Level 1 – Case 13
88
X
OR
H
XH
OR
XH
OR
H 2 O
O
OH
slow
fast
X = O, S
Scheme 13.2
Does the hydrolysis mechanism involve the endocyclic C-X bond cleavage in the
slow step of the reaction? Are oxa- or thiacarbenium ions formed in the slow step
of the hydrolysis process?
To answer these questions, comment the results obtained after a complete Kinetic Isotope Effect (KIE) study on the hydrolysis reactions of methyl D-xylopyranosides 1 and methyl D-thioxylopyranosides 2 (both anomers in each case).
HO
S
HO
OH
OMe
H
HO
O
HO
OH
OMe
H
HO
S
HO
OH
H
OMe
HO
O
HO
OH
H
OMe
D-1
E-1
D-2
E-2
Comment the different types of kinetic isotope data and explain how their values
can be used to obtain information about the mechanism of the reaction.
E Ex xp pe er ri im me en nt ta al l D Da at ta a
Table 13.1. Kinetic Isotope Effects (KIEs) for the acid-catalyzed hydrolysis of methyl xylopyranosides 1 at 80°C (average values).
Type of KIE
D-anomer
E-anomer
Secondary D-D
1.128
1.098
Secondary E-D
1.088
1.042
Leaving group
18
O
1.023
1.023
Anomeric
13 C
1.006
1.006
Ring
18 O
0.983
0.978
Solvent (kD k k 3 O
+
/kH k k 3 O
+ )
2.31
2.24