Organic Chemistry

Section 18.5 Reactions of Carbonyl Compounds with Hydride Ion 745

The reaction is carried out at (the temperature of a dry ice–acetone bath). At
this cold temperature, the initially formed tetrahedral intermediate is stable, so it does
not eliminate the alkoxide ion. All of the unreacted hydride donor is removed from the
solution before the solution warms up. Therefore, when the tetrahedral intermediate
eliminates the alkoxide ion, there is no reducing agent to react with the aldehyde. If,
however, the reaction is carried out at room temperature, a primary alcohol is
obtained—the same product obtained when an ester reacts with
The reaction of a carboxylic acid with forms a single primary alcohol.

In the first step of the reaction, a hydride ion reacts with the acidic hydrogen of the
carboxylic acid, forming and a carboxylate ion. We have seen that nucleophiles do
not react with carboxylate ions because of their negative charge. However, in this case,
an electrophile is present that accepts a pair of electrons from the carboxylate
ion and forms a new hydride donor. Then, analogous to the reduction of an ester by
two successive additions of hydride ion take place, with an aldehyde being
formed as an intermediate on the way to the primary alcohol.

Acyl chlorides, like esters and carboxylic acids, undergo two successive additions
of hydride ion when treated with

Amides also undergo two successive additions of hydride ion when they react with
The product of the reaction is an amine. Primary, secondary, or tertiary
amines can be formed, depending on the number of substituents bonded to the nitrogen

LiAlH 4.

C

O

1. LiAlH 4
   CH 2. H^3 O+
   3 CH 2 CH 2 Cl

CH 3 CH 2 CH 2 CH 2 OH

butanoyl chloride

1-butanol

LiAlH 4.

LiAlH 4 ,

(AlH 3 )

H 2

acetic acid

ethanol

benzoic acid benzyl alcohol

1. LiAlH 4
   CH 3 OH

CH 3 CH 2 OH

1. LiAlH 4


2. H 3 O+


3. H 3 O+

OH

C CH 2 OH

O

C

O

LiAlH 4

LiAlH 4.

• 78 ° C
  3-D Molecule:
  Diisobutylaluminum hydride

H 3 O+

O

+

+

a carboxylic acid

mechanism for the reaction of a carboxylic acid with hydride ion

an alcohol an aldehyde

CH 3 COH H

+ H 2

AlH 3

AlH 3

O

CH 3 C

OAlH 2

O CH 3 CH

O H

H H

Al

CH 3 CO

O

O

−

CH 3 CH 2 O

AlH 2 O−

CH 3 CH 2 OH − CH 3 CH

HAlH 3

−

−

−

−

hydride ion

removes an

acidic proton

new hydride

donor

second addition

of hydride ion