Organic Chemistry I

optical
activity
, and the compound with such activity is labelled as
optical active
. The stereoisomer that is optical active is also
called as
optical isomer
.
Chiral compound is optical active. Achiral compound is optical inactive.
The sample containing a chiral compound rotates the plane of polarization of plane-polarized light, the direction and
angles of the rotation depends on the nature and concentration of the chiral substances. The rotation angles can be
measured by using polarimeter (later in this section).
For a pair of enantiomers with same concentration, under the same condition, they rotate the plane of polarization
with the
same angles
but in
opposite
direction
, one is clockwise and the other is counterclockwise.
5.4 Optical Activity | 161

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Figure 5.4b Clockwise rotation/enantiomer dextrorotatory vs. counterclockwise rotation/
enantiomer levorotary
The enantiomer rotates the plane of polarization clockwise is said to be
dextrorotatory
(
Latin
, means to the right),
and labelled with the prefix (
d
) or (
+
). The enantiomer rotates the plane of polarization counterclockwise is said
to be
levorotatory
(
Latin
, means to the left), and labelled with the prefix (
l
) or (
–
). The
d/l
(or
+/-
) indicate the
direction in which an optical active compound rotates the plane of polarization of plan-polarized light, that has to
be determined by experiment to measure the optical rotation.
d/l
(
or
+/–) symbol has nothing to do with R/S
. R/
S indicates the arrangement of the groups around the chirality center, that can be determined by knowing the exact
spatial arrangement of the groups. That means compound with