Advanced Topic 753 of 1102 Organic Chemistry
CIP Priority Rules
Cahn-Ingold-Prelog rules assign R or S configuration using atomic number priorities.
In-depth explanation of CIP Priority Rules
The Cahn-Ingold-Prelog rules assign a priority to each group attached to a stereocentre, then describe its configuration as R or S. Priority is decided first by the atomic number of the atom directly attached, so heavier atoms rank higher, and if two atoms are equal the comparison moves outward through the substituents. Multiple bonds are treated by counting duplicate atoms. The molecule is viewed with the lowest-priority group pointing away, and if the sequence of the three other groups from highest to lowest priority runs clockwise the centre is R, and if anticlockwise it is S.
Going deeper: CIP Priority Rules
Key ideas: Rank substituents by atomic number at the stereocentre (I > Br > Cl > S > P > F > O > N > C > H). If tied, move outward along each branch comparing sets of atoms in order, exploring the highest branch first. Double bonds count as duplicated atoms. Orient the lowest priority away from you and trace 1 to 2 to 3: clockwise is R, anticlockwise is S.
Common mistakes and connections: If the lowest priority group points toward you, reverse the answer. Another error is comparing whole groups instead of atom by atom at the first difference, or deciding by group size. Isotopes rank by mass (D above H). Links to E-Z notation, Fischer projections and naming chiral compounds.
Visualization of CIP Priority Rules
CIP priority follows atomic number at the first point of difference: I > Br > Cl > S > O > N > C > H. The interactive lesson shows this as a live animation or chart.
Worked example and use case
In CHFClBr, the priorities are Br greater than Cl greater than F greater than H.
Frequently asked questions about CIP Priority Rules
What is CIP Priority Rules?
Cahn-Ingold-Prelog rules assign R or S configuration using atomic number priorities.
Is CIP Priority Rules a beginner, intermediate or advanced topic?
CIP Priority Rules is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before CIP Priority Rules?
Study these earlier topics in Organic Chemistry first: Meso Compounds, Racemic Mixture, Optical Activity.
What comes after CIP Priority Rules?
The next topics in Organic Chemistry are Fischer Projections and Conformations.
Can I practise CIP Priority Rules with quizzes?
Yes. The interactive lesson includes a self-check question on CIP Priority Rules, and the Organic Chemistry quiz draws on every topic in this area.
Where CIP Priority Rules fits in Organic Chemistry
CIP Priority Rules is topic 25 of 148 in Organic Chemistry.