The observation of optical activity and the measurement of optical rotation distinguish one enantiomer (+) from the other (−).
ABSOLUTE CONFIGURATION
The
observation of optical activity and the measurement of optical rotation
distinguish one enantiomer (+)
from the other (−). The sign of
rotation (+ or −) is thus an experimental way to
differentiate enantiomers. A second way to des-ignate these stereoisomers is to
assign the configurations as R or S based on stereostructures (for enantiomers
which contain a single chiral center). The R and S configurations are relative
configurations based on three-dimensional structures which are drawn on paper.
However, it is not possible to predict a priori whether the R enantiomer (for
example) will be dextrorotatory (+) or levorotatory (−).
The
absolute configuration of an enantiomer is determined only when the optical
rotation of an enantiomer (+
or −) can be matched
with its configuration (R or S). For example, the absolute configuration of
lactic acid has been found to be R-(−) in that the R enantiomer is
levorotatory.
Conversion
to the methyl ester does not change the configuration of the stere-ocenter,
which remains R. However, the rotation is found to be positive so the absolute
configuration is R-(+).
This illustrates that while the relative config-uration (R,S) can be used to
show the structures of stereocenters, the absolute configuration must be known
to show the changes in configuration that occur dur-ing a chemical sequence.
That is, just knowing that a chemical reaction changes the optical rotation in
one direction or another is not sufficient to indicate whether a change in
configuration at a chiral center has occurred.
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