Reduction

Hydrogenolysis reactions, which involve addition of hydrogen across a σ bond and results in cleavage of the σ
bond, have been known in organic chemistry for a long time. The earliest observation of hydrogenolysis
was in 1906 by Padoa and Ponti during studies on the reduction of furfural using a nickel catalyst with
hydrogen gas, where 2- methyltetrahydrofuran and 2-pentanol were noted as byproducts of the reaction .
Since then, hydrogenolysis reactions have been studied extensively, especially for carbon-heteroatom
bonds such carbon-oxygen, carbon-nitrogen, carbon-sulphur and carbonhalogen, and various mechanistic
aspects have been elucidated. In addition, numerous synthetic applications have been developed, such as
protecting group strategies in both natural product and peptide synthesis. More recently, hydrogenolysis is
being utilized to develop sustainable fuels from biomass such as carbohydrates and lignin.

Hydrogenolysis of Furfural

Hydrogenolysis reactions can be performed with either homogeneous or heterogeneous

Catalysts. The mechanism of the reaction is different depending on the choice of catalyst.
For homogeneous catalysis, the reaction takes place within the coordination sphere of the
metal and can be described in terms of the standard organometallic processes of oxidative
addition, hydrometallation, transmetallation (or ligand exchange), β-hydride elimination
(or in some cases, β-carbon elimination) and reductive elimination.
Reductive processes fall into three categories:
1. The removal of oxygen
2. The addition of hydrogen – This processs further subdivided into two types:
a. Hydrogenation – Addition of hydrogen to an unsaturated system.
R-CH=CH2 + H2 catalyst R-CH2-CH3
b. Hydrogenolysis- Addition of hydrogen with concomitant bond rapture.
C6H5-CH2-O-CH2-R + H2 catalyst C6H5-CH3 + R-CH2-OH
c. The gain of electrons

In organic chemistry reductions is defined as a process in which an increase in the hydrogen

content or a decrease in the oxygen content occurs in an organic compound. It is for the reason the
reduction is sometime called as hydrogenation and hydrogenolysis. All reduction processes
involves three phenomenon, removal of oxygen, addition of hydrogen and the gain of electrons.
Hydrogenation means the addition of hydrogen to an unsaturated system whereas hydrogenolysis
means the addition of hydrogen with concomitant sulphur Viz:
R CH CH2 H2 R Catalysis
CH 2 C H 3 ; hydrogenation

H2/Pd/C
CH2 O CH2 CH3 CH3 + CH3-CH2-OH
Hydrogenolysis
CH3
H2/Pd/C
CH2 N CH3 + CH3-NH-CH3
CH3
 C2H5 C2H5 CH3 C2H5 C2H5
H H
- H-O-R H-O-R
Na Na
H
e- e-
-
H

H3C H3C
-
C O + Na C O

H3C H3C

- -
O O OH OH
coupling 2H+
H 3C C C CH3 H 3C C C CH3

CH3 CH3 CH3 CH3

(B)There is and H- transfer. This hydride ion comes from complex metal hydrides of Boron and Aluminium or from
alkoxide of aluminium ie-
-+
O Li+ O Li
O OH
LiAlH4 +
H
H3C C CH3 H3C C CH3 H3C C CH3 H3C C CH3
H OH
H H
-
H Al H
H H
(C)This rotate involves the addition of molecular hydrogen in the presence of some catalyst such as palladium,
Adams catalyst or raney nickel etc. for example
O O
H2/Pt
CH CH C CH2 CH2 C

H2 2 / Ni;
CH 3 CH 2 CH CH CH 3 CH 3 CH 2 CH 2 CH 2 CH 3
 Hydrogenation and dehydrogenation
Hydrogenation and dehydrogenation are the oldest catalytic processes. Catalytic
hydrogenation reactions are used for production of fine chemicals, in
pharmaceutical industry and polymer industry and for the production of edible and
non edible fats and oils. Dehydrogenation reactions are used for production of light
alkene C3-C4 for preparation of acrylonitrile, oxoalcohols or propylene oxides
and production of C4-C8 for detergents. The process is also used for preparation of
polypropylene, styrene, aldehydes, ketones etc
Hydrogenation
Some of the hydrogenation processes are:
1. Hydrogenation of alkenes and alkadienes

Example
a. hydrogenation of ethene to ethane
C2H4 + H2 = C2H6
b. hydrogenation of cyclohexene to cyclohexane
C6H10 + H2 = C6H12
 Hydrogenation of aromatics

3. Hydrogenation of nitrobenzene
Hydrogenation of nitrobenezene shows how wide variety of products can be obtained by using
different catalysts and reaction conditions.
2. Hydrogenation of aromatics

Example : Benzene to cyclohexane which is the precursor for nylon polymer.

C6H6 + 3 H2 = C6H12
3. Hydrogenation of nitrobenzene
Hydrogenation of nitrobenezene shows how wide variety of products can be
obtained by using different catalysts and reaction conditions.

