5.7.20 Reduction of carboxylic acids: preparation of primary
alcohols
Carboxylic acids are considerably less reactive than acid chlorides, aldehydes and ketones towards reduction. They cannot be reduced by catalytic
hydrogenation or sodium borohydride (NaBH 4 ) reduction. They require the
use of a powerful reducing agent, e.g. LiAlH 4 . The reaction needs two
hydrides (H
À ) from LiAlH 4 , since the reaction proceeds through an
aldehyde, but it cannot be stopped at that stage. Aldehydes are more easily
reduced than the carboxylic acids, and LiAlH 4 reduces all the way back to
1
alcohols.
RCH 2 OH
R C OH
O
ii. H 3 O +
i. LiAlH 4
5.7.21 Reduction of acid chlorides
Preparation of primary alcohols
Acid chlorides are easy to reduce than carboxylic acids and other carboxylic
acid derivatives. They are reduced conveniently all the way to 1
alcohols by
metal hydride reagents (NaBH 4 or LiAlH 4 ), as well as by catalytic hydrogenation (H 2 /PdÀ ÀC).
R C
O
Cl
ii. H 3 O +
i. NaBH 4 or LiAlH 4
1 o Alcohol
RCH 2 OH
H 2 /Pd-C
Acid chloride
1 o Alcohol
RCH 2 OH
Preparation of aldehydes
Sterically bulky reducing agents, e.g. lithium tri-t-butoxyaluminium
hydride, can selectively reduce acid chlorides to aldehydes at low temperatures (À78
C). Lithium tri-t-butoxyaluminium hydride, LiAlH(O-t-Bu) 3 ,
has three electronegative oxygen atoms bonded to aluminium, which
makes this reagent less nucleophilic than LiAlH 4 .
R C H
O
R C
O
Cl
(CH 3 ) 3 CO Al
H
OC(CH 3 ) 3
OC(CH 3 ) 3
i. LiAlH(O-t-Bu) 3 , −78 o C
ii. H 3 O +
Aldehyde
Lithium tri-t-butoxyaluminium hydride
Li +
_
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CH5 ORGANIC REACTIONS