plane-polarized light
Summary
Plane-polarized light has its electric field oscillating in a single plane, made by passing unpolarized light through a polarizing filter. Chiral, optically active compounds rotate that plane; the direction (+ clockwise, − counterclockwise) must be measured and cannot be read from R/S.
Detail
Meaning: Plane-polarized light is light whose electric field oscillates in one plane, made by passing unpolarized light (oscillating in all planes perpendicular to its path) through a polarizing filter. A solution of a chiral, optically active compound rotates that plane clockwise (dextrorotatory, +) or counterclockwise (levorotatory, −). Enantiomers rotate by equal amounts in opposite directions, so a racemic mixture shows no net rotation.
Equation: [α] = α / (l × c), where [α] is specific rotation, α is observed rotation in degrees (SI: radians), l is path length in decimeters (SI: meters), and c is concentration in g/mL (SI: kg/m³). Specific rotation is conventionally reported in degrees·mL·g⁻¹·dm⁻¹.
On the MCAT: A polarimeter passage gives α, l and c, and you calculate [α] or compare samples. Expect to predict that the mirror-image enantiomer gives the opposite sign, and that a racemic mixture or a meso compound gives zero. Optical purity (enantiomeric excess) = observed [α] ÷ [α] of the pure enantiomer × 100%.
Common confusion: The sign of rotation is not R/S. R/S comes from CIP priorities applied to the structure, while + or − can only be measured. Also, having stereocenters does not guarantee rotation: a meso compound is achiral overall and does not rotate the light.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.