Cahn–Ingold–Prelog priority rules

The Cahn–Ingold–Prelog rules rank groups attached to stereochemical centers by atomic number and successive structural comparisons, enabling consistent R/S and E/Z descriptors.

Atomic number: The number of protons in an atom’s nucleus, which determines its identity as a chemical element. The first point of comparison when attached atoms differ.
Stereochemistry: The study of how the three-dimensional arrangement of atoms affects molecules and their reactions. CIP provides a standardized language within this broader subject.
Robert Sidney Cahn: A British organic chemist who helped develop the Cahn–Ingold–Prelog system for stereochemical nomenclature. His collaboration with Ingold and Prelog gave the priority rules their name.
Cahn–Ingold–Prelog sequence rules: The formal sequence criteria used to assign relative priorities to ligands in stereochemical nomenclature. This formulation applies the ranking method to more complex structures and ties.
D–L system: A relative stereochemical notation historically used for sugars and amino acids. Unlike CIP’s priority-based absolute descriptors, D–L labels rely on reference compounds.
Sequence rule: A priority-ranking procedure that compares atoms outward from a point of attachment until the first difference appears. It explains how priorities are resolved beyond the directly attached atom.
Stereogenic center: A position in a molecule where exchanging two attached groups produces a stereoisomer. Ranking its attached groups is central to assigning many absolute configurations.
Christopher Kelk Ingold: A British chemist known for foundational work in physical organic chemistry and stereochemical nomenclature. He co-developed the conventions that made absolute configuration describable consistently.
Helicity (chemistry): A stereochemical property describing a molecular structure that twists in a right- or left-handed sense. CIP conventions can assign descriptors to helical stereogenic units.
Optical rotation: The rotation of plane-polarized light as it passes through an optically active substance. An R or S descriptor does not by itself predict the sign of optical rotation.

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