Chapter 5 — Quiz

1. The staggered conformation of ethane is more stable than the eclipsed by about: (a) 0.3 kcal/mol (b) 3 kcal/mol (c) 30 kcal/mol (d) 300 kcal/mol

2. A Newman projection shows: (a) A molecule viewed from the side. (b) A molecule viewed along a specific C-C bond. (c) A molecule viewed from above. (d) A 3D ball-and-stick representation.

3. The gauche-anti energy difference in butane is: (a) 0 kcal/mol (b) 0.9 kcal/mol (c) 3 kcal/mol (d) 5.7 kcal/mol

4. The chair conformation of cyclohexane has: (a) Angle strain but no torsional strain. (b) Torsional strain but no angle strain. (c) Both types of strain. (d) Essentially no strain.

5. An axial substituent on cyclohexane is: (a) In the average plane of the ring. (b) Perpendicular to the average plane of the ring. (c) Always higher in energy than an equatorial substituent. (d) Both b and c (for substituents larger than H).

6. A ring flip of cyclohexane: (a) Converts axial substituents to equatorial and vice versa. (b) Breaks C-C bonds. (c) Changes the molecular formula. (d) Does nothing observable.

7. The A-value of a methyl group on cyclohexane is approximately: (a) 0.2 kcal/mol (b) 1.7 kcal/mol (c) 5 kcal/mol (d) 10 kcal/mol

8. Which cycloalkane has the most strain per $CH_2$? (a) cyclohexane (b) cyclopentane (c) cyclobutane (d) cyclopropane

9. The Gibbs free energy equation: (a) $\Delta G = \Delta H + T\Delta S$ (b) $\Delta G = \Delta H - T\Delta S$ (c) $\Delta G = T\Delta S$ (d) $\Delta G = \Delta H$

10. A negative $\Delta G$ means: (a) Reaction is thermodynamically favorable. (b) Reaction is fast. (c) Reaction is endothermic. (d) Reaction has no barrier.

11. At room temperature, a $\Delta G$ of about 1.4 kcal/mol corresponds to $K_{eq}$ of: (a) 0.1 (b) 1 (c) 10 (d) 1000

12. The Arrhenius equation is: (a) $k = A e^{-E_a/RT}$ (b) $k = A e^{E_a/RT}$ (c) $k = A \log(-E_a/RT)$ (d) $k = E_a / T$

13. A reaction has $E_a = 20$ kcal/mol. Doubling the temperature will: (a) Roughly halve the rate. (b) Have no effect on the rate. (c) Substantially increase the rate. (d) Stop the reaction.

14. Increasing temperature by 10 °C typically: (a) Doubles the rate. (b) Halves the rate. (c) Has no effect. (d) Changes the equilibrium constant but not the rate.

15. The transition state of a reaction: (a) Is a stable intermediate that can be isolated. (b) Is the highest-energy configuration on the reaction path. (c) Contains all the products. (d) Is reached only at low temperature.

16. The Hammond postulate says: (a) Reactions always proceed via the lowest-energy TS. (b) The TS resembles whichever of reactants or products is closer to it in energy. (c) TSs cannot be drawn. (d) Reactions at low T are always kinetically controlled.

17. In an exothermic reaction, the TS is: (a) Late (product-like). (b) Early (reactant-like). (c) Exactly halfway. (d) At the highest possible energy.

18. Thermodynamic control is favored by: (a) Low temperature, short reaction time. (b) High temperature, long reaction time (allows equilibration). (c) No solvent. (d) Cold surroundings only.

19. A reaction with $\Delta G = -15$ kcal/mol but $E_a = 50$ kcal/mol: (a) Will run fast at room temperature. (b) Will not run at room temperature on reasonable timescales. (c) Is thermodynamically unfavorable. (d) Has no products.

20. The enthalpy of combustion of methane is approximately: (a) +10 kcal/mol (b) -10 kcal/mol (c) +200 kcal/mol (d) -200 kcal/mol

Answers: 1. b. 2. b. 3. b. 4. d. 5. d. 6. a. 7. b. 8. d. 9. b. 10. a. 11. c. 12. a. 13. c. 14. a. 15. b. 16. b. 17. b. 18. b. 19. b. 20. d.