Quiz 2 - (Summer 2023)

1 CHM 205-65: QUIZ II (Summer 2023) Name (please print): Answer directly on the sheets where space has been provided. WRITE CLEARLY! I. (72 pts) Multiple Choice. Choose the one alternative that best completes the statement or answers the question. 1) How many secondary (2°) carbons are found in 5-ethyl-3,3,4-trimethylheptane? A) 1 B) 4 C) 2 D) 3 E) 6 2) A branched alkane has ________ boiling point relative to the isomeric linear alkane. There are ________ London force interactions in the branched alkane. A) a higher; stronger B) a higher; weaker C) a lower, stronger D) a lower; weaker E) the same; similar 3) The energy barrier for carbon-carbon bond rotation in propane is mainly due to ________. A) angle strain B) bond strain C) muscle strain D) steric strain E) torsion strain 4) Which of the following best describes the molecules of a sample of ethane gas at room temperature? A) Almost all of the molecules are frozen or locked in the eclipsed conformation. B) Almost all of the molecules are frozen or locked in the staggered conformation. C) The molecules are rapidly interconverting between the eclipsed and staggered conformations, but at any one time slightly more of them are present in the eclipsed conformation. D) The molecules are rapidly interconverting between the eclipsed and staggered conformations, but at any one time slightly more of them are present in the staggered conformation. E) None of the above statements is correct. 2 5) From the perspective of viewing down the C2-C3 bond, draw the Newman projection of the highest energy conformation of 2,2-dimethylbutane. 6) Convert the Newman structure below to the equivalent line-angle structure. 7) Which of the following correctly ranks the cycloalkanes in order of increasing ring strain per methylene? A) cyclopropane < cyclobutane < cyclohexane < cyclopentane B) cyclohexane < cyclopentane < cyclobutane < cyclopropane C) cyclohexane < cyclobutane < cyclopentane < cyclopropane D) cyclopentane < cyclopropane < cyclobutane < cyclohexane E) cyclopropane < cyclopentane < cyclobutane < cyclohexane 8) Which of the following statements regarding cyclobutane is correct? A) The lowest energy conformation of cyclobutane is a planar one in which all of the bond angles is 90°. B) The lowest energy conformation of cyclobutane is known as the chair conformation. C) The lowest energy conformation is one in which the bond angles are slightly less than 90° even though this increases angle strain. D) The lowest energy conformation is one in which the bond angles are greater than 90° so that angle strain is significantly reduced. E) None of the above statements is correct. 9) Draw the chair conformer of cyclohexane. Label the axial hydrogens (Ha) and the equatorial hydrogens (He). 3 10) In the lowest energy chair conformation of cis-1,3-dimethylcyclohexane, how many axial positions are occupied by hydrogen atoms? A) 2 B) 3 C) 4 D) 5 E) 6 11) Which of the following is not a possible termination step in the free radical chlorination of methane? A) ∙CH3 + Cl2 → CH3Cl + Cl∙ B) ∙CH3 + Cl∙ → CH3Cl C) ∙CH3 + ∙CH3 → CH3CH3 D) ∙CH3 + wall → CH3-wall E) Cl∙ + wall → Cl-wall 12) If the equilibrium constant (Keq) of a reaction is 0.5 then which of the following that must be true? A) The reaction will have an early transition state. B) Reaction equilibrium will favor the products. C) Gibbs free energy (G) is positive. D) Gibbs free energy (G) is negative. E) None of the above statements is true. 13) Which is a measure of the randomness of a system? A) entropy B) enthalpy C) free energy D) halogenation E) stoichiometry 14) The bond dissociation energy is the amount of energy required to break a bond ________. A) homolytically B) heterolytically C) so as to produce the more stable pair of ions D) via hydrogenation E) none of the above 4 15) Energy is ________ when bonds are formed and is ________ when bonds are broken; therefore, bond dissociation energies are always ________. A) released; consumed; exothermic B) released; consumed; endothermic C) consumed; released; exothermic D) consumed; released; endothermic E) consumed; released; isothermic 16) Predict the enthalpy (ΔH) value for the theoretical reaction below, and indicate whether it is endothermic or exothermic. The bond dissociation energy for each bond in Kcal/mol is shown below each reactant and product. A-B 63 + C-D 88 → A-C 47 + B-D 96 A) +8 Kcal/mol, endothermic B) -8 Kcal/mol, exothermic C) +16 Kcal/mol, endothermic D) +8 Kcal/mol, exothermic E) none of the above 17) Consider the reaction (CH3)3CBr + CH3CH2OH → Experimentally one finds that if the concentration of (CH3)3CBr is tripled, the rate of the reaction triples. One also finds that if the concentration of CH3CH2OH is doubled, the rate of the reaction is unchanged. Which of the following correctly describes the kinetics of this reaction? A) The reaction is third order in (CH3)3CBr. B) The reaction is first order in CH3CH2OH. C) The reaction is second order overall. D) The reaction is first order overall. E) none of the above 18) Consider the three-step mechanism for the reaction of A through intermediates B and C to produce D shown below. A→B B→C C→D Ea = 15 kcal/mol, ΔH° = 13 kcal/mol Ea = 10 kcal/mol, ΔH° = -6 kcal/mol Ea = 2 kcal/mol, ΔH° = -20 kcal/mol Which of the three steps is rate-limiting? A) The reaction of A to B. B) The reaction of B to C. C) The reaction of C to D. D) All three steps occur at the same rate; there is no rate-limiting step. E) The most exothermic step is rate-limiting.