INTRODUCTION

Reduction is addition of hydrogen to an unsaturated group (such as

carbon-carbon double bond, a carbonyl group or an aromatic ring). In
other words, reduction means hydrogenation or hydrogenolysis.

Reduction can be carried out in following ways:

(a) Metal hydrides reduction

(b) Metal/proton (acid) reduction
(c) Catalytic hydrogenation
(d) Miscellaneous Reductions
 Lithium aluminium hydride(LiA H4) LAH

LAH Can reduce those functional group which has multiple bond in b/w
two different electronegative atom

O
|| H
LAH
C CH3—C—CH3
H3C CH3
OH
 Mechanism

LiAlH4 → Li+ + AlH4–

AlH4– → AlH3 + H–

H H
H― H
CH3—C—CH3 CH3—CH—CH3 From H2O
CH3—C—CH3
⊝O
O OH
Key Point

LiAlH4 (LAH) Lithium aluminium hydride [LiAlH4 / Ether or THF] :

LAH is most common and versatile reducing reagent. It is sensitive to
protic solvent and therefore used in ether.

4RCHO + LiAlH4 4R ― CH2OH + LiOH + Al(OH)3

Mechanism of LiAlH4

∶O∶
⊝
Li+ O Li+
H H
C Al R C H + AlH3
R H H H
H

Li+
Li+ O
H
R―H2C―O H R―C―H
⊝

Al R―CH2―O ―Al H
H H

O Li+
⊝
R―H2C―O ⊝ H 2RCHO
C Al
+R H R―H2C―O H
H Li
 Mechanism of LiAlH4

R―CH2OH OCH2―R
Al Li+
R―CH2O OCH2―R

H2O
4R―CH2OH

4RCHO + LiAlH4 4R ― CH2OH + LiOH + Al(OH)3

EXAMPLE

O O
|| LAH || LAH
C C RCH2 — OH
R Z R H

Where Z = —OR,-OH, —Cl

 O OH
LAH
4R—C—R 4R—CH—R + LiOH + Al (OH)3

O
LAH
2R—C—OR’ 2R—CH2—OH + 2R’OH

O
3LAH
4R—C—OH 4R—CH2—OH

O
LAH
2R—C—Cl 2R—CH2—OH

O O
LAH
R—C—O —C—R 2R—CH2OH
EXAMPLE

O
LAH
R—C—OH R—CH2—OH

O O
LAH
R—C—O —C—R 2R—CH2OH
EXAMPLE

O
|| LAH
R C NH2 R—CH2—NH2

|
EXAMPLE

LAH
R — CH=CH2 X

LAH
R—N3 R — NH2 + N2

LAH
R — CH2 — NO2 R — CH2 — NH2

LAH
R—C N R — CH2 — NH2
EXAMPLE
O
HO OH
O LAH

Cyclic ester
(Lactones)
EXAMPLE

O O—
|| LAH | H2 O
C C RCH2 — OH
R H |
R H H

D2O

RCH2 — OD
Key Point

Alkene, alkyne, benzene rings are not reduced by LiAlH4 but it is

reported that Cinnamic double bond can be reduced by LiAlH4 / THF in
few cases like :

(i) LiAlH4–THF
Ph—CH=CH—CHO Ph—CH2—CH2—CH2OH
(ii) H2O
[Cinnamaldehyde]
LAH can reduce 1∘ and 2∘ alkyl halide into Alkane but not 3∘

LAH
CH3—CH2—Cl CH3—CH3

LAH
CH3 — CH—Cl CH3—CH2—CH3

—
CH3

CH3
— — LiAlH4
CH3
CH3—C—Cl CH2=C
E2 CH3
CH3
Reagent Reactant Product

LiAlH4 Aldehyde 1° alcohol

LiAlH4 Ketone 2° alcohol

LiAlH4 Acid 1° alcohol

LiAlH4 Acid anhydride 1° alcohol

LiAlH4 Acid chloride 1° alcohol

Reagent Reactant Product

LiAlH4 Ester 1° alcohol

LiAlH4 Cyanide 1° amine

LiAlH4 Amide 1° amine

LiAlH4 Isocyanide 2° amine

LiAlH4 Nitro 1° amine

❑ NCERT REFERENCE
 EXAMPLE

The major product of the following reaction is

LiAlH4
CH3CH=CHCO2CH3 [JEE-MAIN-2019]

(1) CH3CH2CH2CHO (2) CH3CH=CHCH2OH

(3) CH3CH2CH2CO2CH3 (4) CH3CH2CH2CH2OH

Solution

Ans : (2)
Example [JEE 2014]

In the reaction,
LiAlH4 PCl5 AlC. KOH
CH3COOH A B C, Final product C is.

