Intermediate Topic 752 of 1102 Organic Chemistry
Optical Activity
The ability of chiral compounds to rotate plane-polarized light.
In-depth explanation of Optical Activity
Optical activity is the ability of a chiral compound to rotate the plane of plane-polarized light passing through it. It is measured with a polarimeter, and the specific rotation is calculated as [alpha] equals the observed rotation divided by the product of concentration and path length. A compound that rotates light clockwise is labelled dextrorotatory, shown as (+), while one that rotates it anticlockwise is levorotatory, shown as (-). The sign of rotation is determined experimentally and bears no fixed relationship to the R or S configuration.
Going deeper: Optical Activity
Key ideas: Specific rotation is observed alpha divided by (path length in dm times concentration in g per mL), measured with a polarimeter at the 589 nm sodium D line and quoted with temperature. Dextrorotatory is plus (clockwise), levorotatory is minus. Rotation is additive for mixtures and depends on solvent, temperature and wavelength. Only chiral substances, not racemates or meso forms, are active.
Common mistakes and connections: Students use cm for path length instead of dm, or the wrong concentration unit. Another error is relating the sign of rotation to R or S, or to D and L, which are unrelated to sign. Links to enantiomeric excess, mutarotation in sugars (glucose changes from 112 to 52.7 degrees) and chirality.
Visualization of Optical Activity
Observed rotation grows linearly with path length and concentration: alpha = [alpha] x l x c, as measured in a polarimeter. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Glucose is dextrorotatory, shown as (+), while fructose is levorotatory, shown as (-).
Frequently asked questions about Optical Activity
What is Optical Activity?
The ability of chiral compounds to rotate plane-polarized light.
Is Optical Activity a beginner, intermediate or advanced topic?
Optical Activity is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Optical Activity?
Study these earlier topics in Organic Chemistry first: Diastereomers, Meso Compounds, Racemic Mixture.
What comes after Optical Activity?
The next topics in Organic Chemistry are CIP Priority Rules and Fischer Projections.
Can I practise Optical Activity with quizzes?
Yes. The interactive lesson includes a self-check question on Optical Activity, and the Organic Chemistry quiz draws on every topic in this area.
Where Optical Activity fits in Organic Chemistry
Optical Activity is topic 24 of 148 in Organic Chemistry.