Chapter 7 — Quiz
Twenty-five questions on stereochemistry — chirality, R/S, enantiomers vs diastereomers, meso compounds, optical activity.
1. A chiral center is: (a) any carbon in a ring (b) any sp³ carbon (c) an sp³ carbon with four different groups (d) a double-bonded carbon
2. The R/S system: (a) measures optical rotation (b) labels stereocenters by priority (CIP rules) (c) describes ring sizes (d) is only for sugars
3. In CIP priority, $-NH_2$ vs $-CH_3$: which has higher priority? (a) -NH₂ (N has higher atomic number than C) (b) -CH₃ (c) Equal (d) Depends on context
4. In CIP priority, $-OH$ vs $-Cl$: which has higher priority? (a) -OH (b) -Cl (Cl has higher atomic number than O) (c) Equal (d) Depends on context
5. Enantiomers: (a) have different physical properties (mp, bp, etc) (b) have same physical properties in achiral environment, opposite optical rotation (c) always rotate light clockwise (d) cannot exist in the same molecule
6. Diastereomers: (a) are mirror images (b) have same melting point (c) are stereoisomers that are NOT mirror images (d) don't exist
7. A meso compound: (a) has no stereocenters (b) has stereocenters but an internal mirror plane makes it achiral overall (c) is always a mixture (d) rotates light very strongly
8. A racemic mixture: (a) has one enantiomer only (b) has 50:50 of both enantiomers (c) is always dextrorotatory (d) has no stereocenters
9. Racemic ibuprofen has optical rotation: (a) zero (b) +40° (c) -40° (d) depends on temperature
10. How many stereocenters does aspirin (acetylsalicylic acid) have? (a) 0 (b) 1 (c) 2 (d) 3
11. Specific rotation $[\alpha]$ is: (a) always positive (b) independent of concentration (c) normalized against concentration and path length (d) only measured for solids
12. In Fischer projections: (a) horizontal bonds point away from viewer (b) vertical bonds point toward viewer (c) horizontal bonds point toward viewer, vertical away (d) no convention exists
13. (E) and (Z) for alkenes: (a) are same as R/S (b) describe geometry; higher-priority groups on opposite (E) or same (Z) sides (c) are randomly assigned (d) apply only to rings
14. A compound with 2 stereocenters has: (a) exactly 2 stereoisomers (b) up to 4 stereoisomers (some may be meso, reducing this) (c) exactly 1 stereoisomer (d) exactly 3
15. (R)-glyceraldehyde is: (a) identical to D-glyceraldehyde (b) the enantiomer of D-glyceraldehyde (c) a meso compound (d) not a real compound
16. Natural amino acids (except glycine) are: (a) D (b) L (c) racemic (d) no fixed configuration
17. Glycine has: (a) one stereocenter (b) two stereocenters (c) zero stereocenters (achiral; the α-C bears two H's) (d) an undefined center
18. The two enantiomers of thalidomide: (a) have identical IR spectra in achiral solvent (b) have identical biological activity (c) have different molecular formulas (d) cannot be made in the lab
19. Enantiomeric excess (ee) of a 90% (R) / 10% (S) sample: (a) 80% ee (b) 90% ee (c) 100% ee (d) 50% ee
20. A compound with 3 independent stereocenters has up to: (a) 4 stereoisomers (b) 6 stereoisomers (c) 8 stereoisomers (d) 16 stereoisomers
21. In CIP priority, what is the priority of $-COOH$ vs $-CH_2OH$? (a) -COOH > -CH₂OH (at the central C, O,O,O beats O,H,H) (b) -CH₂OH > -COOH (c) Equal (d) Depends on context
22. Why does (R)-2-bromobutane and (R)-2-chlorobutane potentially have different signs of optical rotation, despite both being (R)? (a) The R/S descriptor is structural; the sign of rotation depends on the actual electrons interacting with light (b) (R) is always (+) (c) Halogens have specific rotation rules (d) They cannot have different signs
23. Three modern strategies for obtaining a single enantiomer of a drug: (a) Resolution by diastereomer formation, chiral HPLC, asymmetric synthesis (b) Always use racemic (c) Distillation (d) None — all drugs are racemic
24. Atropisomerism refers to: (a) chirality from a stereocenter (b) chirality from restricted rotation around a single bond (c) chirality from an alkene (d) optical rotation of solutions
25. Why must drug enantiomers be tested separately? (a) They have different biological activities (one may be active, one may be toxic) (b) The FDA has a regulation requiring it (c) Pharmaceutical companies make more money this way (d) Both (a) and (b)
Answer Key
| # | Answer | Explanation |
|---|---|---|
| 1 | c | Stereocenter = sp³ C with 4 different groups |
| 2 | b | CIP rules; structural label |
| 3 | a | -NH₂ wins (N > C at attached atom) |
| 4 | b | -Cl wins (Cl Z=17 > O Z=8) |
| 5 | b | Identical except optical rotation |
| 6 | c | Not mirror images |
| 7 | b | Stereocenters + internal mirror = achiral |
| 8 | b | 50:50 mix of two enantiomers |
| 9 | a | Zero (rotations cancel) |
| 10 | a | Aspirin has no stereocenters |
| 11 | c | Normalized for concentration and path length |
| 12 | c | Horizontal = out, vertical = in |
| 13 | b | E/Z = geometric; CIP priority on each sp² C |
| 14 | b | Up to 2² = 4 |
| 15 | a | (R) = D for glyceraldehyde (by definition) |
| 16 | b | All natural amino acids are L |
| 17 | c | Glycine α-C: H, H, NH₂, COOH; two H's = no stereocenter |
| 18 | a | Identical in achiral; differ only in optical rotation and chiral interactions |
| 19 | a | ee = 90% − 10% = 80% |
| 20 | c | 2³ = 8 |
| 21 | a | -COOH carbon has O,O,O; -CH₂OH has O,H,H |
| 22 | a | R/S is structural; (+/-) is experimental |
| 23 | a | Three valid strategies |
| 24 | b | Atropisomerism = restricted rotation around a single bond |
| 25 | d | Both biological and regulatory reasons |
Concept connections
- Chirality is at sp³ C with 4 different groups (most common). Other sources: atropisomerism, axial chirality, planar chirality, helical chirality.
- R/S is structural; +/- is experimental. The two are independent.
- Enantiomers ↔ identical physical except optical rotation. Diastereomers ↔ different compounds with different physical properties.
- Meso compounds have stereocenters + internal mirror plane → achiral.
- Biology is chiral: receptors built from L-amino acids; receptors distinguish enantiomers.
- Modern pharmaceutical chemistry invests in asymmetric synthesis to give pure single enantiomers; FDA regulation pushes this direction.
Scoring: 22+ = strong understanding. 18-21 = re-read CIP rules and meso definitions. Below 18 = re-read the chapter; stereochemistry is foundational for the rest of the book.