(A) Ethylene (B) Acetyl chloride

(C) Acetaldehyde (D) Acetylene

Solution

Ans. (A)
Sodium borohydride [NaBH4 / C2H5OH or Ether]

It is more specific than LAH as a reducing agent.

It reduces ketones and aldehydes to the corresponding alcohols

without affecting other functional groups, reduces acid chlorides to 1°
alcohols.

It is effective even in protic solvent like alcohol.

Weak reducing agent as compare to LAH

EXAMPLE

O
|| NaBH4
R C H RCH2 — OH

|
|
O OH
|| NaBH4 |
R C R R CH R

|
|
|
O
|| NaBH4
R C Cl RCH2 — OH
|
|

N TE NaBH4 can also reduce imine group into amine.

EXAMPLE

The major product of the following reaction is: [JEE-MAIN-2019]

O
NaBH4
EtOH

OH OH O OH

(1) (2) (3) (4)

OEt

Solution

Ans : (4)
EXAMPLE

The major product of the following reaction is: [JEE-MAIN-2019]

O
CH3N NaBH4

OH OH
(1) CH3N (2) CH3N

OH OH
(3) CH3N (4) CH3N

Solution

Ans : (3)
EXAMPLE

NO2 LAH NH2

Cl—C HOH2C
||
O SBH

NO2
HOH2C
EXAMPLE
NH

H3O+ LAH
A B
O

O
Solution
O OH

OH OH
COOH H2C
OH
(A) (B)
EXAMPLE

O
|| LiAlH4
—CH = CH — C—OH —CH2—CH2—CH2OH
EXAMPLE

NO2 NH2

LAH

HOOC C—H HOH2C CH2OH

||
O
SBH

NO2

HOOC CH2OH
EXAMPLE
NH2
NH LAH
HOH2C

COOH NH2
SBH

COOH
EXAMPLE
NH

H3O+ LAH
A B
O

Solution
O OH

OH OH
COOH H2C
OH
(A) (B)
Example [JEE 2014]

Consider all possible isomeric ketones including stereoisomers of

MW = 100, All these isomers are independently reacted with NaBH4 (NOTE :
stereoisomers are also reacted separately).
The total number of ketones that give a racemic product(s) is/are.

Solution

Ans. (5)
EXAMPLE
Which of the following reactions is/ are correct?
O OH
O LiAIH4 OH LiAIH4
(A) HO
(B) CH3—C  N CH3—CH2—NH2
2 H2O

O OH
O
CH2—OH O O O O
LiAlH4
(C) O (D) NaBH4
H2O H2O
CH2—OH
O O OH
Solution

Ans. (A,B,C,D)
❑ NCERT REFERENCE
Diisobutyl Aluminium Hydride [DIBAL-H / Inert solvent]

Al DIBAL-H is used at —78°C.

H

O O
DIBAL-H
R—C—OR R—C—H
—78°C
Diisobutyl aluminium hydride is parallel to LAH (Lithium aluminium hydride)
as a reducing agent but it is more selective.

Reagent DIBAL-H/(-78°C) DIBAL-H/(-78°C)

Reactant Ester Cyanide
Product Aldehyde Aldehyde
EXAMPLE
O
DIBAL-H H2O/H+
R—C ≡ N —78°C
R—CH = NH R—C—H

O O
DIBAL-H
R—C—Cl R—C—H
—78°C

O
DIBAL-H
R—C—H R—CH2—OH
Room
temperature
Mechanism of DIBAL H

iBu
iBu 
⊝
O Al — iBu
|| O
H — Al — iBu || H
C
C
R Cl
R Cl

iBu
O OH Al — iBu
|| | O
H2O |
C C
| C
R H R Cl |
H R Cl
H
Stable Tetrahedral complex
(at —78°C)
 EXAMPLE

LiAlH4-THF
H2O Ph—CH2—CH2—CH2OH + C2H5OH
(a) Ph—CH=CH—COOC2H5
DIBAL-H
—78°C
Ph—CH = CH2—CHO + C2H5OH
(b) By DIBAL at ordinary temperature esters are reduced to alcohols but at
low temperature esters are reduced to aldehyde.
DIBAL
O 25°C
C6H5CH2OH
||
C6H5—C—OCH3
DIBAL
C6H5CHO
—78°C

(c) LAH reduce RCN to amine but DIBAL reduces it to aldehyde.

