210
Training Chem Set I
Organic Chemistry: Chem 210
Training Set I
The questions are arranged approximately in the order of the coverage of material in the lecture. This set covers the material of the first exam.
1. A bond is made by the overlap of an sp2 hybrid with an s orbital as shown below. What is the best graphical representation of the bond that forms at the same time?
bond
a)
b)
c)
d)
e)
f)
2. Here is a comparison of and bonds. Which statement is false? a) bonds are formed by head-to-head overlap of atomic orbitals, while bonds are formed by lateral overlap. b) bonds have increased electron density on the bond axis, while bonds have no electron density on that axis. c) bonds have always lower energy than bonds. d) A rotation around bonds does not change the overlap, while rotation around bonds leads to the diminished overlap. e) Only one bond, but one or two bonds can exist between two atoms. 3. Which of the following statements about sp2 orbitals is not true? a) they are always present as a set of three sp2 orbitals b) they are usually used to form bonds (sometimes lone pairs) c) they are 33% s d) the ideal angle between two sp2 orbitals is 120
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Chem 210
Training Set I
4. In a hypothetical molecule, a four-bonded carbon atom uses one sp and two sp5 hybrids to make three of its bonds. What is the remaining hybrid that is used to make the fourth bond? a) pure s b) pure p c) sp d) sp2 e) sp3 f) sp4 g) sp5 h) sp6
5. What are the hybridizations of the two oxygens in the acetate anion (CH3CO2)? a) both sp3 b) both sp2 c) both sp2.5 d) one sp2 and one sp3
6. Ammonia undergoes a rapid "flip", wherein the hydrogens move from one side of the nitrogen atom to the other (like an umbrella in the wind), as shown in the diagram below. At the lowest energy points, the lone electron pair on nitrogen resides in an sp2.1 hybrid, but during the flip the hybridization of nitrogen changes continuously to finally reach the hybridization of the starting point again. What is the orbital that the lone pair resides in at the highest energy point during that transformation? H H Energy N H (all atoms are in one plane)
H H H b) p f) sp4 N c) sp g) sp5 d) sp h)
2
H N H H
a) s e) sp3
7. Let's assume that the H-P-H bond angle in PH3 is 90o (it really is 92o). What is the nature of the orbital holding the lone electron pair? a) sp3 hybrid b) sp2 hybrid c) sp hybrid d) pure s e) pure p f) intermediate between sp3 and sp2
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Chem 210
Training Set I
8. Which of the following molecules have at least one sp hybridized carbon atom?
O I
CO II
CH3CN III
CO2 IV V
OH VI
a) I, II, IV, VI
b) II, III, IV, VI
c) III, V, VI
d) III, IV, VI
9. Which molecules have at least one atom that is sp2 hybridized? A: +CH2CH3 B: CH3OCH3 C: CH2CCH2 D: CH3CCH E: CH2CHCH3 a) A and C b) F and H c) B and I d) A, G, and I F: CO G: OCHCH3 H: CH3CN I: CH3NH2
e) B, D, and H f) B, C, F, and G g) C, D, F, and H h) A, C, E, and G
10. In the methyl anion (CH3), the H-C-H bond angle is 106.6o (1/cos(106.6o) = 3.5). What is the hybridization index of the orbital holding the lone electron pair? a) 3.0 b) 2.7 c) 2.3 d) 2.0 e) 1.8
11. Consider the allyl anion. Which orbital diagram represents the LUMO?
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Chem 210
Training Set I
12-15. The seven lowest molecular orbitals of formaldehyde (H2CO) are arranged according to their energy, from the lowest at the bottom (g) to the highest at the top (a). 12. Which orbital will interact the strongest with the empty s orbital of H+ in the protonation reaction? 13. Which orbital will interact the strongest in the reaction of formaldehyde with the hydride (H)? 14. Which is the C-O orbital? 15. Which orbital can be characterized as mainly C-O?