Catalysts
Hydrogenation catalysts can be grouped in two categories as shown in Fig Wide
variation in commercial catalysts is available. Surface area, pore structure and
presence of surface functional groups in supports greatly influence catalysts
properties.
 Catalysts design for hydrogenation catalysts
Raney Ni
Raney Ni is a very important hydrogenation catalyst. Uniqueness of this catalyst is that it
is a bulk unsupported catalyst with high surface area. The advantages include absence of
support, minimum side reactions and more easy reduction of Ni in absence of interaction
with supports. Raney Ni is typically prepared by the following steps:
1. Bulk Ni and Al metals in a 50-50 mixture are melted together.
2. Molten metals are poured into water producing fine grains
3. 20 % NaOH is added to leach Al from alloy leaving a porous matrix rich in Ni 90-95 %
with a high surface area ~ 100 m2/g
4. Raney nickel is stored in an inert atmosphere such as water or fat oil to prevent
reoxidation
 Dissociative adsorption of hydrogen over Pt active site
Kinetics and mechanism
During hydrogenation of alkene over noble metal based catalysts, at first hydrogen get
adsorbed dissociatively on the metal sites Fig. Dumesic et al. reported that adsorption
of hydrogen follows different kinetics at low alkene coverage and at high coverage. For
hydrogenation of ethylene two types of site for hydrogen adsorption was proposed.
The proposed mechanism is shown below. The two types of site for hydrogen
adsorption are represented by ‘S' and ‘*'.The site for adsorption of hydrogen on a
surface partially covered with ethene was represented as ‘S', whereas hydrogen
adsorption site on a clean surface was designated as ‘*'. At adsorption sites, ‘*', both
the hydrogen and ethene can get adsorbed but at ‘S' site only hydrogen can adsorb .
Catalytic hydrogenation –
It involves stirring of substrate with a catalyst in a suitable solvent in an atmosphere of hydrogen. Catalytic
hydrogenation can be classified into two categories
a. Heterogeneous hydrogenation - Numerous heterogeneous catalysts
have been used for catalytic hydrogenation. It involves transition metal catalysts absorbed on a solid
support. The reduction take place at the surface of the catalyst which adsorbs both hydrogen and
organic compound and facilitates their contact.
Elevated temperature and pressure invariably increase the rate of hydrogenation.
The addition of hydrogen is syn. Thus reaction is stereospecific.
b. Homogeneous hydrogenation – Wilkinson’s catalyst ([Ph3 P]3RhCl) is a most useful homogeneous
catalyst of alkenes.
Oxidation of hydrazine
NH2-NH2 H2O2/Cu NH=NH
1. By thermal decomposition of p-toluenesulphonylhydrazine
CH3-C6H5-SO2NH-NH2 Δ CH3-C6H5-SO2H +
NH=NH
2. By thermal decomposition of azo dicarboxylic acid
HOOC-N=N-COOH Δ NH=NH + 2CO2
Diimide is a highly selective reagent for the reduction of carbon –carbon double
bond.

C=C C=C
+
H N=N H N=N

Reduction with alkali metal in liquid ammonia

Alkenes that are substituted with an electron-withdrawing group on olefinic carbon can be reduced with lithium , sodium or
potassium in liquid ammonia at low temperature. The reduction is carried out in the presence of proton donor,i.e., ethyl
alcohol

CH3-CH=CH-CO-CH3 CH3-CH2-CH2-CO-CH3
Na/NHNH,C4Cl
3 25
H OH
 O O
Na/NH3C2H5OH

NH4Cl

O O

Na/NH3,C2H5OH
NH4Cl
O2N- Azo dicarboxylic acid
CH CHCOOH
Δ
O2N-
CH- CH-COOH

Reduction with alkynes

Following methods are used for the reduction of alkynes:
Catalytic hydrogenation
When alkynes undergo catalytic hydrogenation, the first addition of hydrogen yields an alkene and a second addition of
hydrogen gives an alkane
The catalyst may be homogeneous or heterogeneous.
 H2 H2
R-C C-R R-CH=CH-R R-CH2-CH2-R
catalyst catalyst
Hydrogenation of an alkyne may be stopped at alkene stage which is known as partial reduction of alkynes. Partial
reduction of alkynes can be done by heterogeneous hydrogenation with Lindlar’s catalyst [Pd poisoned with Pb+2 and an
amine (quinoline or pyridine)].
Lindlar’s catalyst convert non terminal alkenes into cis –alkenes and give stereoselective reaction.
Reduction with borane or hydroboration of alkynes
A sterically hindered dialkyl borane reacts with alkyne to give vinyl borane. The reaction is stereoselective and
followed syn addition. The vinyl derivative on protonolysis give cis alkenes. Protonolysis carried out in the presence of
boiling acetic acid and inert towards NO2, COOR and halo groups.

C6H5-C C-COO-C2H5 Acetic acid/ Δ C6H5 COOC2H5

Sia2BH/THF C C
H H

C6H5-C C--C2H5 C6H5 C 2H 5

N2 C=C
DIBAL H H