Study guides › C/P › Organic Chemistry
Stereochemistry
Molecules with the same connectivity can differ in three-dimensional arrangement. You will spot stereocenters, assign R/S and E/Z labels, name the relationship between two stereoisomers, and use optical rotation and Fischer projections.
Study this chapter in PrepMed (free)
What this chapter covers
- Chirality & Stereocenters. Chirality questions ask you to spot stereocenters, count stereoisomers, and tell a chiral molecule from an achiral one.
- CIP Priorities & R/S Configuration. R and S labels are computed, not guessed. Learn the priority rules once and every stereocenter becomes routine.
- Enantiomers, Diastereomers & Meso Compounds. Enantiomer, diastereomer, meso, epimer: the MCAT asks which word fits a pair. You can decide from R/S labels alone.
- Optical Activity, Racemic Mixtures & Fischer Projections. Rotation calculations, enantiomeric excess, and Fischer-projection rules all show up as quick MCAT points.
- E/Z Isomerism & Configuration Versus Conformation. Double bonds and rings lock groups in place and create geometric isomers, which are always diastereomers.
Key points to know
- Chiral = not superimposable on its mirror image
- Stereocenter = sp³ atom with four different groups
- Higher atomic number gets higher priority; go outward on ties
- Multiple bonds count as duplicated atoms
- Enantiomers: all centers inverted; same properties except rotation
- Diastereomers: not mirror images; different properties
- [α] = α / (l·c) with l in dm and c in g/mL
- Racemic mixture: 50:50, no net rotation
Try a practice question
Which molecule is chiral?
- mol:CC(C)CC{2-methylbutane}
- mol:CCC(C)CC{3-methylpentane}
- mol:CCC(C)Br{2-bromobutane}
- mol:CC(C)Br{2-bromopropane}
Show the answer
C. Only 2-bromobutane has a carbon with four different groups (H, Br, CH₃, C₂H₅). In the other three, the candidate carbon carries two identical groups (two CH₃ or two C₂H₅).
Common questions
What does this chapter cover?
Stereochemistry covers Chirality & Stereocenters, CIP Priorities & R/S Configuration, Enantiomers, Diastereomers & Meso Compounds, Optical Activity, Racemic Mixtures & Fischer Projections, E/Z Isomerism & Configuration Versus Conformation.
What should I know first?
Chiral = not superimposable on its mirror image Stereocenter = sp³ atom with four different groups Higher atomic number gets higher priority; go outward on ties
24 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.