Topic 15 · page 3 of 28

Optical activity and racemic mixtures

A single enantiomer rotates the plane of plane-polarised light. Its mirror image rotates it by the same angle the other way, so an equal mixture of the two has no effect.

13 min read15.315.4

Read first: Drawing enantiomers

Plane-polarised light 15.3

Ordinary light vibrates in every plane at right angles to its direction of travel. A polariser lets through only the light that vibrates in one plane. This is plane-polarised light. "Monochromatic" means light of a single wavelength, which matters because the size of the rotation depends on the wavelength.

Enantiomers have the same melting point, boiling point, density and chemical reactions with ordinary reagents. They differ in how they affect plane-polarised light.

The polarimeter 15.3

A polarimeter measures the rotation.

A polarimeter drawn left to right: a source of light of one wavelength, a polariser that lets through light vibrating in one plane, a tube holding the solution, which turns the plane through an angle alpha, an analyser that is rotated until the light is back at maximum or minimum, and a detector.
A polarimeter. The solution turns the plane of polarisation, and the analyser is turned to find by how much.
  1. Light of one wavelength passes through the polariser and becomes plane-polarised.
  2. It passes through a tube of the solution. If the solution is optically active, the plane is rotated through an angle α.
  3. The analyser is turned until the detector shows the same brightness as before. The angle it has been turned through is the rotation.

If the solution contains no chiral substance, or a chiral substance as an equal mixture of enantiomers, there is no rotation and the angle is 0°.

Racemic mixtures 15.4

In a racemic mixture the two rotations are equal and opposite, so they cancel. The mixture is not optically active, even though every molecule in it is chiral.

A bar chart of rotation in degrees. Enantiomer A is plus 24 degrees, enantiomer B is minus 24 degrees, and a racemic mixture of equal amounts of A and B is 0 degrees.
Illustrative rotations. The mirror-image forms rotate by equal amounts in opposite directions, so the racemic mixture shows none.

The values in the chart (+24°, −24° and 0°) are illustrative. Real values depend on the compound, the concentration, the length of the tube and the wavelength.

Racemic mixtures matter in practice. Many reactions that start from flat molecules, as page 4 and page 9 show, produce both enantiomers in equal amounts. The two enantiomers of a drug can act differently in the body, because the receptors in the body are themselves chiral.

Check yourself

  1. Define optical activity.
  2. What is meant by plane-polarised monochromatic light?
  3. Define a racemic mixture, and explain why it does not rotate plane-polarised light even though its molecules are chiral.
  4. A pure sample of one enantiomer of 2-chlorobutane rotates the plane by +14° in a polarimeter. State the rotation that you expect for (a) its mirror image at the same concentration and (b) a mixture of equal amounts of the two.
  5. Which of these can be optically active: butan-2-ol, propan-2-ol, 2-chlorobutane, ethanol? Explain your answer.

Answers cover these until you have tried the questions

  1. The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light.
  2. Light that vibrates in one plane only (plane-polarised) and has a single wavelength (monochromatic).
  3. A racemic mixture is an equimolar mixture of the two enantiomers. Each enantiomer rotates the plane by the same angle but in opposite directions, so the rotations cancel and the mixture shows no net rotation.
  4. (a) −14°, the same size in the opposite direction. (b) 0°, because the two rotations cancel in a racemic mixture.
  5. Butan-2-ol and 2-chlorobutane can be optically active, because each has a chiral carbon (C2) with four different groups. Propan-2-ol (two CH₃ on the central carbon) and ethanol (no carbon with four different groups) cannot.