Organic chemistryLesson 3 of 3 in the course Stereochemistry

CIP Priority Rules, R/S and E/Z, and Stereochemical Relationships

5 flashcards · 5 quiz questions · 5 match pairs · 4 fill-in sentences · English

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Summary

The Cahn–Ingold–Prelog (CIP) system provides a consistent way to describe the three-dimensional arrangement of atoms in stereoisomers. This lesson introduces the purpose of CIP descriptors and the basic method for ranking substituents.

The CIP sequence rules are used to assign R or S descriptors to stereogenic centers and E or Z descriptors to double bonds. These descriptors let a systematic name specify a molecule's configuration precisely. A molecule may have several stereocenters and double bonds, so each relevant location is described separately.

All flashcards

How does the CIP system rank directly attached atoms?
The atom with the higher atomic number receives higher priority.
How are isotopes of the same element ranked by CIP rules?
The isotope with the greater mass number receives higher priority.
What do E and Z indicate for a double bond?
Z means the higher-priority groups are on the same side; E means they are on opposite sides.
How are enantiomers distinguished from diastereomers by corresponding stereocenter descriptors?
Enantiomers have all descriptors opposite; diastereomers have at least one the same and one different.
What does an Re or Si label identify?
It labels a face of a trigonal planar group using the priority order of its three attached groups.

Quiz questions

  1. In a CIP comparison of a carbon–carbon double bond, how is each bonded atom treated?Answer: As connected to a duplicate copy of the other bonded atom.
  2. How does CIP handle a path through a ring when it reaches an atom already encountered along that path?Answer: It introduces a phantom atom in a finite hierarchical path tree.
  3. A tetrahedral stereocenter has priority group 4 pointing away. What does a clockwise 1 → 2 → 3 path indicate?Answer: The stereocenter has the R configuration.
  4. Two stereoisomers have corresponding configurations (R,S) and (R,R). What is their relationship?Answer: They are diastereomers because one descriptor matches and one differs.
  5. What do Re and Si labels specify for a trigonal planar group?Answer: The identity of a face, not the product's absolute configuration.

Based on the Wikipedia article Cahn–Ingold–Prelog priority rules, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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