Organic Reactions & Mechanisms (CHEM 220) · Module 1 of 9
Structure & Stereochemistry Foundations
The 3D structure and stereochemical vocabulary every mechanism argument depends on.
Hybridization & Molecular Geometry Review
- Carbon's hybridization state (sp³, sp², sp) determines bond angles (109.5°, 120°, 180°) and geometry.
- sp³ carbons form four single (sigma) bonds; sp² carbons form three sigma bonds plus one pi bond, as in a double bond; sp carbons form two sigma bonds plus two pi bonds, as in a triple bond.
What is the hybridization and approximate bond angle at a carbon double-bonded to another carbon, as in an alkene?
Solution. sp² hybridized, with a bond angle of approximately 120°.
Chirality & Stereocenters
- A stereocenter (typically a carbon) has four different groups attached; a molecule with one stereocenter and no internal symmetry is chiral, meaning it is not superimposable on its mirror image.
- Enantiomers are non-superimposable mirror images of each other, identical in most physical properties but rotating plane-polarized light in opposite directions and often behaving differently with other chiral molecules, like enzymes.
How can you tell if a carbon is a stereocenter?
Solution. Check whether it's attached to four different substituents; if all four are different, it's a stereocenter (also called a chiral center).
R/S Nomenclature (CIP Rules)
- Assign priority to the four groups on a stereocenter using atomic number (higher atomic number = higher priority); if there's a tie, look at the next atoms out.
- With the lowest-priority group pointing away, if priorities 1→2→3 go clockwise, the center is R (rectus); if counterclockwise, it's S (sinister).
What does it mean if a stereocenter is labeled (R)?
Solution. When viewed with the lowest-priority substituent pointing away, the remaining three substituents decrease in priority in a clockwise direction.
Diastereomers & Meso Compounds
- Molecules with two or more stereocenters can have diastereomers: stereoisomers that are not mirror images of each other (unlike enantiomers) and can have different physical properties.
- A meso compound has multiple stereocenters but is achiral overall because it contains an internal plane of symmetry, making it superimposable on its own mirror image.
How is a diastereomer different from an enantiomer?
Solution. Enantiomers are non-superimposable mirror images of each other (opposite configuration at every stereocenter); diastereomers are stereoisomers that are not mirror images — they differ in configuration at only some, not all, stereocenters.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the bond angle around an sp³ hybridized carbon?
2. What makes a carbon a stereocenter?