16. Consider the energy diagram (MO theory) of N2. Which orbital is the highest filled orbital in the energy diagram? a) the orbital built mainly of s atomic orbitals b) the two orbitals (same energy) c) the orbital built mainly of p atomic orbitals d) the * orbital
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Chem 210
Training Set I
17-18. Consider the system of 1,3,5-hexatriene (see below). The molecular orbitals are shown schematically in a random order.
a)
b)
c)
d)
e)
f)
17. Which orbital corresponds to the HOMO of this hexatriene? 18. Which orbital corresponds to the LUMO of this hexatriene? 19. Below are the four lowest energy molecular orbitals of the pentadienyl cation (the highest energy is omitted). Which corresponds to the LUMO?
20. Which of the following best describes the lowest energy molecular orbital of an imine?
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Chem 210 21. Which of the following molecules contain two conjugated bonds?
Training Set I
22. In 1-bromo-3-chloro-1,2-propadiene (BrHC=C=CHCl) each terminal carbon is bonded to halogen and hydrogen atoms. Excluding the central carbon, what is the relative position of the terminal Br-C-H and Cl-C-H groups? a) They are all in the same plane. b) They are in two planes 60o to each other. c) They are in two planes 90o to each other. d) They are in two planes 107o to each other. e) They are in two planes 109.5o to each other. f) They are in two planes 120o to each other. g) They are in two planes that freely rotate versus each other. 23. Assuming that oxygen and sulfur do not form double bonds in SO42, determine the formal charges for this anion (S is the central atom). Atom: a) b) c) d) e) f) g) h) O (all equivalent) +1 +1 +1/2 +1/4 -1 -1 -1/2 -1/4 S -4 -2 -2 -2 +4 +2 +2 +2
24. What is the formal charge on nitrogen in the following species?
a) 1
b) 0
c) +1
d) +2
25. Which of the following compounds has the highest dipole moment?
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Chem 210 26. Which of the following compounds has the shortest C-H bond?
Training Set I
27. Examine only the shortest C--C bond in each of the following molecules. Arrange the molecules in order of increasing bond lengths of their shortest C--C bond. (Hint: Think about how hybridization affects bond lengths.)
H2CCHCHCH2 A
a) b) c) d) A<C<D<B C<D<A<B D<C<A<B D<B<C<A
[H2CCHCH2] B
e) f) g) h)
H2CCCH2 C
HCCH D
B<A<D<C B<C<D<A D<A<C<B B<D<C<A
28. Compare the structure of CO2 and SO2. Which of them has a greater dipole moment and why? a) CO2 has a greater dipole because the electronegativity difference between carbon and oxygen is greater than that between sulfur and oxygen. b) CO2 has a greater dipole because the carbon is sp hybridized, and it forms shorter bonds with more negative charge on oxygens. c) SO2 has a greater dipole because the bond dipoles do not cancel in that molecule. d) SO2 has a greater dipole because one can write several resonance forms that show charge separation. e) Neither molecule has a dipole moment. 29. Which of the following is not a valid resonance structure of the other three?
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Chem 210
Training Set I
30. Some resonance structures of diazomethane are shown below. Which statement about these structures is correct?
H2C N N A
H2C N N B
H2C N N C
H2C N N D
a) The central nitrogen atom is sp hybridized in A and B, but sp2 hybridized in C and D. b) Structure D contributes more to the description of the molecule than structure C. c) All structures satisfy the "octet rule" for all atoms. d) The equilibrium between A and B will be shifted to the left, because the negative charge is more stable on nitrogen than on carbon. e) All structures contribute equally to the description of the molecule. f) None of the statements is correct. 31. Which statement about the reaction of the methyl cation (CH3+) with the methyl anion (CH3) is incorrect? a) This is a Lewis acid Lewis base reaction. b) The HOMO controlling this reaction is the sp3-type orbital on the anion. c) The LUMO controlling this reaction is the sp3-type orbital on the cation. d) Ethane is the product of this reaction. e) There is some change of hybridization of both carbon atoms as the two ions combine. f) The HOMO-LUMO mixing of the two ions will produce two new molecular orbitals in the product, corresponding to the carbon-carbon and * bonds. 32. Which of the following is not a good Lewis base? a) CH3SCH3 b) H2O c) CH3NH2 d) CH3CH3
33. What are the orbitals of the reactants which interact in the Lewis acid-base reaction between BH3 and H2O? a) an empty p orbital on boron and a non-bonding lone pair on oxygen b) an empty sp3 orbital on boron and a non-bonding long pair on oxygen c) a filled orbital on boron and an empty * orbital on oxygen. d) an empty s orbital on the hydrogen and a filled orbital on boron.