LiAlH4-THF
CH3—CH2—NH2
H2O

CH3—C≡N
DIBAL
CH3—CHO
—78°C
❑ NCERT REFERENCE
 EXAMPLE

The major product of following reaction is : [JEE-Main-2019]

(1) AlH (i-Bu)2
R—C≡N ?
(2) H2O
(1) RCHO (2) RCOOH (3) RCH2NH2 (4) RCONH2

Solution

Ans : (1)
EXAMPLE
O
O
(i) DIBAl—H, Toluene, —78C
(ii) H3O
“P” [JEE-MAIN-2021]
(Major Product)

The product "P" in the above reaction is :

OH OH

(1) COOH (2) CHO

O
O—C—H

(3) (4) CHO

Solution
Ans : (2)
 B2H6 or BH3/THF

B2H6/THF gives BH3 which is a electrophile(Lewis acid) so B2H6 react

with Nucleophile except ester.
H H H
B B + THF 2BH3 + THF
H H H
It does not reduce electrophilic functional groups.
E.g.
O
||
R—C—X, R — NO2 , R—X
Reduction by B2H6

Reactant Remark Product

O
|| Not Reduced
R—C—X

R — NO2 Not Reduced

R—X Not Reduced

Reduction by B2H6

Reactant Remark Product

O
|| Reduced — CH2OH
—C—H

O OH
|| Reduced |
—C— — CH —

O
|| Reduced — CH2OH
R— C — OH
O
|| Reduced R — CH2 — NH2
R — C — NH2
N TE

Borone is a highly chemoseletive reagent for the reduction of carboxylic

acids in the presence of other reducible functional groups.
EXAMPLE
The most suitable reagent for the given conversion is : [JEE-MAIN-2020]
CH3 CH3
CONH2 CONH2
C=O CO
?

HO2C HOH2C
CN CN
(1) LiAlH4 (2) NaBH4 (3) H2/Pd (4) B2H6
Solution

Ans : (4)
 Reduction of Alkene by B2H6

(i) BH3 / THF

R—CH=CH2 R—CH—CH3
(ii) CH3COOH or H2O
H
(Alkane)

N TE

(i) BH3 / THF

R—CH=CH2 R—CH—CH2
(ii) H2O2/ OH
H OH
(Alcohol)
reduction of ketone by B2H6

R R
H2 O
O O OH
H R R
H BH2 H
H B
H
❑ NCERT REFERENCE
EXAMPLE

LAH CH2OH
HOH2C

SBH
COOH
COOH HOH2C
OHC

B2H6 CH2OH
CH3COOH HOH2C
Catalytic hydrogenation

Hydrogenation using H2(g) on metal surface or other suitable catalyst

is called catalytic hydrogenation. A catalyst provides a new pathway
for the reaction that involves lower free energy of activation.

Ni or Pd
R—CH=CH—R + H2 R—CH2—CH2—R + heat
 Catalytic hydrogenation

No catalyst

G(1) (hypothetical)
H2+
C=C Catalyst present
(usually multistep)

Free energy

G (2)
Hº = — 120kJ mol—1

—CH—CH—

 
G(1) G (2)

Reaction coordinate
The catalysts used can be divided into two broad classes,
(a) Heterogeneous catalysts
(b) Homogeneous catalysts

Both of which mainly consist of transition metals and their compounds

:
 Heterogeneous catalysts

(i) Heterogeneous catalysts : (catalysts insoluble in the reaction medium)

In heterogeneous catalytic hydrogenation, catalysts are used in

powdered form. Ni,Pt,Pd,Raney nickel, Palladium on charcoal (Pd/C),
PtO2(Adam’s Catalyst) are common heterogeneous catalysts.

Raney Nickel  Is a solid substance that is made up of nickel

obtained from an alloy of nickel and aluminium.
 Homogeneous catalysts

Homogeneous catalysis : (catalysts soluble in the reaction medium).

It uses reactants and catalyst in the same phase.
example : Wilkinsion catalyst

Reagent - [(Ph3P)3RhCl] + H2

• Reduce only alkene or alkyne

Facts of catalysis :

(i) Substrate molecules are assumed to undergo homolysis into atoms at

the surface of the catalyst.

(ii) The substrate is chemisorbed on the surface of the catalyst and

hydrogenation takes place.

(iii) The process is exothermic.

(iv) It gives Syn addition

Alkenes and alkynes on catalytic hydrogenation gives alkanes

Ni, Pt or Pd
C==C + H2 CH—CH

Alkene Alkane

—C C— + 2H2
Ni, Pt or Pd
—CH2—CH2—

Alkyne Alkane
N TE

Greater the crowding at double bond carbon more difficult the

hydrogenation

Rate of Hydrogenation

R R H
CH2 == CH2 > RCH == CH2 > C == CH2 > C == C
R R R
R R
> C == C
R R
 REACTIVITY

O O O
|| || ||
—C—X > — NO2 > — C N > —C CH > —C—H > —C— >— CH = NH

O O
|| ||
> — CH = CH2 > R—C—OR > R—C—NH2

Higher electrophilicity of electron deficient carbon more reactive for

hydrogenation.

