Ester Hydrolysis in Base:
Saponification
Dr. Wolf's CHM 201 & 202
20-1
Ester Hydrolysis in Aqueous Base
O
RCOR'
O
+
HO–
RCO– + R'OH
is called saponification
is irreversible, because of strong stabilization of carboxylate
ion
if carboxylic acid is desired product, saponification is followed
by a separate acidification step (simply a pH adjustment)
20-2
Soap-Making
Basic hydrolysis
of the glyceryl
triesters (from
fats and oils)
gives salts of
long-chain
carboxylic acids.
These salts are
O
soaps.
R1CONa
O
O
CH2OCR1
R2COCH
CH2OCR3
O
NaOH , H2O, heat
O
R2CONa
O
R3CONa
20-3
Which bond is broken when esters are
hydrolyzed in base?
••
••
•• O
RC
– ••
OR' + •• OH
••
••
••
•• O
RC
– ••
OH + •• OR'
••
••
••
nucleophilic acyl substitution.
20-4
Stereochemistry gives the same answer
H
O
CH3C
alcohol has same
configuration at
chirality center as
ester; therefore,
nucleophilic acyl
substitution
C6H5
O
C
NaOH, H2O
O
CH3CONa+ HO
CH3
H
C6H5
C
CH3
Dr. Wolf's CHM 201 & 202
20-5
Mechanism of Ester Hydrolysis
in Base
Involves two stages:
1) formation of tetrahedral intermediate
2) dissociation of tetrahedral intermediate
20-6
First stage: formation of tetrahedral intermediate
O
RCOR' + H2O
HO–
OH
RC
OH
OR'
water adds to the
carbonyl group of the
ester
this stage is
analogous to the
base-catalyzed
addition of water to a
ketone
20-7
Mechanism of formation
of
tetrahedral intermediate
Step 1
••
O ••
H
•• O •
•
•• –
RC
• OR'
•
••
– ••
•• O ••
RC
•• OR'
••
H
O ••
••
Step 2
••
•• –
•• O •
•
H
••
•• O
H
O ••
H
H
O ••
RC
••
•• OR'
••
H
– ••
•• O ••
RC
•• OR'
••
H
O ••
••
Dissociation of
tetrahedral intermediate
Dr. Wolf's CHM 201 & 202
Step 3
••
•• –
•• O •
•
H
••
•• O
H
O ••
H
RC
H
O ••
••
•• OR'
••
H
••
O ••
RC
•• O
••
H
– ••
•• OR'
••
Step 4
••
O ••
RC
•• O •• –
••
HO–
H
••
O ••
OR'
••
H2O
RC
•• O
••
••
H
– ••
•• OR'
••
Key Features of Mechanism
Nucleophilic addition of hydroxide ion to carbonyl
group in first step
Tetrahedral intermediate formed in first stage
Hydroxide-induced dissociation of tetrahedral
intermediate in second stage