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Chem 210 34. For the following two acid base reactions, which statement is true?
Training Set I
a) I is favored to the right, II is favored to the left b) I is favored to the left, II is favored to the right c) I is favored to the right, II is favored to the right d) I is favored to the left, II is favored to the left 35. Consider the following reaction involving two acids shown below: formic acid and HF. Which of the following statements about this reaction are true? A) B) C) D) E) F) G) H) Formic acid is the strongest Brnsted acid in the reaction. HF is the strongest Brnsted acid in the reaction. KF is the strongest Brnsted base in the reaction. KO2CH is the strongest Brnsted base in the reaction. The equilibrium favors the reactants. The equilibrium favors the products. Formic acid has a weaker conjugate base. HF has a weaker conjugate base.
O K+F+ H OH H
O O -K + + HF pKa = 3.2
pKa = 3.8
a) only A and D b) only B and F c) only C and H d) only D and G e) A, D, and F
f) B, D, and H g) A, C, and H h) B, D, E, and H i) A, D, F, and H j) B, C, D, and F
36. How many constitutional isomers of C4H10O contain alcohols (OH groups)? a) 1 b) 2 c) 3 d) 4 e) 5 f) 6
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Chem 210
Training Set I
37. There are nine constitutional isomers with a formula C7H16. How many of these isomers have no secondary hydrogens? a) none b) one c) two d) three e) four f) five
38. According to the IUPAC rules, what is the best name for the following compound?
a) b) c) d) e) f) g)
3-ethyl-4,7-dimethylnonane 3,6-diethylisononane 3,6-diethylisodecane 3,6-diethyl-7-ethylnonane 1,1-diethyl-2,5-dimethylheptane 3,6-diethyl-7-(methylethyl)octane 3-ethyl-4,7-dimethylninenane
39. What is the correct IUPAC name for the following molecule?
a) 4-ethyl-2,5-dimethylnonane b) 5,7-dimethyl-6-ethylnonane c) 3-isobutyl-4-methyloctane d) 4-hexyl-2-methylhexane 40. Which of the following compounds contains at least one secondary alcohol?
a) I, II, IV, VI
b) I, III
c) I, II, III, V
d) I, III, V
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Chem 210
Training Set I
41-46. Fill in the missing words in the passage below with words from the provided list to best preserve the meaning of the paragraph. Some words may be used more than once. The (41) conformation of ethane is 12 kJ/mol more stable than the (42) conformation because of (43) strain. In general, any alkane is most stable when its bonds are (44) . In cycloalkanes three kinds of strain contribute to the overall energy: (1) torsional strain due to bond eclipsing, (2) (45) strain that is the result of repulsive van der Waals interactions, and (3) (46) strain attributable to angle distortions. a) torsional b) rotational c) conformational d) angle e) steric f) hysteric g) eclipsed h) staggered i) repulsive j) aligned
