Chemistry Reference
In-Depth Information
now be becoming familiar to us. However, there
are significant differences between the two types of
process, as we shall see. In
acid-catalysed hydrolysis
of esters
, the process is analogous to the acid-
catalysed formation of esters: it is merely the reverse
reaction.
acid-catalysed hydrolysis of esters
H
+
, + H
+
H
OH
−
OH
OH
O
CH
3
H
3
C
OCH
3
OH
2
H
3
C
H
3
C
OCH
3
O
H
3
C
OCH
3
OH
equilibrium:
loss of proton to solvent, then
reprotonation from solvent;
molecule may have any one of
the three oxygens protonated
ester
protonation of
carbonyl oxygen;
formation of
conjugate acid
H
H
2
O
nucleophilic attack
on to protonated ester
O
OH
H
H
3
C
OH
H
3
C
OH
acid catalyst
regenerated
carboxylic acid
acid is
catalyst
and is regenerated;
equilibrium reaction
The reaction begins with protonation of the
carbonyl oxygen to give the conjugate acid, which
increases the electrophilicity of the carbonyl. This is
necessary because we are using a neutral nucleophile.
Nucleophilic attack follows, giving a protonated
tetrahedral intermediate. In an acidic medium, an
equilibrium will again be set up such that any
one of the three oxygen atoms may be protonated.
This equilibrium also involves loss of protons to
the solvent, followed by reprotonation of another
oxygen using the solvent. As in esterification (see
Section 7.9.1), the process is not internal transfer of a
proton, but requires solvent molecules. For hydrolysis
to occur, the methoxyl needs to be protonated, so
that it can be lost as a methanol leaving group.
Loss of the leaving group is again facilitated by
the resonance effect from a hydroxyl, leading to
regeneration of the carbonyl group in protonated
form. Formation of the uncharged carboxylic acid
regenerates the acid catalyst. As with acid-catalysed
ester formation, the reaction is an equilibrium, and
this equilibrium needs to be disturbed for complete
hydrolysis, typically by using an excess of water, i.e.
aqueous acid.
Box 7.15
Autolysis of aspirin
The analgesic
aspirin
, acetylsalicylic acid, is an ester. In this compound, the alcohol part is actually a phenol,
salicylic acid. Aspirin is synthesized from salicylic acid by treatment with acetic anhydride.
OH
O
CH
3
O
CH
3
H
2
O
(CH
3
CO)
2
O
O
O
CO
2
H
CO
2
H
CO
2
H
salicylic acid
acetylsalicylic acid
aspirin
provides acid catalyst
for ester hydrolysis
H
2
O
OH
CH
3
CO
2
H
CO
2
H
