dextrorotation

Dextrorotation is the property of certain chiral substances to rotate plane-polarized light clockwise (to the right) as it passes through a solution or crystal. The dextrorotatory enantiomer is indicated by the plus sign (+) or by the label d-, while the opposite enantiomer rotates light to the left and is indicated by the minus sign (−) or l-.

The direction and magnitude of rotation are determined experimentally with a polarimeter. The observed rotation α depends on wavelength, temperature, solvent, and concentration. The specific rotation [α] is defined by the equation [α] = α/(l c), where α is the observed angle in degrees, l is the path length in decimeters, and c is the solute concentration in grams per milliliter. Standard reporting uses the sodium D line (589 nm) and a specified temperature, commonly 20°C.

Dextrorotation is about optical activity, which is related but not identical to the stereochemical D/L designation. The D/L system is based on historical reference to glyceraldehyde and describes configuration, while dextrorotation describes the direction of light rotation. Thus a D enantiomer can be dextrorotatory or levorotatory depending on its specific rotation.

Applications of dextrorotation include assessing enantiomeric purity and identifying chiral compounds in chemistry and pharmacology. Optical rotation measurements help characterize mixtures, monitor reactions, and support quality control in the production of enantiomerically enriched substances. The phenomenon was first studied in the 19th century by scientists such as Jean-Baptiste Biot, who developed polarimetry to quantify optical activity.