REDUCTION

The reduction may be defined as the removal of oxygen, addition of hydrogen, and gain of electrons.
Typical example of reduction is conversion of unsaturated compounds into saturated compounds.
Like conversion of ethylene into ethane, acetaldehyde into ethyl alcohol etc. In terms of oxidation
number, change of oxidation number higher oxidation state to lower, oxidation state

Catalytic Hydrogenation

This is one of the most convenient methods applied for the reduction of organic compounds. In this
method the solution of organic compound is shaken with suitable metallic catalyst under an
atmosphere of hydrogen. At the end of reaction the catalyst is removed by filtration. Pure product is
recovered from filtrate by evaporation of solvent. The amount of hydrogen used for reduction can be
measured.
Many functional groups such as C ⎯ C double and triple bond, carbonyl group, carboxylic
group, nitrite group, nitro group, aromatic and hetro cyclic compounds can be reduced with catalytic
hydrogen.
Various catalysts are used like platinum, palladium rhodium, ruthenium, nickel and copper
chromate. Catalytic hydrogenation is carried out under low (1 − 4 atoms, 0 − 100C) or light
pressure (100 − 300 atm, − 100 − 400C).
Hydrogenation proceeded at low pressure with catalyst like raney nickel, platinum, or
palladium or rhodium on a support like carbon, barium sulphate, or calcium carbonate, while higher
pressure hydrogenation can be carried out with raney nickel, copper chromate or palladium on a
carbon.

Reduction of Alkenes
Catalytic Hydrogenation
Reduction of C ⎯ C double bond in the presence of metal catalysts is the easier and take
place mostly under mild conditions, except some highly hindered Alkenes, metal catalyst like raney
Nickel, platinum and pladium are most frequently used for reduction of alkene. In this process
hydrogen and alkenes first associated with catalyst surface. Hydrogen is then transferred to
unsaturated carbon of the organic compound in a process that follows synsterospecifically.
The reaction takes place on the surface of the metal in three steps:

Step 1
Hydrogen molecules adsorbs on the surface of metal. The relatively strong
H ⎯ H sigma bond is broken and new weak M ⎯ H bond are formed.

,
catalyst surface (M)

Step 2
The Pi bond of alkene interacts with the metal catalyst and a hydrogen atom is transferred
from the catalyst surface to one of carbon double bond.
Step 3
A second hydrogen atom is shifted from surface of catalyst toward alkene forming alkanes
and resulting

(M) catalyst
alkene is released from the surface of catalyst.
Note that the hydrogenation of alkene take place in stereospecific manner, both hydrogen
atoms add from the same side of the molecule (Syn addition). For example 1, 2 methyl cyclohexene
gives Cis 1,2-dithyl cyclohexane predominately.

H2/HO2
⎯⎯⎯⎯→
CH3COOH

In catalytic hydrogenation of alkenes platinum and palladium are most effective and
frequently used catalyst. Raney nickel is also used in some cases.
C6H5CH ⎯
⎯ CHCH2OH Error! C6H5CH2CH2CH2OH

Cimnamyl alcohol Dihydrocinnamyl alcohol

(3-phenyl prapanol)

In an alkene, the ease of reduction decrease with the degree of substitution of double bond,
addition occurs on less hindered side.

REDUCTION OF ALKYNES
Catalytic hydrogenation of alkynes gives completely reduced product alkane. This reduction
is faster than alkenes. Platinum, palladium or raney nickel are mostly use for complete reduction of
alkynes into alkanes.
Pt/H2
CH ⎯
⎯
⎯ CH ⎯⎯⎯→ CH3CH3

However, partial hydrogenation can be carried out in quantitative yield with palladium
calcium carbonate, catalyst, also known as Lindlars catalyst. In this catalyst palladium on calcium
carbonates, support, treated with lead acetate which has been deactivated (poisoned) by quinolined
is used on which alkenes are less adsorbed than alkynes. These reductions are highly stereo
selective.

⎯⎯→
H2
1. CH3(CH2)7C ⎯
⎯
⎯ C(CH2)7COOH
Lindlercat alyst
 H2
2. ⎯
CH3CH2CH2C ⎯
⎯ CCH2CH2CH3 ⎯⎯⎯⎯⎯→ CH3CH2CH2C = CCH2CH2CH3
Pd/CaCO3
Pg(OAC)2

REDUCTION OF AROMATIC RING

Vigorous conditions are required for catalytic reduction of aromatic rings, because aromatic
stabilization energy is lost in the process. For example benzene can be reduced to cyclo hexane with
platinum oxide in acetic acid solution.

