This section makes chirality measurable. A chiral molecule is optically active: it rotates the plane of plane-polarized light. In a polarimeter, polarized light passes through a solution of the compound and the plane emerges rotated by an angle. Rotation to the right (clockwise) is dextrorotatory (+ or d); to the left is levorotatory (− or l). Critically, the (+)/(−) sign is an experimental observation and is not predictable from the R/S configuration — a common student misconception. R does not mean (+).

The size of the rotation depends on concentration and path length, so chemists report a standardized specific rotation [α], which is a physical constant of the compound. A pair of enantiomers rotates light by equal magnitude in opposite directions. A racemic mixture (50:50 enantiomers) therefore shows zero net rotation — it is optically inactive despite containing chiral molecules. Enantiomeric excess (ee) quantifies how far a sample departs from racemic: ee = (observed rotation ÷ specific rotation of pure enantiomer) × 100%.

Fischer projections are the flat shorthand for drawing stereocenters, especially in sugars and amino acids. The rules are strict because the drawing encodes 3-D information: horizontal lines point toward the viewer (wedges), vertical lines point away (dashes), and the carbon chain runs vertically with the most oxidized carbon (e.g., the aldehyde) at the top. Assigning R/S from a Fischer projection has a built-in shortcut: assign priorities normally, but because the lowest-priority group is usually on a horizontal (toward you) line, the apparent rotation is reversed — if it looks S, it is actually R, and vice versa. D/L nomenclature (used for sugars and amino acids) is read off the bottom-most stereocenter: –OH (or –NH₂) on the right = D, on the left = L. D/L is a separate system from R/S and from (+)/(−).

"Bend the Light"