This document summarizes various methods of chemical reduction. It discusses heterogeneous catalytic hydrogenation using platinum, palladium, nickel, and copper chromite catalysts. Alkenes are converted to alkanes, and unsaturated functional groups like carbonyls, nitriles, and aromatics can be reduced. Homogenous hydrogenation uses soluble transition metal complexes as catalysts, like rhodium and ruthenium. Rhodium complexes selectively reduce double bonds without affecting other functional groups. Reduction of alkynes, aromatics, aldehydes, ketones, nitriles, oximes and nitro compounds are also summarized. The mechanisms of catalytic and homogenous hydrogenation are provided.

1. REDUCTION
Guided by Prepared by
Ms. Neha Raut Ms. Shweta Raut
M.Pharm 2nd sem .(2019-2020)
Pharmaceutical Chemistry Department
SKB College of Pharmacy, kamptee.

2. Reduction : reduction is defined as addition of hydrogen to
an unsaturated group like a carbon carbon double bond, a carbonyl
group or an aromatic nucleus is known as reduction.
Addition of hydrogen with fission of bond between two atoms is reduction
Eg: conversion of benzyloxy phenol into phenol.
C6H5CH2OC6H5 HOC6H5

3. When an alkene is mixed with hydrogen in
prescence of large no. of catalyst, addition of
hydrogen converts the alkene into an alkane. This
reaction is known as hydrogenation.
Types of hydrogenation:
Hetrogenous Homogenous
hydrogenous hydrogenous

4. Hetrogenous hydrogenous
Catalytic hydrogenation is one of the convenient method for the reduction of organic
cpmpound. The method is consist in stirring the substrate with the catalyst in a suitable
solvent in an atmosphere of hudrogen. Common unsaturated group such as C-C double
bond, C-C triple bond , carbonyl carbon , carboxylic group , nitrile group , nitro group &
aromatic & heterocyclic nuclei can be reduced catalytically under appropriate conditions.
1) Platinum
2) Palladium
3) Nickel
4) Copper chromite

5. Following some of the application of
hetrogenous hydrogenous: highly hindered
alkenes are difficult to reduce and vigorous conditions are
required. The most frequently used catalyst for the reduction
of alkenes are PLATINUM and PALLDIUM catalyst.
Raney nikel is also used in certain cases:
raney Ni/H2
C6H5CH=CHCH2OH C6H5CH2CH2CH2OH
Cinnamyl alcohol EtoH dihydrocinnamyl alcohol
(3-phenylpropanol)

6. In case limoneneif the hydrogenation is stopped after
the absorption of one molecule of hydrogen, the
product obtained p-menthene.
Selective reduction of carbon carbon double bond in compound containing other
unsaturated groups except the presence of triple bond, aromatic nitro compound and
acyl halides.

7. Reduction of alkynes: catalytic
hydrogenation of alkynes gives the completely reduced
, alkynes can also be reduced partially to give Z-alkenes by
palladium-calcium carbonate catalyst deactivated (partially
poisoned) by the addition of lead acetate (LINDLAR CATALYST)
or Pd-BaSO4 deactivated by quinolone.the lead treatment
poisoned the palladium catalyst , rendering it less active and
reaction more selective.

8. Reduction of aromatic compound: Aromatic
compound are difficult to reduced by catalytic hydrogenation
common catalyst for reduction of aromatic compound are platinum,
rhodium and raney nickel.
Benzene reduction with platinum oxide and hydrogen in acetic acid
gives cyclohexane.
Reduction of β-napthol with raney nickel/H2 produce the tetrahydro-β-napthol as
major product.

9. Mandelic acid gives hexahydromandelic acid on reduction with H2 and
rhodium-alumina in methanolic solution, hydrogenolysis to phenyl acetic acis
is the main product with palladium-carbon/H2
Naphthalene can be reduced partially or completely depending upon the reducing
agent , reduction of naphthalene with Pt/H2 gives cis-decalin. However reduction with
Ni/H2 gives trans-decalin.

10. Reduction of aldehyde and ketone: Aliphatic
aldehyde can be easily reduced (RCHO RC H2OH) under
mild condition platinum, raney nickel or ruthenium (in aqueous
solution) aromatic aldehyde and ketones can be reduced to
alcohol with palladium hydrogen. Reduction of carbonyl group
in the prescence of carbon carbon double bond can be
by hydride reducing agent
by Meerwien-Ponndorf-verley method: the reduction of
aldehyde and ketone by heating the carbonyl compounds with
aluminium isopropoxide in the presence of excesss isopropyl
alcohol. the alcohol reduction product, is obtained from the
reaction mixture after acidification, and acetone is generated
from isopropyl alcohol.is called as Meerwein-Ponndorf-Verely
redction.

12. Reduction of nitriles,oximes and nitro compounds: nitriles,
oximes, azides and nitro compound are all reduced to prmiary
The chice of reagent depends on the substrates.

13. Homogenous hydrogenation: homogenious caytalyst are those
that are soluble in reaction mixture. The catalysis occurs not at the surface of
a metal layer, but throughout the solution. Soluble in transition metal
compound containing metal compounds containg organic ligands are used
as homogenous hydrogenation catalysts, they are soluble solvents reaction
which the most effective soluble catalysts are rhodium and ruthenium
complexes clorotris(triphenylphosphine)rhodium (I) [(C6H5)P]3RHCl, and
hydrichlorotris(triphenylphosphine)ruthenium, [(C6H5)P]3RuClH. The
complex is obtained by the reaction of rhodium chloride with excess of
triphenylphosphine in boiling ethanol.this rhodium compound catalyses the
hydrogenation of the terminal double bond, leaving internal double bond
untouched.

14. The rhodium complex is an efficient catalyst for the homogenous
hydrogenation of the conjugated alkenes and alkynes in boiling benzene.
functional group like oxo, cyno ,nitro, chloro and azo are not effected by this
catalyst under the conditions (ordinary temperature and pressure). Reaction
of linalool gives 90% yield of the corresponding dihydro compound.

15. Mechanism:
rhodium complex is dissolve in benzene and hydrogen gas is passed in the solution.
A new octahedral organometallic compound of rhodium it formed that contains metal hydrogen
This complex add on alkene molecule by displacing one of the triphenylphosphine ligands to give π-
complex.
Transfer of hydrogen to C=C bond gives alkane and chlorobis(triphenylphosphine) thodium (I)
the chlorobis(triphenylphosphine) rhodium (I) rects with triphenyphosphamine to regenerate the
chlorotis(triphenylphosphamine)rhodium (I) complex, which start the cycle again.

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