Organic Chemistry

Section 5.7 Optical Activity 193

However, when polarized light passes through a solution of a chiral compound, the
light emerges with its plane of polarization changed. Thus,a chiral compound rotates
the plane of polarization. A chiral compound will rotate the plane of polarization
clockwise or counterclockwise. If one enantiomer rotates the plane of polarization
clockwise, its mirror image will rotate the plane of polarization exactly the same
amount counterclockwise.

A compound that rotates the plane of polarization is said to be optically active. In
other words, chiral compounds are optically active and achiral compounds are
optically inactive.
If an optically active compound rotates the plane of polarization clockwise, it is
called dextrorotatory, indicated by If an optically active compound rotates the
plane of polarization counterclockwise, it is called levorotatory, indicated by
Dextroand levoare Latin prefixes for “to the right”and “to the left,”respectively.
Sometimes lowercase dand lare used instead of and
Do not confuse and with Rand S. The and symbols indicate the di-
rection in which an optically active compound rotates the plane of polarization, where-
as Rand Sindicate the arrangement of the groups about an asymmetric carbon. Some
compounds with the Rconfiguration are and some are
The degree to which an optically active compound rotates the plane of polarization
can be measured with an instrument called a polarimeter(Figure 5.2). Because the

(+) (-).

(+) (-) (+) (-)

(+) (-).

(-).

(+).

direction of light propagation

normal polarizer

light

light

source

plane-polarized

light

plane-polarized

light

sample tube

containing an

achiral compound

the plane of polarization

has not been rotated

direction of light propagation

normal polarizer

light

light

source

plane-polarized

light

plane-polarized

light

sample tube

containing a

chiral compound

the plane of polarization

has been rotated

direction of light propagation

normal polarizer

light

light

source

plane-polarized

light

plane-

polarized

light

sample tube

containing a

chiral compound

analyzer viewer

α

Figure 5.2

Schematic of a polarimeter.

Joseph Achille Le Bel (1847–1930),

a French chemist, inherited his fami-

ly’s fortune, which enabled him to

establish his own laboratory. He and

van’t Hoff independently arrived at

the reason for the optical activity of

certain molecules. Although van’t

Hoff ’s explanation was more precise,

both chemists are given credit for the

work.

Some molecules with the Rconfigura-

tion are and some molecules with

the Rconfiguration are ().

(),