Catalytic hydrogenation is one of the most convenient available for the reduction of organic compounds. compounds. The reduction is carried out easily by stirring or shaking the substrate with the catalyst in a suitable solvent

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Reduction using Catalytic hydrogenation

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Catalytic hydrogenation
1) Catalytic hydrogenation is one of the most convenient available for reduction
of organic compounds. Reduction is carried out easily by stirring or shaking
the substrate with the catalyst in a suitable solvent or without a solvent if the
substance being reduced is a liquid in an atmosphere of hydrogen gas. Once
the reaction is completed, the catalyst is filtered off and the product is
recovered from the filtrate, often in a high state of purity.
2) In many cases reaction proceeds smoothly at room temperature and at
atmospheric or slightly elevated pressure. However, in some cases, high
temperatures (100–200°C) and pressures (100–300 atmospheres) are
necessary, requiring special high-pressure equipment.
3) Catalytic hydrogenation may result simply in the addition of hydrogen to one
or more unsaturated groups in the molecule, or it may be accompanied by
fission of a bond between atoms. The latter process is known as
hydrogenolysis.
4) Under appropriate conditions, catalytic hydrogenation can be used to reduce
unsaturated groups such as alkenes, alkynes, carbonyl groups, nitriles, nitro
groups and aromatic rings.
5) Certain groups, notably allylic and benzylic hydroxyl and amino groups and
carbon–halogen single bonds readily undergo hydrogenolysis, resulting in
cleavage of the bond between the carbon and the heteroatom.
H2,Pd/C
MeOH
100 %
6) An alternative procedure that is sometimes advantageous is ‘catalytic
transfer hydrogenation’, in which hydrogen is transferred to the substrate
from another organic compound. The reduction is carried out simply by
warming the substrate and hydrogen donor such as isopropanol or a salt of
formic acid) together in the presence of a catalyst, usually palladium.
7) Catalytic-transfer hydrogenation can show different selectivity towards
functional groups from that shown in catalytic reduction with molecular
hydrogen.

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Catalyst for hydrogenation
1) The most commonly used catalyst in the laboratory for catalytic
hydrogenations are the platinum palladium and nickel and sometimes
rhodium, Iridium and ruthenium and are used either as the finely divided
metal or more commonly, supported on a suitable carrier such as activated
carbon, alumina or barium sulphate.
2) Platinum is often used in the form of its oxide PtO2 (Adams’ catalyst), which
is reduced to metallic platinum by hydrogen in the reaction medium.
H2,PtO2
AcOH
72 %
3) Most platinum metal catalysts (with the exception of Adams’ catalyst) are
stable and can be kept for many years without appreciable loss of activity,
but can be deactivated by many substances, particularly by compounds of
divalent sulphur.
4) Catalytic activity is sometimes increased by addition of small amounts of
platinum or palladium salts or mineral acid. The increase in the activity may
simply be the result of neutralization of alkaline impurities in the catalyst.
Reduction Selectivity
1) Many hydrogenations proceed satisfactorily under a wide range of
conditions, but where a selective reduction is wanted, conditions may be
more critical.
2) The choice of catalyst for a hydrogenation depends on the activity and
selectivity required. In general, the more active the catalyst the less
discriminating it is in its action and for greatest selectivity reactions should
be run with the least active catalyst and under the mildest possible
conditions consistent with a reasonable rate of reaction.
3) The rate of a given hydrogenation may be increased by raising the
temperature, by increasing the pressure or by an increase in the amount of
catalyst used, but all these factors may result in a decrease in selectivity.
4) Hydrogenation of ethyl benzoate with copper chromite catalyst under the
appropriate conditions leads to benzyl alcohol by reduction of the ester
group, while Raney nickel gives ethyl cyclohexane carboxylate by selective
attack on the benzene ring.

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H2,Raney Nickel
50°C, 100 atm
H2,CuCr2O4
160°C, 250 atm
5) Both the rate and, sometimes, the course of a hydrogenation may be
influenced by the solvent used. The most common solvents are methanol,
ethanol and acetic acid, although other solvents can be used. Many
hydrogenations over platinum metal catalysts are favoured by strong acids.
6) Example, reduction of nitro styrene in acetic acid–sulfuric acid is rapid and
gives 2-phenyl-ethylamine (90% yield), but in the absence of sulfuric acid
reduction is slow and the yield of amine is poor. Not all functional groups are
reduced with equal ease. Below table shows the approximate order of
decreasing ease of catalytic hydrogenation of a number of common groups.