47. Which equilibrium is not rapid at room temperature?
48. Which is the lowest energy conformation of butane?
49. What is the energy difference between the following conformational isomers?
a) 3.6 kcal/mol
b) 4.5 kcal/mol
c) 2.6 kcal/mol
d) 1,8 kcal/mol
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Chem 210
Training Set I
50. What energy diagram would you expect for rotation about the central bond of 2methylbutane?
51. Which of the following molecules have dipole moments? A: gauche conformation of 1,2-dibromoethane B: anti conformation of 1,2-dibromoethane C: trans-1,4-dibromocyclohexane D: cis-1,4-dibromocyclohexane E: tetrabromomethane F: dibromomethane a) A and B e) A, D, and F b) A and D f) A, C and E c) B and E g) B, C and E d) C and F h) B, D and E
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Chem 210
Training Set I
52. The carbon atoms of cyclopropane have been described as having sp3 hybridization, but with "bent" bonds. What is true about the valence-bond picture of cyclopropane? a) The hybrids making the C-C bonds have increased s character as compared to those of ethane. b) The hybrids making the C-C bonds have increased p character as compared to those of ethane. c) The bend C-C bonds have increased electron density above and below the plane of carbon ring (like bonds). d) The hybrids making the C-C bonds have more s character than the hybrids making the C-H bond in this compound. e) All the strain of cyclopropane is due to the angle strain. 53. Which of the two molecules can exist in a more stable form? a) trans-1,4-dimethylcyclohexane b) cis-1,4-dimethylcyclohexane 54. What is the energy difference between the two structures (the negative value means that the structure on the right is more stable)?
a) 1.15 kcal/mol
b) 1.15 kcal/mol
c) 2.3 kcal/mol
d) 2.3 kcal/mol
55. One of the two chair conformations of cis-1-chloro-3-methylcyclohexane is more stable than the other by 15.2 kJ/mol. What is the energy cost of a 1,3-diaxial interaction between a chlorine and a methyl group?. a) 2.1 kJ/mol b) 3.7 kJ/mol c) 5.4 kJ/mol d) 7.9 kJ/mol e) 10.0 kJ/mol f) 13.5 kJ/mol g) 15.2 kJ/mol
56. Which of the following chair cyclohexane structures has the lowest energy?
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Chem 210 57. Which is the most stable form of 1-ethyl-4-methylcyclohexane? Note: Me = -CH3, Et = -CH2CH3
a)
H Me H H H H H Et
Training Set I
b)
H Et
H
H H H
H
Me
c)
Me H
H
H H Et
d)
Et H
H
H Me H
H
H
H
H
e)
Et H
H
H H H
f)
H H
H
H Et H
Me
H
Me
H
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Chem 210
Training Set I
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Chem 210
Training Set I
Answer key: Set I
1. D 2. C 3. A 4. G 5. B 6. B 7. D 8. B 9. H 10. D 11. A 12. B 13. A 14. C 15. G 16. C 17. E 18. A 19. A 20. B 21. C 22. C 23. F 24. C 25. B 26. C 27. C 28. C 29. D 30. F 31. C 32. D 33. A 34. A 35. H 36. D 37. B 38. A 39. A 40. D 41. H 42. G
43. A 44. H 45. E 46. D 47. B 48. C 49. C 50. A 51. E 52. B 53. A 54. D 55. E 56. D 57. C
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Chem 210
Practice Exam 1A
Organic Chemistry: Chem 210
Practice Exam 1A
There are 30 questions on this exam. Check that you have done all of the problems and filled in the first 30 bubbles on the scantron. Most questions are worth 4 points; there are several twopoint questions clearly labeled in the text. The maximum score on this exam is 100 points. Instructions Answer sheet 1) On the scantron, you need to clearly fill: your name and your student number, section number: (it is 001) test form (white = test form A; yellow = test form B). 2) Use a #2 pencil Exam policy 1) No electronic devices of any kind, such as calculators, cell phones, or even more advanced digital watches, are allowed. Possession of such devices during the exam, whether in use or not, is grounds for awarding a zero on the exam. 2) Molecular models are allowed (no instruction pages are permitted, however). 3) There are some blank pages at the end of the test that can be used as scratch paper. 4) Relevant tables, including the periodic table, are attached at the end of this exam. 5) Numerical values given in one question apply only to that question, and should not be used in other questions, unless there is a specific instruction to do so. If necessary, the values from the provided tables should be used, even if they differ from values that you may remember from different sources. 6) The exam results are based strictly on scantron's marks. No extraneous information is used to adjust the scores. Mark your choices with extra care. 7) Feel free to take this copy of the exam with you. The answer key will be posted on the web after the exam (under "News"). Hints 1) As you read the question, underline or circle key words to highlight them for yourself. 2) Questions have only one correct answer, unless indicated otherwise. No partial credit will be given. 3) There is no penalty for guessing.
Page 1 of 10
Chem 210
1. (4 pts) What is the IUPAC name for the following molecule?
