Chapter 11 — Quiz
Twenty-five questions for self-assessment.
1. $S_N1$ kinetics is: (a) second order overall (b) first order in substrate, zero order in nucleophile (c) zero order in substrate (d) third order
2. $S_N1$ stereochemistry at the reacting carbon: (a) inversion (b) racemization (with possible slight inversion bias) (c) retention (d) random non-stereospecific
3. Which substrate is most reactive in $S_N1$? (a) $CH_3-Br$ (b) $CH_3CH_2-Br$ (c) $(CH_3)_2CH-Br$ (d) $(CH_3)_3C-Br$
4. Best solvent for $S_N1$: (a) hexane (b) ether (Et₂O) (c) DMSO (polar aprotic) (d) water (polar protic)
5. Rate-determining step of $S_N1$: (a) nucleophilic attack (b) ionization to form the carbocation (c) proton transfer (d) loss of the leaving group AFTER nucleophile attack
6. Carbocation stability order (most stable first): (a) 3° > 2° > 1° > methyl (b) methyl > 1° > 2° > 3° (c) 1° > 2° > 3° > methyl (d) all equal
7. Methyl halides do $S_N1$: (a) yes, but slowly (b) no — the methyl cation is too unstable (c) only in DMSO (d) only with strong acids
8. Carbocation rearrangement is: (a) impossible in $S_N1$ (b) a signature of $S_N1$, often observed (c) typical of $S_N2$ instead (d) only happens in radical reactions
9. A benzylic carbocation is stable because: (a) hyperconjugation only (b) resonance with the aromatic ring (c) it is anti-aromatic (d) it forms a 5-member ring
10. $S_N1$ rate depends on [nucleophile]: (a) yes, strongly proportional (b) no — nucleophile is not in the rate law (c) only at high concentration (d) inversely proportional
11. A 1,2-hydride shift converts: (a) a primary cation to a tertiary cation (if the geometry allows) (b) only tertiary cations to other tertiary cations (c) doesn't actually happen (d) inverts stereochemistry
12. Adding LiClO₄ (an inert salt) to a solvolysis: (a) accelerates SN1 substantially (~5×) by stabilizing the cation (b) accelerates SN2 substantially (c) has no effect (d) shuts down the reaction
13. Solvolysis of t-butyl chloride in water vs. in DMSO: (a) much faster in water (polar protic supports cation) (b) much faster in DMSO (c) about the same (d) doesn't react in either
14. Why does $(CH_3)_3CCH_2Cl$ (neopentyl chloride) give t-butyl methyl ether (rearranged) on solvolysis in methanol? (a) Because it does $S_N2$ (b) Because the primary cation that forms first rearranges via 1,2-methyl shift to a tertiary cation, which is captured by methanol (c) Because methanol attacks the carbon bearing chloride (d) Because the Cl wanders off
15. The Hammond postulate predicts that the TS₁ of $S_N1$ resembles: (a) the reactant (b) the carbocation intermediate (c) the product (d) the transition state of $S_N2$
16. Why do tertiary alcohols react faster than primary alcohols with HBr? (a) The tertiary cation forms faster (more stable cation) (b) Tertiary OH is a better leaving group (c) Tertiary alcohols are more electrophilic (d) Tertiary alcohols ionize spontaneously
17. A carbocation is which hybridization? (a) sp³ (b) sp² (c) sp (d) all of the above
18. Which best describes the geometry of a carbocation? (a) Tetrahedral (109.5°) (b) Trigonal planar (120°) (c) Linear (180°) (d) Bent
19. Why is the ion-pair effect responsible for inversion bias in $S_N1$? (a) The leaving group blocks one face briefly before diffusing away (b) The nucleophile attacks before the cation forms (c) Steric effects from the cation (d) The solvent inverts the cation
20. Which is a good substrate for $S_N1$? (a) $C_6H_5CH_2Br$ (benzyl bromide) (b) $CH_3CH_2Cl$ (ethyl chloride) (c) $CH_3I$ (methyl iodide) (d) Bromobenzene
21. A 2° benzylic alcohol with HCl gives substitution. What mechanism? (a) $S_N2$ probably (b) $S_N1$ (the 2° benzylic cation is stable due to resonance) (c) Won't react (d) Always $S_N1$ regardless of conditions
22. In aqueous solvolysis, t-butyl bromide gives mostly t-butyl alcohol but ~15% isobutylene. Why? (a) Some radical decomposition. (b) The cation can either be captured by water ($S_N1$) or lose a proton to give the alkene ($E1$ — Chapter 12). (c) Different starting materials. (d) The Cl⁻ attacks back.
23. Which experimental observation distinguishes $S_N1$ from $S_N2$? (a) The kinetic order in nucleophile (1 for SN2; 0 for SN1) (b) The stereochemistry (inversion for SN2; racemization for SN1) (c) The substrate preference (primary for SN2; tertiary for SN1) (d) All of the above
24. Trityl cation $(C(C_6H_5)_3^+)$ is: (a) Unstable, never observed. (b) Highly stable due to resonance with all three phenyls; isolable as a salt. (c) An intermediate in DNA replication. (d) An aromatic system.
25. The Grunwald-Winstein equation $\log(k/k_0) = mY$ tells you: (a) The rate constant for any reaction. (b) The solvent ionizing power's effect on rate, with $m$ characterizing the mechanism. (c) The pKa of any compound. (d) The activation energy.
Answer key
- b. 2. b. 3. d. 4. d. 5. b. 6. a. 7. b. 8. b. 9. b. 10. b. 11. a. 12. a. 13. a. 14. b. 15. b. 16. a. 17. b. 18. b. 19. a. 20. a. 21. b. 22. b. 23. d. 24. b. 25. b.
Scoring guide: - 22+ correct: ready for Chapter 12. - 18–21: review carbocation stability and rearrangement sections. - Below 18: re-read the chapter, especially the comparison table with SN2.