Optical Activity

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 oscillate

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

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 clockwise

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 asymmetric

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 compound

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

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(-)

Dextro(+) and levo( -)are Latin prefixes for “to the right” and “to the left,” respectively.

Sometimes lowercase d and l are used instead of and

Do not confuse and with R and S. The and symbols indicate the direction

in which an optically active compound rotates the plane of polarization, whereas

R and S indicate the arrangement of the groups about an asymmetric carbon. Some

compounds with the R configuration 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.

amount of rotation will vary with the wavelength of the light used, the light source for

a polarimeter must produce monochromatic (single wavelength) light. Most polarimeters

use light from a sodium arc (called the sodium D-line; wavelength ). In

a polarimeter, monochromatic light passes through a polarizer and emerges as polarized

light. The polarized light then passes through an empty sample tube (or one filled

with an optically inactive solvent) and emerges with its plane of polarization unchanged.

The light then passes through an analyzer. The analyzer is a second polarizer

mounted on an eyepiece with a dial marked in degrees. When using a polarimeter, the

analyzer is rotated until the user’s eye sees total darkness. At this point the analyzer is

at a right angle to the first polarizer, so no light passes through. This analyzer setting

corresponds to zero rotation.

The sample to be measured is then placed in the sample tube. If the sample is optically

active, it will rotate the plane of polarization. The analyzer will no longer block

all the light, so light reaches the user’s eye. The user then rotates the analyzer again

until no light passes through. The degree to which the analyzer is rotated can be read

from the dial and represents the difference between an optically inactive sample and

the optically active sample. This is called the observed rotation it is measured in

degrees. The observed rotation depends on the number of optically active molecules

the light encounters in the sample. This, in turn, depends on the concentration of the

sample and the length of the sample tube. The observed rotation also depends on the

temperature and the wavelength of the light source.

Each optically active compound has a characteristic specific rotation. The specific

rotation is the number of degrees of rotation caused by a solution of 1.0 g of the compound

per mL of solution in a sample tube 1.0 dm long at a specified temperature and

wavelength. The specific rotation can be calculated from the observed rotation using

the following formula:
 Measuring Optical Activity

When rotation is quantified using a polarimeter it is known as an observed rotation, because rotation is affected by path length (l, the time the light travels through a sample) and concentration (c, how much of the sample is present that will rotate the light).  When these effects are eliminated a standard for comparison of all molecules is obtained, the specific rotation, [a].

[a] = 100a / cl    when concentration is expressed as g sample /100ml solution

Specific rotation is a physical property like the boiling point of a sample and can be looked up in reference texts.    Take a look at are problem Enantiomers will rotate the plane of polarisation in exactly equal amounts (same magnitude) but in opposite directions.

Dextrorotary designated as d or (+), clockwise rotation (to the right)
Levorotary designated as l or (-), anti-clockwise rotation (to the left)

If only one enantiomer is present a sample is considered to be optically pure.  When a sample consists of a mixture of enantiomers, the effect of each enantiomer cancels out, molecule for molecule.

For example, a 50:50 mixture of two enantiomers or a racemic mixture will not rotate plane polarised light and is optically inactive.  A mixture that contains one enantiomer excess, however, will display a net plane of polarisation in the direction characteristic of the enantiomer that is in excess.

Determining Optical Purity

The optical purity or the enantiomeric excess (ee%) of a sample can be determined as follows:

Optical purity = % enantiomeric excess = % enantiomer₁ - % enantiomer₂
                     = 100 [a]_mixture / [a]_{pure sample}

ee%  =  100 ([major enantiomer] - [minor enantiomer]) / ([major enantiomer] + [minor enantiomer])

where [major enantiomer] = concentration of the major enantiomer
          [minor enantiomer] = concentration of the minor enantiomer

Diasteromeric substances can have different rotations both in sign and in magnitude. 

where is the specific rotation; T is temperature in °C; is the wavelength of the incident

light (when the sodium D-line is used, is indicated as D); is the observed rotation;

l is the length of the sample tube in decimeters; and c is the concentration of the

sample in grams per milliliter of solution.

For example, one enantiomer of 2-methyl-1-butanol has been found to have a specific

rotation of Because its mirror image rotates the plane of polarization the

same amount but in the opposite direction, the specific rotation of the other enantiomer

must be

PROBLEM 12_

The observed rotation of 2.0 g of a compound in 50 mL of solution in a polarimeter tube

50-cm long is What is the specific rotation of the compound?

Knowing whether a chiral molecule has the R or the S configuration does not tell us

the direction the compound rotates the plane of polarization, because some compounds

with the R configuration rotate the plane to the right and some rotate the

plane to the left We can tell by looking at the structure of a compound whether it

has the R or the S configuration, but the only way we can tell whether a compound is

+or –

Optical Purity and Enantiomeric Excess

Whether a particular sample consists of a single enantiomer or a mixture of enantiomers

can be determined by its observed specific rotation. For example, an enantiomerically

pure sample—meaning only one enantiomer is present—of (S)- -2-bromobutane will

have an observed specific rotation of because the specific rotation of (S)- -

2-bromobutane is If, however, the sample of 2-bromobutane has an observed

specific rotation of 0°, we will know that the compound is a racemic mixture. If the observed

specific rotation is positive but less than we will know that we have a

mixture of enantiomers and the mixture contains more of the enantiomer with the S configuration

than the enantiomer with the R configuration. From the observed specific rotation,

we can calculate the optical purity (op) of the mixture.

optical purity = observed specific rotation/

specific rotation of the pure enantiomer

REFERENCES

1.0 Morrison. Robert. T, and Boyd. Robert. N, "Organic Chemistry (6th ed)". Prentice-Hall Inc
1.1 (1992).Crocker,Ernest C (1992).Application of the Octet Theory To Single-Ring Aromatic Compounds
1.2 Macmurry,John(2007).Organic Chemistry(7th edition).Brooks-Cole
1.3   Organic Chemistry. Marc Loudon, 3^rd edition The Benjamin/Cummings Publishing Company,Inc., California, 1995.
1.4  Organic Chemistry. John MucMurry. 3^rd edition. Brooks/Cole Publishing Company, 1992