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194
1.
Protonation of the glycosidic oxygen (weakly basic). Fast reaction
2.
The linkage between C1 and the protonated oxygen atom is cleaved. This
heterolysis is the rate-limiting step (slow). The released substituent (the
‘aglycon’) (HO-R) is released.
3.
As a consequence a carbocation is formed at C1. It is resonance stabilized
with the ring oxygen. The partial double bond changes the ring
conformation (planar ‘halfchair’ structure C1-C2-C5-O5).
4.
The carbocation reacts with a molecule of water, and splits of a proton.
The result is a reducing sugar (hemiacetal).
5.
Note that H
+
is recycled, i.e. it acts like a catalyst.
Acid hydrolysis is reversible. At high concentrations of reducing sugars
‘reversion products’ may be formed by reaction between C1 and one of the
OH- groups in another sugar. For analytical uses (sugar analysis) it is
therefore important to use a sufficiently low concentration to avoid by-products
during acid hydrolysis.
O
H
HO
H
HO
H
H
OH H
O
OH
R
+ H
+
O
H
HO
H
HO
H
H
OH H
H
O
OH
R
O
H
HO
H
HO
H
H
OH H
OH
HO-R
Slow
(rate limiting step)
Carbocation (carbonium ion)
Partial double bond to C1-O5 =>
planar structure
+ H
2
O
O
H
HO
H
HO
H
H
OH H
OH
2
OH
- H
+
O
H
HO
H
HO
H
H
OH H
OH
OH
Glycoside (acetal)