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Hydrogenation of alkenes
Hydrogenation of carbon–carbon double bonds takes place easily and, in most
cases, can be done under mild conditions. Only a few highly hindered alkenes
are resistant to hydrogenation and even these can generally be reduced under
more vigorous conditions. Palladium and platinum are the most-frequently used
catalysts. Both are very active and the preference is determined by the nature
of other functional groups in the molecule and by the degree of selectivity
required.
74 %
H2,10 % Pd/C
EtOH
A few different conditions can be employed to minimize hydrogenolysis, such
as the addition of ethylenediamine (en) and THF as solvent
H2,5 % Pd/C
EtOH
Ethylene diamine
93 %
Rhodium and ruthenium catalysts may alternatively be used and sometimes
show useful selective properties. Rhodium allows hydrogenation of alkenes
without hydrogenolysis of an oxygen function.
H2,5 % Rh-Al2O3
EtOH
The ease of reduction of an alkene decreases with the degree of substitution of
the double bond and this sometimes allows selective reduction of one double
bond in a molecule which contains several other double bonds.
For example, limonene can be converted into p-menthene by reduction of
disubstituted terminal alkene in almost quantitative yield by hydrogenation
over platinum oxide. In contrast, the isomeric diene having two disubstituted
double bond gives only the completely reduced product without selectivity.

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H2, PtO2
EtOH
Limonene
H2, PtO2
EtOH
99 %
p-menthene No selectivity
Selective reduction of carbon–carbon double bonds in compounds containing
other unsaturated groups can usually be accomplished, except in the presence
of triple bonds, aromatic nitro groups and acyl halides.
Stereochemistry and mechanism
Hydrogenation of an unsaturated compound takes place by adsorption of the
compound on to the surface of the catalyst, followed by transfer of hydrogen
from the catalyst to the side of the molecule that is adsorbed on it. Adsorption
onto the catalyst is largely controlled by steric factors, and it is found in general
that hydrogenation takes place by cis addition of hydrogen atoms to the less-
hindered side of the unsaturated centre.
For example, hydrogenation of the E-alkene gives the racemic dihydro
compound by cis addition of hydrogen, while the Z-alkene gives the meso
isomer.
H2, Pd
EtOH
98 %
MesoZ-alkene
H2, Pd
EtOH
98 %
RacemicE-alkene
Hydrogenation of the ketone gave products formed by cis addition of hydrogen
to the more accessible side of the double bonds.
H2,PtO2
AcOH
83 % 17 %

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The hydrogenation of substituted cyclic alkenes is irregular in many cases in
that substantial amounts of trans-addition products are formed, particular
with palladium catalysts.
For example, the bicyclic alkene on hydrogenation over palladium in acetic acid
gives mainly trans-decalin and the alkene 1,2-dimethylcyclohexene gives
variable mixtures of cis- and trans-1,2- dimethyl cyclohexane depending on the
conditions.
H2,Pd/C
AcOH
21 % 79 %
H2,Pd/C
AcOH
16 % 46 %
H2,PtO2
AcOH
82 % 18 %
The reason for the formation of the trans products is thought to be because of
migration of the double bond in a partially hydrogenated product on the
catalyst surface. Although catalytic hydrogenation of alkenes may be
accompanied by migration of the double bond, no evidence of migration
normally remains on completion of the reduction.
Hydrogenation of alkynes
Catalytic hydrogenation of alkynes takes place in a stepwise manner and both
the alkene and the alkane can be isolated. Complete reduction of alkynes to the
saturated compound is easily accomplished over platinum, palladium or Raney
nickel. A complication which sometimes arises, particularly with platinum
catalysts, is the hydrogenolysis of hydroxyl groups to the alkyne.

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H2, Pt
The partial hydrogenation of alkynes to Z-alkenes is achieved with palladium-
calcium deactivated by addition of lead acetate (Lindlar’s catalyst) or quinoline.
It is aided by the fact that the more electrophilic alkynes are absorbed on the
electron-rich catalyst surface more strongly than the corresponding alkenes. An
important feature of these reductions is their high stereoselectivity.
Lindlar catalyst
Hydrogenation of aromatic compounds
Reduction of aromatic rings by catalytic hydrogenation is more difficult than that
of most other functional groups, and selective reduction is not easy.
Hydrogenation of phenols, followed by oxidation of the resulting cyclohexanols
is a convenient method for the preparation of substituted cyclohexanones.
i) 55 atm,H2,EtOH
5 %Rh-Al2O3,AcOH
ii) CrO3, H2SO4
Acetone, H2O
Reduction of benzene derivatives carrying oxygen or nitrogen functions in
benzylic positions is complicated by the easy hydrogenolysis of such groups,
particularly over palladium catalysts. Preferential reduction of the benzene ring
in these compounds is best achieved with ruthenium or rhodium catalysts,
which can be used under mild conditions.

