Optical Isomerism Lab

Pearson Edexcel IAL Chemistry · Topic 15 · 15.1–15.4

Support

Mirror bench

A molecule and its mirror image. Can you turn one so it sits exactly on top of the other?

A
B
Drag a molecule to turn it · click a ball to select that group

Drawings to 3D

Turn wedge-and-dash drawings into 3D models, then test whether two drawings are the same molecule or a pair of enantiomers.

A
B
Drag a molecule to turn it · click a ball to select that group

Plane-polarised light

Watch what an optically active sample does to light. Drag the scene to look from any angle.

Drag to look around · pinch or ctrl + scroll to zoom
Plane of polarisation leaving the sample
40° clockwise
Brightness seen by the observer
light wave and its plane(+) molecule(−) moleculeanalyser setting

What is happening

  1. Light is a transverse wave. Ordinary light vibrates in every plane at right angles to its direction of travel.
  2. The polariser lets through only the vibrations in one plane. This is plane-polarised light.
  3. A single enantiomer rotates the plane of polarisation. The other enantiomer rotates it by the same angle in the opposite direction.
  4. A racemic mixture has equal amounts of both, so the rotations cancel: no overall rotation.

Clockwise, seen by an observer looking towards the light, is dextrorotatory (+). Anticlockwise is laevorotatory (−). The angles here are for illustration only.

Watch the molecules

✗ Common exam mistake: “The enantiomers rotate in opposite directions in plane-polarised light.”

✓ Correct: “Each enantiomer rotates the plane of plane-polarised light by the same angle, but in opposite directions.”

In every sample the molecules tumble about at random, with or without the light. Pause the light and they keep tumbling. What turns is the plane the light vibrates in: follow the yellow ribbon through the tube.

Homework challenge

Ten questions with instant feedback. Press "New set" for a fresh set of questions. You can open any drawing question in the 3D bench to check why.