Organic Chemistry

diisobutylaluminum

hydride

DIBALH

CH 3 CHCH 2 Al CH 2 CHCH 3

CH 3 H CH 3

744 CHAPTER 18 Carbonyl Compounds II

Esters undergo two successive reactions
with hydride ion and with Grignard
reagents.

The reaction of a Class I carbonyl compound (i.e., a carbonyl compound with a

group that can be replaced by another group) with hydride ion involves two successive

reactions with the nucleophile. (Recall that Class I carbonyl compounds also undergo

two successive reactions with a Grignard reagent; see Section 18.4.) Sodium borohy-

dride is not a sufficiently strong hydride donor to react with the less reactive

(compared with aldehydes and ketones) esters, carboxylic acids, and amides, so esters,

carboxylic acids, and amides must be reduced with lithium aluminum hydride

a more reactive hydride donor.

Because lithium aluminum hydride is more reactive than sodium borohydride, it is

not as safe or as easy to use. Since it reacts violently with protic solvents, lithium alu-

minum hydride must be used in a dry, aprotic solvent.

The reaction of an ester with produces two alcohols, one corresponding to

the acyl portion of the ester and one corresponding to the alkyl portion.

When an ester reacts with hydride ion, the first reaction is a nucleophilic acyl

substitution reaction because an ester has a group that can be substituted by hydride

ion. The product of this reaction is an aldehyde. The aldehyde then undergoes a

nucleophilic addition reaction with a second equivalent of hydride ion, forming an

alkoxide ion, which when protonated gives a primary alcohol. The reaction cannot

be stopped at the aldehyde stage because an aldehyde is more reactive than an ester

toward nucleophilic attack.

Chemists have found that if diisobutylaluminum hydride (DIBALH) is used as the

hydride donor at a low temperature, the reaction can be stopped after the addition of

one equivalent of hydride ion. This reagent, therefore, makes it possible to convert es-

ters into aldehydes, which is initially surprising, since aldehydes are more reactive

than esters toward hydride ion.

1-propanol methanol

methyl propanoate

an ester

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

C CH 3 CH 2 CH 2 OH + CH 3 OH

O

LiAlH 4

(LiAlH 4 ),

(NaBH 4 )

3-D Molecule:

Methyl propanoate

C

O

CH 3 CH 2 H

H 3 O+

HAlH 3

+

−

an ester

mechanism for the reaction of an ester with hydride ion

an aldehyde

a primary alcohol

HAlH 3 + CH 3 O−

O

H

CH 3 CH 2 C OCH 3

−

O

H

CH 3 CH 2 CH 2 OH CH 3 CH 2 CH

−

−

C

O

CH 3 CH 2 OCH 3

product of

nucleophilic

addition

product of nucleophilic

acyl substitution

1. [(CH 3 ) 2 CHCH 2 ] 2 AlH, − 78 °C
   CH 2. H^2 O
   3 CH 2 CH 2 CH 2 OCH 3 + CH 3 OH
   methyl pentanoate pentanal

C

O

CH 3 CH 2 CH 2 CH 2 H

C

O