They do not reduce —COOH .

Hydrogenation Products

Substrate Product

RCOCl RCH2OH

RNO2 RNH2

RC ≡CR RCH2CH2R

RCHO RCH2OH

RCH=CHR RCH2CH2R

RCOR RCHOHR

RCN RCH2NH2
❑ NCERT REFERENCE
EXAMPLE
O
CN H2/Pd

HO CH2–NH2 OH
CH2–NH2
I
(1) (2)

HO CH2–NH2 O
H CH2–NH2
(3) (4)

Solution

Ans : (2)
EXAMPLE
D

D2/Ni/ CH2—D
1 eq.
EXAMPLE
Reagent(s) which can be used to bring about the following transformation
is (are) (JEE Advanced 2016)

O O O O
C O C

O H O OH
COOH COOH
(1) LiAlH4 in (C2H5)2O (2) BH3 in THF
(3) NaBH4 in C2H5OH (4) Raney Ni/H2 in THF

Solution

Ans : (3,4)
EXAMPLE

The major product of the following reaction is: [JEE-MAIN-2019]

O

OEt (i) Ni/H2

CN (ii) DIBAL—H

(1) O (2) N
CHO

(3) NH (4) OH
NH2
Solution
O
O
C
OEt OEt
H2/Ni
CN CH2—NH2

DIBAL-H

O
C
N H
CH2—NH2

Ans : (2)
N TE

H2/Pt
EXAMPLE

H2/Ni(eq)
1 eq.

(1,4 addition)
Solution
EXAMPLE
LAH CH2OH
HOH2C

SBH COOH
HOH2C
COOH
OHC
H2/Ni COOH
HOH2C

B2H6 CH2OH
CH3COOH HOH2C
EXAMPLE
CH3 CH3
CH3
CH3

H2/Ni


CH3 CH3
CH3 CH3
3 products
EXAMPLE

CH3 CH3 CH3

H2/Ni


CH3 CH3 CH3

2 products
Lindlar’s catalyst (H2/Pd,CaCO3,quinoline)

It is a poisoned palladium catalyst. It is composed of powdered

calcium carbonate coated with palladium and poisoned with
quinoline. It is used to carry out partial reduction of alkyne to alkene
and acid halide to aldehyde.

Stereochemistry : Syn addition.

R1 R2
C =C
Pd/CaCO3
R1—CC—R2 + H2 S-quinoline
H H
cis-Alkene
 Hydrogenation in presence of H2/Pd/BaSO4 reduces alkyne to alkene
and acid halide to aldehyde. It is poisoned palladium catalyst,
composed of powdered barium sulphate coated with palladium,
poisoned with quinoline or sulphur.

Stereochemistry : Syn addition.

R1 R2
R—CC—R + H2
Pd/BaSO4
S-quinoline C= C
H H
cis-Alkene
EXAMPLE

H3C CH3
H2,Pd,BaSO4,S
(a) CH3—CC—CH3 C C
H H
cis-2-butene

O
|| H2,Pd,BaSO4,S
(b) CH3—C—Cl CH3—CHO

N TE

Hydrogenation of acyl chloride gives aldehyde known as the Rosenmund

reduction.
Example [IIT 1992]

Hydrogenation of benzoyl chloride in the presence of Pd and BaSO4 gives :

(A) Benzyl alcohol (B) Benzaldehyde

(C) Benzoic acid (D) Phenol

Solution

Ans. (B)
 H
H

H
EXAMPLE (P)
H2/Pd/CaCO3 /Quinoline
P Product(s)
The number of isomeric product (s) obtained in this reaction is
(A) 1 (B) 2 (C) 4 (D) 8

Solution

Ans. (A)
❑ NCERT REFERENCE
 P2 catalyst (Ni2B/H2)

Nickel boride Ni2B (P-2 catalyst) (made from Nickel acetate and
sodium borohydride) is an excellent alternative catalyst for the
conversion of alkyne into alkene.
Stereochemistry : Syn addition.
NaBH4
Ni(OCOCH3)2 Ni2B.
C2H5OH

