Chapter 10 — Quiz

Open Organic Chemistry Project

Chapter 10 — Quiz

Twenty-five questions for self-assessment. Answers at the end. Don't peek.

1. $S_N2$ kinetics is: (a) first order overall (b) second order overall (c) zero order in nucleophile (d) third order overall

2. The $S_N2$ mechanism proceeds via: (a) carbocation intermediate (b) carbanion intermediate (c) concerted, single-step transition state (d) radical chain

3. Stereochemistry at the reacting carbon in $S_N2$: (a) retention (b) racemization (c) inversion (Walden inversion) (d) random

4. $S_N2$ favors which substrate? (a) tertiary (b) secondary (c) primary (d) methyl (most reactive overall)

5. Best solvent for $S_N2$: (a) water (b) methanol (c) DMSO/DMF (polar aprotic) (d) hexane

6. Best leaving group of these: (a) F⁻ (b) Cl⁻ (c) I⁻ (d) HO⁻

7. Order of nucleophilicity in polar aprotic solvent: (a) I⁻ > Br⁻ > Cl⁻ > F⁻ (b) F⁻ > Cl⁻ > Br⁻ > I⁻ (c) all equal (d) depends only on charge

8. The transition state in $S_N2$ is: (a) a stable intermediate that can be isolated (b) trigonal-bipyramidal at the central carbon, with partial bonds to Nu and X (c) a tetrahedral intermediate (d) a free carbocation

9. An alcohol does not undergo $S_N2$ directly because: (a) HO⁻ is too small (b) HO⁻ is a terrible leaving group ($pK_a$ of conjugate acid is 15.7) (c) C–OH bond is too short (d) Alcohols are too polar

10. Methyl vs. tertiary $S_N2$ rate ratio is approximately: (a) 1 (b) 10:1 (c) 1000:1 (d) $10^9$:1

11. $S_N2$ on a cyclohexyl bromide requires: (a) the bromine in the axial position (so backside attack is possible) (b) the bromine in the equatorial position (c) ring opening (d) impossible — $S_N2$ doesn't work on cyclohexanes

12. After $S_N2$ inversion, the $R/S$ label: (a) always flips (b) always stays the same (c) flips only if the priority order of groups changes (d) is undetermined

13. Iodide as nucleophile in a Finkelstein reaction works in acetone because: (a) Acetone is a polar aprotic solvent (fast $S_N2$) AND NaBr precipitates out, driving the equilibrium forward (b) Acetone is a radical initiator (c) Acetone catalyzes the reaction (d) None of the above

14. SAM's role in biological methylation: (a) the donor of methyl group via $S_N2$ at the methyl carbon (b) a proton donor (c) a redox reagent (d) a leaving group

15. A bulky strong base (like t-butoxide) on a primary alkyl halide: (a) gives fast $S_N2$ (b) gives slow $S_N2$ because of steric blockade; favors $E2$ instead (c) gives no reaction (d) does $S_N1$

16. The α-effect refers to: (a) An α-H that is unusually acidic. (b) A nucleophile with a lone pair on the atom adjacent to the attacking atom, which makes it surprisingly reactive. (c) The α-position in a sugar. (d) None of the above.

17. If a reaction's rate doubles when [Nu] doubles, the reaction is: (a) zero-order in nucleophile (b) first-order in nucleophile (consistent with $S_N2$) (c) second-order in nucleophile (d) the kinetics is undetermined

18. A reaction shows rearrangement products. This rules out: (a) $S_N2$ (which has no carbocation intermediate to rearrange) (b) $S_N1$ (c) $E2$ (d) all radical mechanisms

19. Which of the following is the BEST nucleophile in DMSO at room temperature? (a) $H_2O$ (b) $HO^-$ (c) $RS^-$ (thiolate, e.g., glutathione) (d) $Cl^-$

20. The Williamson ether synthesis proceeds by: (a) Acid-catalyzed dehydration (b) $S_N2$ of an alkoxide on an alkyl halide (c) Free-radical bromination (d) Friedel-Crafts alkylation

21. Bromobenzene ($C_6H_5Br$) does not undergo $S_N2$ because: (a) The carbon is $sp^2$ (aromatic), so backside attack is geometrically blocked (b) The bromine is too small (c) Aromatic compounds always undergo SN1 (d) Steric crowding from the ring

22. When sulfur mustard alkylates DNA, the active electrophilic species is: (a) sulfur mustard itself (b) an episulfonium ion (3-member ring formed by intramolecular $S_N2$) (c) chloride ion (d) sulfide

23. Activation energy of typical $S_N2$ reactions is approximately: (a) 5 kcal/mol (b) 18 kcal/mol (c) 50 kcal/mol (d) 100 kcal/mol

24. Why are nitrogen mustards used as anticancer drugs: (a) They selectively kill only cancer cells via SN1 (b) They alkylate DNA, cross-linking it and preventing replication; cancer cells (rapidly dividing) are more sensitive (c) They block protein synthesis (d) They activate the immune system

25. Distinguishing $S_N2$ from $S_N1$ on a 2° substrate, you would: (a) Vary the [nucleophile] and watch for rate response (yes for $S_N2$, no for $S_N1$) (b) Check stereochemistry of the product (inversion vs. racemization) (c) Compare reaction rates in polar aprotic vs. polar protic solvent (d) All of the above are diagnostic

Answer key

b. 2. c. 3. c. 4. d. 5. c. 6. c. 7. b. 8. b. 9. b. 10. d. 11. a. 12. c. 13. a. 14. a. 15. b. 16. b. 17. b. 18. a. 19. c. 20. b. 21. a. 22. b. 23. b. 24. b. 25. d.

Scoring guide: - 22–25 correct: You're ready for Chapter 11. - 18–21: Review the mechanism diagrams and substrate-effect tables. - 14–17: Re-read sections 10.2–10.6 carefully. - Below 14: Re-read the entire chapter and do at least 20 of the exercises before moving on. $S_N2$ is the foundation of every later mechanism chapter.