Chapter 8 — Quiz
Twenty-five questions on stereochemistry in reactions: inversion, retention, racemization, syn/anti, asymmetric synthesis.
1. SN2 produces: (a) inversion of configuration (b) racemization (c) retention (d) no change
2. SN1 produces (in pure form): (a) inversion (b) racemization (planar carbocation; equal attack from both faces) (c) retention (d) anti-addition
3. Catalytic hydrogenation of alkenes (H₂/Pd) is: (a) syn (both H's add same face from metal surface) (b) anti (c) stepwise (d) radical
4. Bromine (Br₂) addition to alkenes is: (a) syn (b) anti (via bromonium ion intermediate, attacked from opposite face) (c) random (d) always gives meso
5. A prochiral carbonyl (sp² C) has how many distinct faces? (a) 1 (b) 2 (re and si) (c) 3 (d) 4
6. "Stereospecific" means: (a) one stereoisomer of starting material gives one stereoisomer of product (different starting → different product) (b) one stereoisomer from any starting material (c) racemic product always (d) no stereo control
7. "Enantioselective" means: (a) one enantiomer preferred over the other (b) both enantiomers equal (c) racemic (d) the same as diastereoselective
8. E2 elimination requires geometry: (a) syn-periplanar (b) anti-periplanar (c) gauche (d) random
9. OsO₄ dihydroxylation is: (a) syn (both -OH groups on same face) (b) anti (c) radical (d) anion
10. Diels-Alder cycloaddition is: (a) concerted, syn (and stereospecific) (b) stepwise (c) anti (d) radical
11. A reaction gives 90% (S) and 10% (R). Its enantiomeric excess is: (a) 50% (b) 80% (c) 90% (d) 100%
12. Bromonium ion opening by Br⁻: (a) syn (same face) (b) anti (opposite face — back-side attack on the cyclic intermediate) (c) racemic (d) doesn't happen
13. Hydroboration-oxidation gives: (a) Markovnikov + anti (b) anti-Markovnikov + syn (c) Markovnikov + syn (d) anti-Markovnikov + anti
14. A racemic SN2 product can occur if: (a) The substrate has a planar geometry (b) The mechanism is actually a mix of SN1 and SN2 (some via planar cation) (c) The nucleophile is achiral (d) Always — SN2 always gives racemic product
15. A "concerted" mechanism means: (a) one-step; bonds break and form in the same TS (b) two steps with an intermediate (c) three or more steps (d) requires light
16. "Asymmetric induction" refers to: (a) the use of chirality to make a molecule chiral (b) the rotation of plane-polarized light (c) the inverse of synthesis (d) a bond rotation
17. A chiral catalyst makes one enantiomer: (a) by preferring one face of the prochiral substrate (b) by destroying the other enantiomer (c) by cooling the reaction (d) randomly
18. Kinetic resolution: (a) separates enantiomers based on different reaction rates (b) makes a racemic mixture (c) is the same as DKR (d) always gives 100% ee
19. Dynamic kinetic resolution (DKR) combines: (a) kinetic resolution + racemization (continually replenishes the consumed enantiomer) (b) two SN2 reactions (c) a catalyst + a chiral solvent (d) hydrogenation + dehydration
20. Felkin-Anh model predicts diastereoselectivity in: (a) SN2 (b) addition to chiral α-substituted carbonyls (c) E2 elimination (d) Diels-Alder
21. The thalidomide α-carbon racemizes in aqueous solution because: (a) The α-H is acidic (pKa ~11-13); enolization removes and re-adds the proton, scrambling the stereocenter (b) The molecule is unstable (c) Water is a strong base (d) Enzymes invert the molecule
22. Pure (R)-thalidomide is unsafe because: (a) The α-carbon racemizes in vivo, regenerating the (S) (teratogenic) enantiomer (b) The (R) form is also teratogenic (c) The drug is unstable (d) Both (a) and (b) (in fact, partially)
23. Diastereotopic protons: (a) appear identical in NMR (b) appear as separate NMR signals (distinguishable even by achiral reagents) (c) interchange by rotation (d) are always equivalent
24. "Stereospecific" is a stronger claim than "stereoselective" because: (a) Stereospecific implies the mechanism dictates stereochemistry; stereoselective is a kinetic preference (b) They are synonymous (c) Stereospecific is a higher % ee (d) Stereospecific only applies to SN2
25. A useful diagnostic: observed inversion implies: (a) Concerted backside attack mechanism (SN2-like) (b) Stepwise mechanism with planar intermediate (c) Radical chain (d) E1 elimination
Answer Key
| # | Answer | Explanation |
|---|---|---|
| 1 | a | SN2 = backside attack → inversion |
| 2 | b | SN1 = planar cation → racemization |
| 3 | a | H₂/Pd: syn from metal surface |
| 4 | b | Bromonium → anti attack by Br⁻ |
| 5 | b | re and si faces |
| 6 | a | Stereospecific: starting → product, 1:1 |
| 7 | a | One enantiomer preferred |
| 8 | b | E2 requires anti-periplanar |
| 9 | a | OsO₄: cyclic ester → syn diol |
| 10 | a | Diels-Alder: concerted, syn, stereospecific |
| 11 | b | ee = |
| 12 | b | Bromonium opens by back-side attack → anti |
| 13 | b | Hydroboration: anti-Markovnikov + syn |
| 14 | b | Mixed mechanism gives partial racemization |
| 15 | a | Concerted = one-step |
| 16 | a | Use chirality of catalyst to induce chirality |
| 17 | a | Chiral catalyst prefers one face |
| 18 | a | Different rates separate enantiomers |
| 19 | a | KR + racemization = DKR |
| 20 | b | Felkin-Anh for chiral α-carbonyls |
| 21 | a | α-H is acidic; enolization racemizes |
| 22 | a | Pure (R) racemizes in vivo to (S) |
| 23 | b | Diastereotopic = different signals |
| 24 | a | Stereospecific implies mechanism control |
| 25 | a | Inversion = concerted SN2 |
Concept connections
- Mechanism predicts stereochemistry. Concerted = stereospecific; stepwise with planar intermediate = racemized.
- Cyclic TS or surface delivery → syn addition. Cyclic intermediate opened by back attack → anti addition.
- Re/si faces describe the two faces of a sp² carbon (carbonyl or alkene).
- Stereospecific > stereoselective: stereospecific is mechanism-mandated; stereoselective is kinetic.
- Asymmetric synthesis uses chiral catalysts to enforce face selectivity.
- Thalidomide story: α-stereolability via enolization → pure (R) racemizes in vivo. Mechanism explains why pure-enantiomer drug isn't safe.
Scoring: 22+ = strong understanding. 17–21 = re-read syn/anti and stereospecific definitions. Below 17 = re-read the chapter.