CONFIGURATIONAL ISOMERS
89
Box 3.11
Configurations and conformations: avoiding confusion
At this stage, a word of caution: do not confuse conformation with configuration. Different conformations
interconvert easily; different configurations do not interconvert without some bond-breaking process. We
commented above that changing the conformation did not affect the spatial sequence about chiral centres, and used
ephedrine as a rather trivial and obvious example. Rotation about single bonds did not change the configuration
at either chiral centre.
To emphasize this point, look at the following relationships for trans-3-methylcyclohexyl bromide.
Br
H
Br
H
H
Br
H
Br
CH 3
H
CH 3
H
CH 3
H
Br
Br
Br
Br
CH 3
H
eq
eq
ax
ring flip
ring flip
ax
eq
eq
ax
enantiomer:
has different configuration
at each centre
conformer:
has same configuration
at each centre
don't confuse conformation with configuration
ax
these do not
interconvert
Ring flip of the upper left structure produces an alternative conformer. Ring flip does not change the
configuration. The axial–equatorial relationship (conformation) is modified, but the up–down relationship
(configuration) is still there. The enantiomer of this structure has the alternative configuration at both chiral
centres, but it cannot be produced from the first structure by any simple isomerization process. However, it is still
conformationally mobile. The figure thus shows the conformational isomerism for two different configurational
isomers, the enantiomeric pair.
A common mistake that can be made when one is trying to draw the different conformers that arise from ring flip
in a cyclohexane compound (see Box 3.3) is to remember vaguely that axial groups become equatorial, and vice
versa, and to apply this change without flipping the ring. Of course, as can be seen from looking at the compounds
below, transposing the equatorial bromine to axial and the axial methyl to equatorial changes the configuration
at both centres, so we have produced the enantiomer. This is a configurational isomer and not a conformer.
Br
H
CH 3
H
eq
ax
H
Br
CH 3
H
ax
eq
changing axial to equatorial and vice versa without
ring flip creates the enantiomer, not a conformer
Br
Br
89
Box 3.11
Configurations and conformations: avoiding confusion
At this stage, a word of caution: do not confuse conformation with configuration. Different conformations
interconvert easily; different configurations do not interconvert without some bond-breaking process. We
commented above that changing the conformation did not affect the spatial sequence about chiral centres, and used
ephedrine as a rather trivial and obvious example. Rotation about single bonds did not change the configuration
at either chiral centre.
To emphasize this point, look at the following relationships for trans-3-methylcyclohexyl bromide.
Br
H
Br
H
H
Br
H
Br
CH 3
H
CH 3
H
CH 3
H
Br
Br
Br
Br
CH 3
H
eq
eq
ax
ring flip
ring flip
ax
eq
eq
ax
enantiomer:
has different configuration
at each centre
conformer:
has same configuration
at each centre
don't confuse conformation with configuration
ax
these do not
interconvert
Ring flip of the upper left structure produces an alternative conformer. Ring flip does not change the
configuration. The axial–equatorial relationship (conformation) is modified, but the up–down relationship
(configuration) is still there. The enantiomer of this structure has the alternative configuration at both chiral
centres, but it cannot be produced from the first structure by any simple isomerization process. However, it is still
conformationally mobile. The figure thus shows the conformational isomerism for two different configurational
isomers, the enantiomeric pair.
A common mistake that can be made when one is trying to draw the different conformers that arise from ring flip
in a cyclohexane compound (see Box 3.3) is to remember vaguely that axial groups become equatorial, and vice
versa, and to apply this change without flipping the ring. Of course, as can be seen from looking at the compounds
below, transposing the equatorial bromine to axial and the axial methyl to equatorial changes the configuration
at both centres, so we have produced the enantiomer. This is a configurational isomer and not a conformer.
Br
H
CH 3
H
eq
ax
H
Br
CH 3
H
ax
eq
changing axial to equatorial and vice versa without
ring flip creates the enantiomer, not a conformer
Br
Br