Optical Activity
Definition and meaning of Optical Activity in chemistry.
Optical activity is the ability of a chiral compound to rotate the plane of polarized light as it passes through a solution. Enantiomers (non-superimposable mirror images) exhibit optical activity but rotate light in opposite directions.
In more detail
Chiral molecules lack any internal mirror plane, center of symmetry, or other improper symmetry element (an Sn improper rotation axis) and therefore cannot be superimposed on their mirror images. When plane-polarized light passes through a solution of an optically active compound, the chiral environment causes the light's oscillation plane to rotate. The direction (dextrorotatory, indicated as + or d, or levorotatory, indicated as - or l) and magnitude of rotation depend on molecular structure, concentration, path length, wavelength, and temperature. Optical activity is measured using an instrument called a polarimeter.
Key facts
| Molecular requirement | Overall molecular chirality is required; most often this comes from a stereocenter (commonly a carbon bonded to four different groups), but chirality can also arise without a stereocenter, as in axially chiral allenes and biphenyls |
|---|---|
| Property relationship | Enantiomers have equal but opposite rotations; racemic mixtures show no net rotation |
| Measurement unit | Specific rotation [α], in degrees, equal to the observed rotation divided by path length in decimeters times concentration in grams per milliliter (units of deg·mL·g⁻¹·dm⁻¹) |
| Field | Organic Chemistry |
(+)-Tartaric acid, a naturally occurring enantiomer found in grapes, rotates plane-polarized light clockwise (dextrorotatory), while its enantiomer, (-)-tartaric acid, rotates light counterclockwise (levorotatory) by the same magnitude.
Frequently asked questions
Does optical activity prove a compound is a single enantiomer?
Not necessarily. Any amount of optical activity indicates the presence of enantiomeric excess, but a sample may contain a mixture of enantiomers rather than being pure. Optical purity must be calculated from the specific rotation compared to a known pure enantiomer.
Can non-chiral molecules show optical activity?
No. Optical activity requires the molecule to lack any improper symmetry element, such as a mirror plane or center of symmetry. Racemic mixtures (equal amounts of both enantiomers) show no net optical activity because the opposing rotations cancel completely.