R R
Ni2B/H2
R—C C—R C=C
H H
Syn addition
EXAMPLE

R—CN
H2−Pd/BaSO4
S-quinoline
R—CH=NH
EXAMPLE

Pd—C / Quinoline CH = CH2

H2

Pt / H2
EXAMPLE

Me H
Me
Me H
Me
Me H
H H
H2/Pd-BaSO4
Me S-quinoline
H H
H
 REDUCING AGENTS AND THEIR ROLE
LAH in NaBH4 B2H6
Group Product H2 / Catalyst D
Ether in H2O in THF
—CHO —CH2OH + + + +
>C = O >CH — OH + + + +
—CO2H —CH2OH + — + —
—CO2R —CH2OH + — + +
—COCl —CH2OH + + — +
—CONH2 —CH2NH2 + — + +
(RCO)2O RCH2OH + — + +
—CN —CH2NH2 + — + +
>C = NOH —CH2NH2 + — — +
>C = C< >CH—CH< — — + +
—C  C— —CH=CH— — — + +
1°RX RH + — — +
 Metal/proton (acid) reduction

Reduction by dissolving metals is based on the fact that the metal acts
as a source of electrons.

Step-1
Metals give electrons to the electrophilic species and form anion

Step-2

Proton is abstracted from the acidic source.

—
e — H
A—B A• + B: A• + B—H

—
e
⊝
H
A=B A• — B: A• —
BH
EXAMPLE

(1) LAH

(2) Ni/H2
R—NO2 R—NH2
(3) Zn + HCl

(4) Fe + HCl

(5) Cu + HCl Metal + acid

(6) Sn + HCl

(7) TiCl3 + HCl

❑ NCERT REFERENCE
 Reduction of alkyl halide by metal and acid

Reducing agent
R—X R—H

Reducing agents may be

 Zn + HCl

 Sn + HCl
Metal + acid
 Fe + HCl
 Zn + CH3COOH
❑ NCERT REFERENCE
EXAMPLE

Zn/H+
1 CH3—CH2—Cl CH3—CH2—H

Zn/H+ CH3—CH2—CH2—H
2 CH3—CH2—CH2—Cl

Zn/H+
3 CH3—CH2—CH—CH3 CH3—CH2—CH2—CH3
Br

CH3 Zn/H+ CH3

4 CH—CH2CH2—I CH—CH2CH3
CH3 CH3
 Mendius reduction (complete reduction)

N TE
Alkyl cyanides ( nitriles ) can be reduced to 1° amine and isocyanides are
reduced to 2° amine .
Reagent — 4H + Na/ROH

4H + Na/ROH

LAH
R―C≡N R―CH2―NH2

H2/Ni or Pt

Sn/HCl
❑ NCERT REFERENCE
Mendius reduction (complete reduction)

4H + Na/ROH

LAH

 ⊝
R―N≡C R―NH―CH3
H2/Ni or Pt

Sn/HCl
 Birch reduction

[Na or Li/NH3(liq.) or (ethyl alcohol)] :

Alkyne and Aromatic Compounds are reduced by Na or Li/NH3.
Alkynes are reduced to trans alkene.
Stereochemistry : anti-addition.

Na/NH3
R H
R—C  C—R C C (anti addition)
H R
 Birch reduction

R1 H
Na/Liq. NH3
R1—C ≡ C—R2 C= C

H R2
Alkyne trans-Alkene

Mechanism

R1 R1 H
Na C=C H−NH2 C =
R1−C≡C−R2
R2 C R2
To minimize the
e— repulsion Na

R1 H R1 H
C= C H2N −H C=C
H R2 R2
Example [JEE 2014]

The reagent needed for converting is :

Ph H
Ph–CC–Ph C== C
H Ph

(A) H2 / Lindlar Cat. (B) Cat. hydrogenation

(C) LiAlH4 (D) Li / NH3

Solution

Ans. (D)
Example [JEE-Advance 2022]

The number of isomeric tetraenes (NOT containing sp-hybridized carbon

atoms) that can be formed from the following reaction sequence is
________.
1. Na, liquid NH3
2. Br2 (excess)
3. alc. KOH
Solution

Ans. (2)
❑ NCERT REFERENCE
 BIRCH REDUCTION IN CONJUGATED DOUBLE BOND

H
2
4 Na/liqNH3
1
3 
H

Mechanism
Na
⊝
⊝

⊝
Na liqNH3

H2C

H
liqNH3 CH3

H3C
 Birch reduction in Benzene Ring

Benzene ring is reduced at 1, 4-position.

Typical example of reduction for aromatic system :
Presence of alkyl, alkoxy, amines reduces the benzene ring at ortho
position.

