Chemistry Reference
In-Depth Information
Although this section has been termed hydrogen
exchange, it is important to realize that we could also
visualize this simply as an
enolate anion acting as
abase
. This is also true of the next section, and in
some of the following sections we shall encounter
enolate anions acting as nucleophiles.
will generate a planar enol or enolate anion, and
regeneration of the keto form may then involve
supply of protons from either face of the double
bond, so changing a particular enantiomer into its
racemic form. Reacquiring a proton in the same
stereochemical manner that it was lost will generate
the original substrate, but if it is acquired from
the other face of the double bond it will give the
enantiomer, i.e. together making a racemate. Note
that removal and replacement of protons at the other
α
10.1.2 Racemization
The process of hydrogen exchange shown above
has
-carbon, i.e. the methyl, will occur, but has no
stereochemical consequences.
-carbon is chiral and
has a hydrogen attached. Removal of the proton
implications
if
the
α
CH
3
CH
3
H
3
C
H
3
C
NaOH or HCl
S
RS
CH
3
CH
3
H
aq EtOH
H
O
O
racemic
product
chiral centre must be
α
to carbonyl and
contain an H substituent
planar enolate anion
in base:
CH
3
CH
3
H
3
C
H
3
C
H
3
C
CH
3
S
RS
CH
3
CH
3
H
H
H
3
C
O
O
O
HOH
HO
during reverse reaction, proton
can be added to either face
in acid:
planar enol
CH
3
H
3
C
CH
3
H
3
C
H
3
C
CH
3
H
+
+
H
+
−
S
RS
CH
3
CH
3
racemic
ketone
chiral
ketone
H
H
H
3
C
HO
HO
OH
H
to the carbonyl and
must contain a hydrogen substituent. If there is more
than one chiral centre in the molecule with only one
centre
The chiral centre must be
α
not be affected by enolization, so the product will
be a mixture of
diastereoisomers
of the original
compound rather than the racemate.
α
to the carbonyl, then the other centres will
CH
3
CH
3
CH
3
NaOH or HCl
S
S
R
Ph
R
Et
R
Ph
Et
+
R
Ph
Et
H
aq EtOH
H
H
H
CH
3
H
CH
3
H
CH
3
O
O
O
mixture of two diastereoisomers
chiral centre not
to
carbonyl and unaffected
α