PtO2/H2
⎯⎯⎯⎯⎯→
CH3COOH

Many other catalysts like rhodium and raney nickel are also used. Substituted benzenes are
more readily reduced as compare to benzene. For example

H2/Raney Ni
⎯⎯⎯⎯⎯→

Benzene derivatives carrying oxygen or nitrogen functions easily undergo hydrogenolysis

particularly over platinum catalysts however best results are achieved with catalysts which can be
used under mild conditions i.e. ruthenium or rhodium.

Raney/Ni
⎯⎯⎯⎯⎯→
90 275 atm

H2/Rh-AlO3
⎯⎯⎯⎯⎯→

Reduction of aromatic ring with catalytic method has many limitations such as it requires
high pressure and under these conditions olefin double bond and carbonyl groups are also reduce.
Dissolving metal methods or Birch reduction is used to convert aromatic compound having
benzoid ring into a 1,4 cyclodexadines. This reaction is performed with metals like sodium lithium
and potassium in alcohols or liq. ammonia.
REDUCTION OF ALDEHYDE/KETONES
Reduction into Alcohols with Metal Hydrides
In this method hydrogen atom will be added to each atom of carbonyl double bond,
converting the aldehydes and ketones into alcohols. Metal hydrides are used to reduce
aldehyde/ketones, are lithium aluminum hydride and borohydride of sodium, potassium and lithium.

Overall 2H atoms are added across the C ⎯ ⎯ C to give HC ⎯ OH. In step one, the nucleophilic
H in hydride reagent adds to electrophilic C in polar carbonyl group in aldehyde, electron from the
O
||
C move toward oxygen creating an intermediate metal alkoxide complex. In second step alkoxide
oxygen give primary alcohol products by simple acid base reaction.

Examples
O O OH OH
|| || NaBH4 | |
(i) CH3 ⎯ C ⎯ CH2 ⎯ C ⎯ CH3 ⎯⎯⎯→ CH3 ⎯ C H ⎯ CH2 ⎯ C H⎯ CH3

(86%)
NaBH4
(ii) CH3CH ⎯
⎯ CHCHO ⎯⎯⎯→ CH3CH ⎯
⎯ CH ⎯ CH2OH
ether
(85%)
Clemenensen Reduction

The carbonyl group of aldehyde and ketones is change into methyl or methylene when react
with zinc amalgam in the presence of HCl. This method is known as Clemenensen reduction. The
reaction mixture consists of carbonyl compound amalgamated zinc and concentrated HCl. HCl is
used as a source of hydrogen. Water and an immiscible solvent like xylene, toluene produces three
phase system in which most of carbonyl compound remain in upper hydrocarbon layer, carbonyl
compound in aqueous layer, is reduced on metal surface. The objective of three phase system is to
avoid bimolecular condensation by maintaining low concentration of protonated carbonyl compound
at the metal surface

O O
|| || Zn/Hg
1. Ph ⎯ C ⎯ CH2 ⎯ CH2 ⎯ C OH ⎯⎯⎯⎯→ Ph(CH2)3COOH
toulene HCl
 benzyl propionic acid 85%
 phenyl butyric acid
O
|| Zn/Hg
2. Ph ⎯ C ⎯ CH2CH2CH3 ⎯⎯⎯⎯→ PhCH2CH2CH2CH3
toulene HCl

REDUCTION OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Following are different methods available for the reduction of carboxylic acids into different
compounds.

(i) Reduction into Alcohols

Metal hydrides like lithium aluminum hydride are used for conversion of carboxylic acids
and their derivatives into alcohols. The general mechanism involves the transfer of hydride ion to
more electropositive carbon atom. The carbon atom of carboxyl group is in a relatively high oxidation
state. So carbonic acid rapidly reduces into primary alcohols with metal hydride reagent like lithium
aluminum hydride. Initial product consists of metal salts which must be further hydrolyzed into final
product alcohol.
O
|| LiAlH4
1. CH3CH2 ⎯ C ⎯ OH ⎯⎯⎯⎯→ CH3CH2CH2OH
H3O+

LiAlH4
2. ⎯⎯⎯⎯→
H3O

REDUCTION OF NITROGEN CONTAINING COMPOUNDS

Following are the various methods used for the production of anilines from nitro compounds.
• Catalytic hydrogenation with Pd on carbon or raney nickel.
• Sodium hydro sulphide, sodium sulfide or hydrogen sulphide
• Zinc. • Titanium (IV) chloride.
NO2 NH2

H2/Pt

200 0C

COOH COOH ⎯⎯→