Na/ liq NH3


Mechanism

Na

H H

liq NH3
liq NH3 Na
 

H H H H H H
 Birch reduction

(electron withdrawing group

G = —NO2, —COOH, —CHO, —CN)
G
G H
Na/NH3


H H
G = E.W.G

Presence of nitro, cyano, carboxylic, keto or aldehyded group reduces the

benzene ring at ipso position.
Mechanism

G G G

Na liq. NH3


G = E.W.G
G G G

liq. NH3 Na


H H H H H H
 Birch reduction

(electron releasing group

—R, —OR, —NH2)

G G
H
Na/NH3 H

H
H
G = E.D.G
 Mechanism

G = E.D.G

G G G

Na liq. NH3


G
G G
H
liq. NH3 Na
H  H
H H
H H
H
Rate of reaction with Na/liq NH3

EDG EWG

(i) (ii) (iii)

de-stabilize stabilize
carbanion carbanion
formed formed

(iii) > (i) > (ii)

EXAMPLE

COOH COOH

Na/liq. NH3


NH2 NH2

Na/liqNH3

 Bouvealt-Blanc reduction

Reagent-[Na/C2H5OH]

Reduction of aldehydes, ketones, acid halides, esters or cyanide by

means of excess of Na/C2H5OH is called Bouvealt-Blanc reduction.

Reduces all which are reduced by LAH except -COOH

O OH
Na/EtOH
C C
Reagent Na/C2H5OH Na/C2H5OH Na/C2H5OH Na/C2H5OH Na/C2H5OH

Reactant Aldehyde Ketone Cyanide Ester acid halide

Product 1° alcohol 2° alcohol 1° amine 1° alcohol 1° alcohol

Mechanism

⊝ ⊝
O O OH OH O
Na EtOH Na IMPE
C C C C CH
⊝

EtOH

IMPE = intramolecular proton exchange

OH
C
EXAMPLE

Me Me
Na/EtOH
R―O ― C C―OH HOH2C C―OH + ROH
O O O
Stephen’s Reduction (SnCl2/HCl)

Partial reduction

Reagent - SnCl2 / HCl followed by H3O

O
(i) SnCl2/HCl (ii) H3 O+
R—C  N R—CH = NH R—C—H
 Stephen’s Reductions

When reduction of cyanide is carried out with acidified stannous

chloride (SnCl2/HCl) at room temperature, imine hydrochloride is
obtained. Which on subsequent hydrolysis with boiling water gives
aldehyde. This specific type of reduction of nitrile is called stephen’s
reduction.

LiAlH4 or Na/C2H5OH
R—CH2NH2
(Complete Reduction)
(1°Amine)
R—CN
(i) SnCl2/HCl (ii) H2O
(Partial Reduction) R—CHO
Aldehyde
❑ NCERT REFERENCE
EXAMPLE

Sn/HCl
R—CH2NH2
(Complete Reduction) (1°Amine)
R—CN
(i) SnCl2/HCl (ii) H2O
R—CHO
(Partial Reduction) Aldehyde
 EXAMPLE

CN CH2NH2
(i) SnCl2 + HCl(g)
1.
(ii) LiAlH4

CN CH2OH
(i) SnCl2 + HCl(aq)
2.
(ii) LiAlH4
 Clemmensen Reduction

It is used to prepare alkanes from carbonyl compounds (Aldehyde and

ketones) in absence of acid sensitive groups.

Reagent : Zn-Hg/Conc. HCl

Function : Aldehyde/Ketone to Alkane

O
Zn—Hg/Conc. HCl
CH3—C—CH3 
CH3—CH2—CH3

O
In this reaction —C— is converted into —CH2—
 Mechanism

2
  Zn → Zn + 2e⊝
⊝
2H
C=O H C = O—H C—OH C—OH

sp2
 ⊝ Zn → Zn2 + 2e⊝  
H CH—OH2
CH2 CH CH
—H2O

sp3
 Zn—Hg/conc.HCl
(a) R — CHO RCH3 + H2O


O
|| Zn—Hg/conc.HCl
(b) R — C — R’ RCH2R’ + H2O

N TE

If other acid sensitive group is also attached then H+ will react

with that group.

Clemmensen reduction is avoided for compounds which have

acid sensitive group.
[Like: Alcohol, Alkene, Alkyne, Ether, Amine, Amide].
EXAMPLE
O
C—CH3 CH2—CH3
Zn—Hg
Conc. HCl

O
C—CH3 CH2—CH3
Zn—Hg
Conc. HCl

HO Cl
❑ NCERT REFERENCE
EXAMPLE

H Zn—Hg
Conc. HCl
O O CH3
 Wolff-Kishner Reduction

Reagent : N2H4/OH/
H2C—OH
H2C—OH
Function : Aldehyde/Ketone to Alkane
Using high boiling
solvent (ethylene glycol)