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H2,Rh-Al2O3
H2,Pd/C
Hydrogenation of aldehydes and ketones
Hydrogenation of the carbonyl group of aldehydes and ketones is easier than
that of aromatic rings, but not as easy as that of most carbon–carbon double
bonds.
For aliphatic aldehydes and ketones, reduction to the alcohol can be carried out
under mild conditions over platinum or Raney nickel.
Ruthenium is also an excellent catalyst for reduction of aliphatic aldehydes and
can be used to advantage with aqueous solutions. Palladium is not very active
for hydrogenation of aliphatic carbonyl compounds, but is effective for the
reduction of aromatic aldehydes and ketones
Prolonged reaction time particularly at elevated temperatures or in the
presence of acid leads to hydrogenolysis and can therefore be used as a method
for the reduction of aromatic ketones to methylene compounds.
Hydrogenation of nitriles, oximes and nitro compounds
Functional groups like nitriles, oximes, azides are readily reduced by catalytic
hydrogenation into primary amines. Reduction of nitro compounds takes place
easily and is generally faster than reduction of alkenes or carbonyl groups. Raney
nickel or any of the platinum metals can be used as the catalyst, and the choice
is governed by the nature of other functional groups in the molecule.
H2,5 % Pd/C
EtOH,H2SO4
Nitriles are reduced with hydrogen and platinum or palladium at room
temperature, or with Raney nickel under pressure.

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However, large amounts of secondary amines may also be formed as an impurity
due to the side reaction of the amine with the intermediate imine.
H2
Catalyst
H2
Catalyst
H2
Catalyst
-NH3
With the platinum-metal catalysts, the above reaction can be suppressed by
conducting the hydrogenation in acid solution or in acetic anhydride, which
removes the amine from the equilibrium as its salt or as its acetate. For reactions
with Raney nickel, where acid cannot be used, secondary amine formation is
prevented by addition of ammonia.
Reduction of oximes to primary amines takes place under conditions similar to
those used for nitriles, with palladium or platinum in acid solution, or with Raney
nickel under pressure.
Homogeneous hydrogenation
1) The stereochemistry of reduction may not be easy to predict, since it
depends on chemisorption and not on reactions between molecules. Some
of these difficulties have been overcome by the introduction of soluble
catalysts, which allow hydrogenation in homogeneous solution.
2) A number of soluble-catalyst systems have been used, but the most common
are based on rhodium and ruthenium complexes, such as [(Ph3P)3RhCl]
(Wilkinson’s catalyst) and [(Ph3P)3RuClH].
3) Wilkinson’s catalyst is an extremely efficient catalyst for the homogeneous
hydrogenation of non-conjugated alkenes and alkynes at ordinary
temperature and pressure. Functional groups such as carbonyl, cyano, nitro
and chloro are not reduced under these conditions. Mono- and disubstituted
double bonds are reduced much more rapidly than tri- or tetrasubstituted
ones, permitting the partial hydrogenation of compounds containing
different kinds of double bonds.

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H2, [(Ph3P)3RhCl]
benzene
80 %
H2, [(Ph3P)3RhCl]
benzene
4) An important practical advantage of this catalyst is that it does not bring
about hydrogenolysis, thus allowing the selective hydrogenation of carbon–
carbon double bonds without hydrogenolysis of other susceptible groups in
the molecule.
H2, [(Ph3P)3RhCl]
benzene
93 %
5) Wilkinson’s catalyst has a strong affinity for carbon monoxide and
decarbonylates aldehydes, therefore alkene compounds containing
aldehyde groups cannot normally be hydrogenated with this catalyst under
the usual conditions. For example, cinnamaldehyde is converted into styrene
in 65% yield, and benzoyl chloride gives chlorobenzene in 90% yield.
6) Addition of hydrogen to Wilkinson’s catalyst promotes oxidative addition of
hydrogen. Dissociation of a bulky phosphine ligand and co-ordination of the
alkene is followed by stepwise stereospecific cis transfer of the two hydrogen
atoms from the metal to the alkene by way of an intermediate with a carbon–
metal bond.
7) Diffusion of the saturated substrate away from the transfer site allows the
released complex to combine with dissolved hydrogen and repeat the
catalytic reduction cycle.
8) Another useful catalyst is the iridium complex [Ir(COD)py(PCy3)]PF4 (COD =
1,5-cyclooctadiene; py = pyridine; Cy = cyclohexyl). It reduces tri- and

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tetrasubstituted double bonds as well as mono- and disubstituted ones,
although not so rapidly, and it appears to be unaffected by sulphur in the
molecule. A valuable feature of this catalyst is the high degree of stereo
control that can be achieved in the hydrogenation of cyclic allylic and
homoallylic alcohols. (Hydrogens add from the same side as that of hydroxyl
group).
H2, [Ir(COD)py(Pcy3)]PF4
H2, [Ir(COD)py(Pcy3)]PF4
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