Stereochemistry describes how atoms are arranged in three dimensions, which can change a molecule’s properties even when the molecular formula is the same. This cheat sheet helps students assign R/S configurations at chiral centers and E/Z configurations in alkenes. These skills are essential for organic chemistry, reaction mechanisms, naming compounds, and understanding biological molecules.
A clear reference reduces confusion when working with wedges, dashes, priorities, and double bonds.
The core idea is to rank attached groups using the Cahn-Ingold-Prelog priority rules, then use the spatial arrangement to assign a label. For R/S, priorities through are assigned around a stereocenter, and the direction from gives the configuration when group points away. For E/Z, the higher-priority group on each carbon of a double bond is identified.
If the high-priority groups are on the same side, the alkene is ; if they are on opposite sides, it is .
Key Facts
- A stereocenter is often a tetrahedral carbon bonded to four different groups, giving a possible chiral center.
- Cahn-Ingold-Prelog priority is assigned by atomic number, so higher atomic number means higher priority, such as .
- If two atoms directly attached to the stereocenter are the same, compare the next atoms outward as ordered sets until the first point of difference is found.
- Multiple bonds are treated as if the atom is bonded to duplicate atoms, so is treated like and for priority comparison.
- For R/S assignment, place priority away; if is clockwise, the configuration is .
- For R/S assignment, place priority away; if is counterclockwise, the configuration is .
- If priority points toward you, assign the apparent direction from and then reverse it.
- For E/Z alkenes, means the two higher-priority groups are on the same side of the double bond, and means they are on opposite sides.
Vocabulary
- Chiral
- A molecule or object is chiral if it is not superimposable on its mirror image.
- Stereocenter
- A stereocenter is an atom where exchanging two attached groups creates a different stereoisomer.
- Enantiomers
- Enantiomers are non-superimposable mirror-image stereoisomers.
- Diastereomers
- Diastereomers are stereoisomers that are not mirror images of each other.
- Cahn-Ingold-Prelog rules
- The Cahn-Ingold-Prelog rules rank groups by atomic number and connectivity to assign and configurations.
- Configurational isomer
- A configurational isomer is a stereoisomer that cannot be converted to another stereoisomer without breaking and reforming bonds.
Common Mistakes to Avoid
- Ranking by size instead of atomic number is wrong because Cahn-Ingold-Prelog priority depends first on atomic number, not group bulk or mass.
- Forgetting to point priority away gives the wrong R/S label because clockwise and counterclockwise only apply directly when the lowest-priority group is behind the stereocenter.
- Using cis/trans when E/Z is required can be wrong because E/Z depends on priority, not whether identical groups are present.
- Ignoring atoms beyond the first attached atom leads to incorrect priority ties because groups with the same first atom must be compared outward until a difference appears.
- Treating double bonds as ordinary single bonds is wrong because multiple bonds are counted as duplicate attachments for Cahn-Ingold-Prelog priority comparisons.
Practice Questions
- 1 Assign priorities through for a stereocenter bonded to , , , and .
- 2 A stereocenter has priority pointing away, and the path is clockwise. What is the configuration?
- 3 For an alkene, the higher-priority groups on the two double-bond carbons are on opposite sides. Is the configuration or ?
- 4 Explain why a molecule with one stereocenter can have a non-superimposable mirror image, but an alkene needs restricted rotation and different priority groups to show stereochemistry.