O
NH2NH2/OH
CH3—C—CH3 CH3—CH2—CH3 + N2

 Mechanism

-H2O •• ⊝
C = O + H2 N—NH2 C = N—NH2
B
 C = N—NH
 2 —BH
sp

 
BH ⊝ ⊝ ••
B BH ⊝
CH2 •• CH CH—N = N CH—N = NH •• C —N = NH
—B -N2  —B
 —BH (A)
sp3
N TE

It reduce aldehydes and ketones to alkanes, but

does not reduce alkene and alkyne

This is a base catalysed reaction ,so

simultaneously dehydrohalogenation (-HX) of RX
also take place

N TE

Wolff-kishner reduction is avoided for compounds which posses base

sensitive groups. [Like : Halogens, Acid halide, Esters, Anhydride, Acid]
EXAMPLE

N2H4/OH—

H
X
Mechanism

O
NH2NH2 OH ⊖ ⊖
C C==N—NH2 C==N—NH
Hydrazone

⊖ OH ⊖ H2O ⊖
C—N==N C—N==NH C—N==NH
H H

⊖
H2O
H
C + N2 C
H
H
❑ NCERT REFERENCE
EXAMPLE
N2 H 4
OH—

Cl
O
Zn - Hg
Con. HCl
 EXAMPLE
Given below are two statements : [JEE Mains–2023]
O
Statement I : H2 N under Clemmensen reduction HOOC
O conditions will give
O
Statement II : under Wolff-Kishner reduction
Cl condition will give Cl

In the light of the above statements, choose the correct answer from the
options given below :
(1) Statement I is false but Statement II is true
(2) Both Statement I and Statement II are false
(3) Statement I is true but Statement II is false
(4) Both Statement I and Statement II are true
Solution
Ans. (3)
 Reduction by transfer hydrogenation

Reagent N2H4/H2O2 Or N2H2

NH2—NH2 C—C
C== C H2O2
H H
Syn addition
Mechanism

H H
H—N—N—H + H2O2 H—N == N—H + 2H2O
di-imide
H
C N C H
+ N N
C N C H
H
6 MCTS
 EXAMPLE

H CN
N2 H 2 C
H2O2
O

N2 H 4 CN
H CN H3C
C OH—

LAH
CH2NH2
HOH2C
Example [IIT 2000]

The appropriate reagent for the following transformation:

O
CH2CH3
CH3
HO HO
(A) Zn(Hg), HCl (B) NH2NH2, OH⊝
(C) H2/Ni (D) NaBH4

Solution

Ans. (B)
 Mozingo Reaction

SH
Reagent = + Dry HCl, followed
By Raney Ni
SH
EXAMPLE

+O=C
HS SH S

Raney Ni/H2

SH H
+ C
H
SH
 MPV Reduction

Meerwein-Pondorf-Verley reduction (MPV reduction) (Reduction by

isopropyl alcohol and aluminium isopropoxide): It is selective
reduction of ketones and aldehydes to alcohol, even in the presence of
other functional groups using Aluminium isopropoxide in isopropyl
alcohol.

Al(OCHMe2)3
R—C—R’ + CH3—CH—CH3 R—CH—R’ + CH3—C—CH3
|| | | ||
O OH OH O
 CH3
O OH Al —O—CH OH O
CH3
3
C + CH CH + C
R R CH3 CH3 R R CH3 CH3

In this reaction, carbonyl is reduced into corresponding alcohol by

aluminium isopropoxide taken in isopropanol.
EXAMPLE

Al(OCHMe2)3
+ CH3—CH—CH3
|
OH

OH

+ CH3—C—CH3
||
O
 Reduction by Red P and HI

Red P +HI
R—I R—H + I2


Mechanism

HI/red P H+I
HI


HI/red P
R—I R+I


HI/red P
R+H R—H


HI/red P
H+I H—I


2I I2
 Reduction by Red P and HI

All organic compound containing O convert into Alkane Containing N

Convert into amine
Red P +HI (2 mole)
R—OH 
R—H

O
Red P +HI (4 mole)
R—C—R R—CH2 — R


O
Red P +HI (6 mole)
R—C—OH R—CH3

EXAMPLE

O
Red P +HI
R—CH2—O —C—R 
2R—CH3

O
Red P +HI
R—C—NH2 R—CH2 —NH2


Red P +HI
R—CN R—CH2 —NH2


O
Red P +HI
R—C—Cl 
R—CH3
 No. of moles of (Red P + HI ) required for Reduction of alcohol,
aldehyde, ketone, carboxylic acid in presence of Red P and HI

