CONFIGURATIONAL ISOMERS
101
as indicated. Accordingly, we can now transform
stereostructures into Fischer projections, and vice versa.
The only significant restrictions are
• we should draw the longest carbon chain vertical;
• we should place the carbon of highest oxidation state
at the top.
However, when we come to manipulate Fischer projections, we may need to disregard these restrictions in the
interests of following the changes.
Manipulations we can do to a Fischer projection
may at first glance appear confusing, but by reference
to a model of a tetrahedral array, or even a sketch
of the representation, they should soon become quite
understandable, perhaps even obvious. The molecular
manipulations shown are given to convince you of the
reality of the following statements.
• Rotation of the formula by 180
◦ gives the same
molecule.
• Rotation of any three groups clockwise or anticlockwise gives the same molecule.
A
B
C
D
C
D
A
B
180º
≡
B
D
A
C
≡
B
D
A
C
D
B
C A
≡
≡
B
D
A C
180º
A
B
C
D
B
C
A
D
≡
D
B
C A
B
D
A
C
≡
D
C
A B
≡
≡
C
D
B
A
• Exchange of any two groups gives the enantiomer.
A
B
C
D
A
C
B
D
B
D
A
C
≡
D
B
C A
D
C
B A
D
C
B
A
mirror
≡
B
D
A
C
B
D
C
A
≡
exchange of two
groups gives
enantiomer
mirror image of
enantiomer is
original isomer
original isomer
enantiomer
enantiomer
• Rotation of the formula by 90
◦ gives the enantiomer.
A
B
C
D
D
A
B
C
90º
B
D
A
C
≡
D
B
C A
D
A
B
C
C
A
B D
≡
mirror
B
D
C A
≡
one exchange gives
enantiomer, second
exchange restores
original isomer
original isomer
enantiomer
enantiomer
It is also surprisingly easy to assign R or S configurations to chiral carbons in the Fischer projections; but,
because horizontal lines imply wedged bonds (towards
you) and vertical lines imply dotted bonds (away from
you), there are important guidelines to remember:
• if the group of lowest priority is on the vertical line,
a clockwise sequence gives the R configuration;
• if the group of lowest priority is on the horizontal
line, a clockwise sequence gives the S configuration.
101
as indicated. Accordingly, we can now transform
stereostructures into Fischer projections, and vice versa.
The only significant restrictions are
• we should draw the longest carbon chain vertical;
• we should place the carbon of highest oxidation state
at the top.
However, when we come to manipulate Fischer projections, we may need to disregard these restrictions in the
interests of following the changes.
Manipulations we can do to a Fischer projection
may at first glance appear confusing, but by reference
to a model of a tetrahedral array, or even a sketch
of the representation, they should soon become quite
understandable, perhaps even obvious. The molecular
manipulations shown are given to convince you of the
reality of the following statements.
• Rotation of the formula by 180
◦ gives the same
molecule.
• Rotation of any three groups clockwise or anticlockwise gives the same molecule.
A
B
C
D
C
D
A
B
180º
≡
B
D
A
C
≡
B
D
A
C
D
B
C A
≡
≡
B
D
A C
180º
A
B
C
D
B
C
A
D
≡
D
B
C A
B
D
A
C
≡
D
C
A B
≡
≡
C
D
B
A
• Exchange of any two groups gives the enantiomer.
A
B
C
D
A
C
B
D
B
D
A
C
≡
D
B
C A
D
C
B A
D
C
B
A
mirror
≡
B
D
A
C
B
D
C
A
≡
exchange of two
groups gives
enantiomer
mirror image of
enantiomer is
original isomer
original isomer
enantiomer
enantiomer
• Rotation of the formula by 90
◦ gives the enantiomer.
A
B
C
D
D
A
B
C
90º
B
D
A
C
≡
D
B
C A
D
A
B
C
C
A
B D
≡
mirror
B
D
C A
≡
one exchange gives
enantiomer, second
exchange restores
original isomer
original isomer
enantiomer
enantiomer
It is also surprisingly easy to assign R or S configurations to chiral carbons in the Fischer projections; but,
because horizontal lines imply wedged bonds (towards
you) and vertical lines imply dotted bonds (away from
you), there are important guidelines to remember:
• if the group of lowest priority is on the vertical line,
a clockwise sequence gives the R configuration;
• if the group of lowest priority is on the horizontal
line, a clockwise sequence gives the S configuration.