Organic Chemistry

When light is filtered through
two polarized lenses at a 90°angle
to one other, no light is transmitted
through them.

192 CHAPTER 5 Stereochemistry

PROBLEM 11

Indicate whether each of the following structures has the Ror the Sconfiguration:

a. c.

b. d.

5.7 Optical Activity

Enantiomers share many of the same properties—they have the same boiling points,

the same melting points, and the same solubilities. In fact, all the physical properties

of enantiomers are the same except those that stem from how groups bonded to the

asymmetric carbon are arranged in space. One of the properties that enantiomers do

not share is the way they interact with polarized light.

What is polarized light? Normal light consists of electromagnetic waves that oscil-

late in all directions. Plane-polarized light(or simply polarized light), in contrast, os-

cillates only in a single plane passing through the path of propagation. Polarized light

is produced by passing normal light through a polarizer such as a polarized lens or a

Nicol prism.

You experience the effect of a polarized lens with polarized sunglasses. Polarized

sunglasses allow only light oscillating in a single plane to pass through them, so they

block reflections (glare) more effectively than nonpolarized sunglasses.

In 1815, the physicist Jean-Baptiste Biot discovered that certain naturally occurring

organic substances such as camphor and oil of turpentine are able to rotate the plane of

polarization. He noted that some compounds rotated the plane of polarization clock-

wise and others counterclockwise, while some did not rotate the plane of polarization

at all. He predicted that the ability to rotate the plane of polarization was attributable to

some asymmetry in the molecules. Van’t Hoff and Le Bel later determined that the

molecular asymmetry was associated with compounds having one or more asymmet-

ric carbons.

When polarized light passes through a solution of achiral molecules, the light

emerges from the solution with its plane of polarization unchanged. An achiral com-

pound does not rotate the plane of polarization. It is optically inactive.

direction of light propagation

normal polarizer

light

light

source

plane-polarized

light

light waves oscillate

in a single plane

light waves oscillate

in all directions

CH 3 CH 2 CH 2 CH 3

CH 2 CH 2 CH 2 CH 3

CH 2 CH 3

HO H

CH 2 CH 2 CH 3

CH 2 OH

CH 3 H

Br

CH 2 CH 3

CH 3 CH 2 CH 2 Br

CH(CH 3 ) 2

CH 3

Born in Scotland,William Nicol
(1768–1851)was a professor at the
University of Edinburgh. He devel-
oped the first prism that produced
plane-polarized light. He also devel-
oped methods to produce thin slices
of materials for use in microscopic
studies.