Chemistry Reference
In-Depth Information
This enediol can be regarded as a common enol tautomer for two different keto structures. In other words, there
are two ways in which this enediol can tautomerize back to a keto form, and the reaction thus appears to shift the
position of the carbonyl group. The reaction is enzyme catalysed, which allows the normal equilibrium processes
to be disturbed.
It is nice to see this series of reactions being repeated in the glycolytic pathway, this time accounting for
the transformation of glucose 6-phosphate into fructose 6-phosphate. Although the substrates are different, the
reacting portion of the molecules is exactly the same as that in the glyceraldehyde 3-phosphate to dihydroxyacetone
phosphate transformation. Again, this is an enzyme-catalysed reaction.
10.1.1 Hydrogen exchange
of a proton from the
-carbon and supply of a proton
from solvent to the carbonyl oxygen. Accordingly,
this removal/supply of protons can be observed using
isotopes of hydrogen, either radioactive tritium or the
stable deuterium, which can be detected easily via
NMR techniques.
α
The intermediacy of enols or enolate anions may
be demonstrated by
hydrogen exchange reactions
(see Section 4.11.2). Both acid-catalysed and base-
catalysed tautomerism mechanisms involve removal
O
O
O
H
2
O
D
2
O
H
3
C
CH
3
H
3
C
CH
3
H
3
C
CH
3
DCl or NaOD
HCl or NaOH
H
H
H
H
D
D
D
D
H
H
H
H
pentan-3-one
if large excess D
2
O used
will completely deuteriate
α
-positions only
can reverse by using
large excess H
2
O
Thus, pentan-3-one can be deuteriated using a large
excess of D
2
O, with either acid (DCl) or base
(NaOD) catalyst; the acid or base catalyst should also
be deuteriated to minimize dilution of label. After
suitable
equilibration,
usually
requiring
prolonged
heating,
the
α
-positions
will
become
completely
labelled with deuterium.
supply of deuteron
in base:
O
OD
O
O
DOD
H
H
D
etc.
H
H
abstraction
of proton
enolate
anion
in acid:
D
OD
O
O
OD
OD
D
D
D
H
H
H
H
H
etc.
H
H
enol
Two mechanisms are shown above. The base-
catalysed mechanism proceeds through the eno-
late anion. The acid-catalysed process would be
formulated as involving an enol intermediate. Note
that the terminal hydrogens in pentan-3-one are
not exchanged, since they do not participate in the
enolization process. Of course, it is also possible
to re-exchange the labelled hydrogens by a simi-
lar process using an excess of ordinary water, a
process that might be exploited to determine or con-
firm the position of labelling in a deuterium-labelled
substrate.
