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Chirality

Optical Activity

Most of the physical properties of a pair of enantiomers have identical values. For instance, enantiomers have the same boiling point, melting point, solubility, refraction index, and many other properties. All these properties do not depend on the small and only difference between enantiomers, the absolute configuration.

Fig.1
Pair of enantiomers of 2-butanol.

How can one distinguish between two enantiomers?

This can be illustrated with the help of daily objects. Look at your hands. The difference between your chiral hands is noticeable, for instance when you are putting on a glove or shaking another hand. The left glove does not fit the right hand. Or try to shake a left hand with your right hand, it does not fit. The difference is not noticeable, for instance when you are carrying a plastic bag or cleaning a table with a cloth. By inspecting these situations, one realizes that the difference of chiral objects is noticeable any time they are interacting with other chiral objects. The difference is not noticeable when they are interacting with other achiral objects.

Enantiomers behave the same way. They are chiral and they interact differently with other chiral molecules but not with other achiral molecules. For instance, enantiomers show different reaction rates in reactions with other chiral molecules and different solution behaviour in pure chiral solvents.

Information
Enantiomers differ in only one physical property, the interaction with plane-polarized light. The plane of plane-polarized light is rotated when the light passes through a solution of an enantiomer. This property of a chiral compound is called optical activity. The rotation is called optical rotation.

The optical rotation can be measured with a polarimeter. Two enantiomers always show opposite optical activities, that is, they rotate the plane of polarized light by equal amounts in opposite directions. The optical rotation of an equimolar mixture of a pair of enantiomers is zero because the optical rotations of the enantiomers cancel out one another. An equimolar mixture of a pair of enantiomers is called racemate. A racemate is optically inactive.

Polarimeter

Tab.1
Fig.2
Interaction of an achiral molecule with polarized light.
Fig.3
Interaction of a chiral molecule with polarized light.

What is the cause of the rotation of the plane of polarized light?

The cause of the rotation of the plane of polarized light cannot be easily explained and we do not try to explain it here. However, why does a solution of a chiral compound rotate the plane of polarized light and a solution of an achiral compound not? As moving, charged particles, the electrons in a molecule produce electromagnetic fields, which interact with the electromagnetic field of light. This interaction indirectly rotates the plane of the polarized light by a small amount. The direction and quantity of this small rotation depend on the orientation of the molecule in the solution. A macroscopic sample contains a huge number of molecules. Therefore, in a solution of an achiral compound, each small rotation caused by one molecule is cancelled by the rotation caused by another molecule with a mirror-image orientation. Such a solution shows no optical activity. In a solution of a pure enantiomer, the rotation caused by one molecule cannot be cancelled because there is no molecule with a mirror-image orientation as an enantiomer is nonsuperimposable on its mirror image. This results in a net rotation of the plane of polarized light. The solution of a pure enantiomer is optically active.

Exercise 1

Exercise 2

Exercise 3

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