ion followed by protonation. Reductions that follow the first path are generally effected by metal, the source of the electrons, and a proton donor, which may be water, an alcohol or an acid. However, in the absence of proton source, it can undergo dimerization or polymerization.

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Reduction by dissolving metals
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Reduction by dissolving Metals
Chemical methods of reduction can take place by addition of electrons to the
unsaturated compound followed by transfer of protons or can take place by
addition of hydride ion followed by protonation.
Reductions that follow the first path are generally effected by a metal, the
source of the electrons, and a proton donor, which may be water, an alcohol or
an acid. However, in absence of proton source it can undergo dimerization or
polymerization.
The metals commonly employed in these reductions include the alkali metals,
calcium, zinc, magnesium, tin and iron. The alkali metals are often used in
solution in liquid ammonia or as suspensions in inert solvents such as ether or
toluene, frequently with addition of an alcohol or water to act as a proton
source.
Reduction of carbonyl compounds
Reduction of ketones to secondary alcohols can be effected by catalytic transfer
hydrogenation or by complex hydrides or by sodium and an alcohol.

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One feature of the sodium–alcohol method is that with cyclic ketones it normally
gives rise exclusively or predominantly thermodynamically more stable alcohol.
Trans Cis
(More stable) (Less stable)
The ratios of the more-stable (trans) product (equatorial substituents) and the
cis product formed by reduction of 2-methylcyclohexanone with different
reducing agents are given in Table below
The ketone 4-tert-butylcyclohexanone similarly gives the more stable trans-4-
tertbutylcyclohexanol almost exclusively on reduction with lithium and
propanol in liquid ammonia.
The high proportion of the more-stable product (equatorial hydroxyl group)
formed in the reduction with a metal–alcohol is thought to arise from the
preference for the intermediate radical anion (or other intermediate) to adopt
the configuration with the equatorial oxygen atom.
.Na . EtOH
-
Na
EtOH

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The reductive coupling of two aldehydes or ketones is referred to as the
McMurry or pinacol coupling reaction and gives rise to a 1,2-diol (a pinacol) or
an alkene (McMurry)
Pinacol Coupling
2
Mg,HgCl2
McMurry Coupling
2
TiCl4,Zn
Dioxane
Reflux
Carboxylic esters can be reduced by sodium and alcohols to form primary
alcohols, known as the Bouveault–Blanc reaction, but has now been largely
replaced by reduction with lithium aluminium hydride.
When the reaction is carried out in the absence of a proton donor, for example
with sodium in xylene or liquid ammonia, dimerization takes place known as
Acyloin condensation, resulting in medium and large rings ring.
Na
Xylene
Reflux
H3O+
The Clemmensen reduction of aldehydes and ketones to methyl or methylene
groups takes place by heating with zinc and hydrochloric acid.
A non-miscible solvent can be used and serves to keep the concentration in the
aqueous phase low, and thus prevent bimolecular condensations at the metal
surface.

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Zinc, HCl
Reduction of conjugated double bond with metal and ammonia or an amine
Reduction of conjugated double bond possible because the intermediate anion
can be stabilized by electron delocalization. The best reagent is a solution of an
alkali metal in liquid ammonia, with or without addition of an alcohol called as
Birch reduction conditions.
Under these conditions conjugated alkenes, α, β unsaturated ketones and even
aromatic rings can be reduced to dihydro derivatives.
Birch reductions are usually carried out with solutions of lithium or sodium in
liquid ammonia. Any added alcohol can act as a proton donor to buffer against
the accumulation of the strongly basic amide ion.
Solutions of alkali metals in liquid ammonia contain solvated metal cations and
electrons.
Conjugated dienes are readily reduced to the 1,4-dihydro derivatives with
metal–ammonia reagents in the absence of added proton donors.
The protons required to complete the reduction are supplied by the ammonia.
Na,NH3
Reduction of α, β unsaturated ketones gives the saturated ketone or saturated
alcohol, depending on the conditions. In absence of alcohol as proton source
saturated ketone is obtained, however in presence of ethanol as proton source,
the saturated alcohol is obtained.

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NH4Cl
EtOH
Li,NH3,
80%
Li,NH3,Ether
92%
Reduction of aromatic compounds with metal and ammonia
One of the most useful synthetic applications of metal–ammonia–alcohol
reducing agents is in the reduction of benzene rings to 1,4-dihydro derivatives.
The presence of an alcohol as a proton donor is necessary in these reactions, for
the initial radical anion is an insufficiently strong base to abstract a proton from
ammonia.
The alcohol also acts to prevent the accumulation of the strongly basic amide
ion, which might bring about isomerization of the 1,4-dihydro compound to the
conjugated 1,2-dihydro isomer (which would be further reduced to
tetrahydrobenzene).
EtOH
.
-
.
Li,NH3, EtOH Li,NH3,
-
Particularly useful synthetically is the reduction of methoxy- or amino
substituted benzenes to dihydro compounds, which are readily hydrolyzed to
cyclohexenones.
The Birch reduction of benzenes containing electron-donating substituents
takes place to give the 1,4-dihydro compounds in which the two new hydrogen
atoms avoid the carbon atoms to which the electron-donating substituents are
attached.
This selectivity can be rationalized in terms of the relative electron densities of
the carbon atoms in the intermediate radical anion.
An electron-donating substituent destabilizes an adjacent negative charge and
therefore the site of highest electron density is not alpha to such substituents.

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EtOH.
-
.
Li or Na EtOH
Liq.NH3,
Li or Na
Liq.NH3,
-
The reduction of benzene rings substituted by electron-withdrawing carbonyl
groups gives rise to 1, 4-dihydrobenzoic acid derivatives, in which the
intermediate anion is alpha to the anion-stabilizing substituent.
These intermediate carbanions can be alkylated in the alpha position to the
carbonyl group.
H3O+
-Na H3O+
Liq.NH3,
C7H15Br
Selective reduction of a benzene ring in the presence of another reducible group
is possible if the other group is first protected in some way. Ketones, for
example, may be converted to acetals or enol ethers to protect them from
reduction. Selective reduction of less-electron-rich aromatic rings occurs in
bicyclic aromatic compounds.
EtOH
Li,NH3,Ether
98%
Stereo selective Birch reduction is possible and a number of examples have been
reported, particularly for selective alkylation of the intermediate enolate anion

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i)K, NH3,tBuOH
ii) EtI
Reduction of alkynes with metal and ammonia
Alkynes are reduced to alkenes with metal-ammonia as reducing agents. The
reduction is completely stereoselective and the only product from a
disubstituted alkyne is the corresponding E-alkene.
Na,NH3
THF
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