Alcohol

Red P +HI Red P +HI

R—OH 
R—I 
R—H + I2

No. of moles of (Red P +HI) required = 2

Aldehyde

O OH OH
Red P +HI Red P +HI
R—C—H  R—C—H  R—C—H
I H
 Red P +HI

H I
Red P +HI
R—C—H 
R—C—H
H H

No. of moles of (Red P + HI) required = 4

Ketone

O OH OH
Red P +HI Red P +HI
R—C—R  R—C—R  R—C—R
I H
 Red P +HI

H I
Red P +HI
R—C—R  R—C—R
H H

No. of moles of (Red P + HI) required = 4

Carboxylic Acid

O O OH
2 (Red P +HI) Red P +HI
R—C—OH 
R—C—H  R—C—H
I
 Red P +HI

H I OH
Red P +HI Red P +HI
R—C—H R—C—H R—C—H
 
H H H

No. of moles of (Red P +HI) required = 6

EXAMPLE

Calculate the number of moles of (Red P +HI )required for reduction of :

CH2−COOH
I
X(Red P +HI)

I−H2C CH2−COCH3
OH

Solution

Ans : (14)
EXAMPLE
Calculate the number of moles of (Red P +HI )required for reduction of :
O
C−H
CH−OH
X(Red P +HI)
CH−OH 

CH−OH

CH−OH

CH2−OH

Solution

Ans : (14)
EXAMPLE
Which of the following reagent can be used for this conversion ?
O

(1) Zn—Hg/ con. HCl (2) NH2NH2/OH/

(3) Red P + HI (4) All of these

Solution

Ans : (4)
EXAMPLE

CN
OH
(i) H+
+ O (ii) LAH
(iii) H3O+
OH
Solution

OH CN CN
O
H+
+ O
OH O

Used as protecting reagent

LAH

CH2—NH2
O
OH H3O+
+ O
O
OH
CH2—NH2
EXAMPLE
NH2—NH2
CH2== CH—CH2—NO2 (A)
H2O2

H2 Pd

(B)

Solution

(A) CH3—CH2—CH2—NO2

(B) CH3—CH2—CH2—NH2
EXAMPLE

(i) H2/Pd-BaSO4
R—C  N
(ii) H3O+

Solution
O
H3 O+
R–C  N + H2 + Pd/BaSO4 R–CH=NH C
R H
aldehyde
EXAMPLE

D
D
N2 H 4 H
H2O2
H
D
D
EXAMPLE

Conc.HSO4

 

H

1,2- H  Shift

Pt+H2  
EXAMPLE

O OH
|| Br Br
NaBH4

+ H
O
O
MeOH
EXAMPLE

O
|| NaBH4
H—C HOCH2

PBr3

BrCH2
EXAMPLE

LiAlH4
HOOC COOH (A)

NaBH4 O
Ca(OH)2 
(B) (C) (D)
(i) LiAlH4
(ii) D2O

(E)
Br2/CCl4 H3O+
(K) (J)  (I)  H 3 O+
minor
(F) minor
(i) mCPBA
(ii) H3O+

(G)
Solution

(A) HO2HC CH2OH

COOH Ca(OH)2 COO—  O

COOH COO—
(C) (D)
Solution

LiAlH4 D2O
O O— OD
H3O+

(D) OH (E)
H
H
mCPBA
H H3O+ H
OH
(±)
(F)
(G) minor
Solution

O— H2O OH
O (i) NaBD4
D D
(D) (I)

 H3O+

D D
Br Br
D
Br2/CCl4
Br Br
H H
(J)
(K) minor
 Name Reagent Function

H
Wolf kishner (i) N2H4/(ii) KOH, D O
H

Clemenson H
Zn—Hg / HCl O
Reduction H
SH
Mozingo Dry HCl, followed
H
Reduction By Raney Ni O
SH H
Stephen's R—C  N R—CH = O
SnCl2 / HCl followed by H3O
Reduction

CH3 CH3
MPV Reduction Al—O—CH HO—CH O OH
CH3 CH3
 Name Reagent Function

C=C C―C
Hydroboration H H
B2H6/AcOH, H2O
Reduction
O O
H H
Bouvoult Blanc
Na / EtOH R—COO—R RCH2OH+ROH
Reduction

Transfer C=C C―C

N2H4/H2O2
Hydrogenation
H H

O O
Rosenmund R— C— Cl R— C— H
H2,Pd—BaSO4
Reduction — C  C— — C = C—
H H
 Name Reagent Function
H
Birch Reduction Na / Liq. NH3 — C  C— —C=C—
H
—COOR
H—Al—CH2—CH—CH3 —CN
CH3 —COCl —CHO
2 O O
DIBAL-H (—78°C)
followed by H2O —C—O—C—
25°C
—CHO —CH2OH

R—CO2H RCH3
R—CH=O RCH3
Red phosphorus in R—C—R RCH2R
Red P + HI
presence of HI
O
R—OH R—H