hydride sources do not reduce alkene double bonds. Therefore, when a
compound contains both a C À À
À À O group and a C À À
À À C bond, selective reduction
of one functional group can be achieved by proper choice of the reagent.
CH C CH 2
O
C
O
C
H 2
CH CHCH 2 C
O
C
H 2
OH
CH CHCH 2 CH 2 OH
C
H 2
OH
NaBH 4
i. LiAlH 4
ii. H 3 O +
OCH 3
EtOH
OCH 3
Stereochemistry of hydride reduction Hydride converts a planar sp
2
-
hybridized carbonyl carbon to a tetrahedral sp
3 -hybridized carbon. Thus,
hydride reduction of an achiral ketone with LiAlH 4 or NaBH 4 gives a
racemic mixture of alcohol when a new stereocentre is formed.
C
O
C 2 H 5
C
H 3
C
C
H 3
C 2 H 5
O
H
C
C
H 3
C 2 H 5
H
O
C
H 3
C 2 H 5
OH
H
C
H 3
C 2 H 5
H
OH
H
H
Attack from bottom face
Attack from top face
(R)-2-Butanol
(S)-2-Butanol
LiAlH 4
LiAlH 4
H 3 O +
H 3 O +
5.7.17 Clemmensen reduction: preparation of alkanes
This method is used for the reduction of acyl benzenes to alkyl benzenes,
but it also reduces aldehydes and ketones to alkanes.
C R
O
CH 2 R
Zn(Hg), HCl
Acyl benzene
Alkyl benzene
H 2 O
C Y
R
O
Y
CH 2
R
Y = H or R
Alkane
Zn(Hg), HCl
H 2 O
Sometimes the acidic conditions used in the Clemmensen reduction are
unsuitable for certain molecules. In these cases, Wolff–Kishner reduction is
employed, which occurs in basic conditions.
5.7.18 Wolff–Kishner reduction: preparation of alkanes
This method reduces acyl benzenes as well as aldehydes and ketones, but
does not reduce alkenes, alkynes or carboxylic acids. Hydrazine reacts with
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CH5 ORGANIC REACTIONS