Practice Exam 1A
a) 2,4,5,7-tetramethylnonane b) 1,1,3,5,6-pentamethylheptane c) 3,5-dimethyl-isobutylhexane d) 2,3,5,7-tetramethyloctane e) 2,4,6,7-tetramethyloctane
2.
(4 pts) Of the trimethylcyclohexanes shown below, which is lowest in energy?
a)
b)
c)
d)
e)
3.
(4 pts) Which of the following molecules has a dipole moment?
CCl4 a)
Cl C C C C H b)
H Cl
BrC CBr
c)
CH2Cl2 H d)
F C e) C
H F
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Chem 210
4.
Practice Exam 1A
(4 pts) The carbon attached to the OH functional group in cholesterol would be correctly classified as a ____________ carbon.
HO
a) primary b) secondary c) tertiary d) quaternary e) penitentiary
5.
(4 pts) What is the energy difference between these two conformers of trans-3-chloro-1-methylcyclohexane? A positive value means that the structure on the left is more stable.
CH3 Cl CH3
a) b) c) d) e) f) 2.4 kJ/mol 5.8 kJ/mol 4.8 kJ/mol 2.4 kJ/mol 5.8 kJ/mol 4.8 kJ/mol
Cl
6.
(4 pts) A carbon atom forms four bonds to the unspecified number of other atoms. To form these bonds, the carbon atom uses a combination of hybrid and/or atomic orbitals. Which of the following combinations of orbitals is not possible? a) sp2, sp2, sp2, sp5 b) sp2, sp2, sp5, sp5 c) sp3, sp3, sp, p d) sp3, sp3, sp2, sp5 e) sp2, sp2, sp2, p
7.
(4 pts) Which of the underlined atoms in the molecules shown below have sp hybridization? A. CH2CHCH3 B. CH2CCHCl C. CH3CH2+ a) b) c) d) e) D and F D, E, and F A, C, and D B, D, and E A, B ,C, and F D. HCCCH3 E. CH3CN F. (CH3)2CNNH2
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Chem 210
8-9. Consider the following acid-base reaction:
Practice Exam 1A
N
+
Cl Al Cl Cl aluminum trichloride
Cl N Al Cl Cl
pyridine
8.
(4 pts) Identify the critical HOMO in the reactants: a) a orbital of pyridine b) a p orbital of aluminum trichloride c) a non-bonding orbital on nitrogen d) a orbital of aluminum trichloride
9.
(4 pts) Identify the critical LUMO in the reactants: a) a p orbital of aluminum trichloride b) a non-bonding orbital on nitrogen c) a orbital of aluminum trichloride d) a orbital of pyridine
10. (4 pts) How do the two highest-energy occupied molecular orbitals of acetylene (HCCH) differ? One is a molecular orbital with both s and p character, and the other is a bond constructed from p-orbitals. b) They are both constructed out of p-orbitals, with one forming a bond and the other forming a bond. c) They are both formed from p-atomic orbitals that are 120 apart. d) One is bonding and the other is antibonding. e) They are both -bonding orbitals of the same energy and oriented 90 from each other. a) 11. (4 pts) In allene (H2C=C=CH2), the terminal carbons are sp2 hybridized. Each of the terminal H2C groups marks, therefore, a plane, and the two "terminal" planes are 90o from each other. Other compounds with double bonds on successive carbons may also exist. They are called cumulenes. What is the relationship between the two terminal H2C groups in a cumulene containing three consecutive double bonds (H2C=C=C=CH2)? a) b) c) d) e) They are in perpendicular planes. They are in two planes 60 from each other. They are in the same plane. They are in two planes 120 from each other. They are in two planes 109o from each other.
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Chem 210
Practice Exam 1A
12. (4 pts) What is the respective formal charge on the oxygen, the carbon (in bold), and the nitrogen in the molecule shown below? Consider only the resonance structure that is shown below; lone pairs on oxygen and nitrogen are not drawn. O C a) -1 0 b) 0 -1 c) 0 0 d) 0 -1 e) -1 +1 N +1 0 0 +1 0
O H 3C C C N H
13. (4 pts) What is the hybridization of the oxygen and the carbon (in bold) in the molecule shown below. Consider all important resonance contributors; lone pairs on oxygen and nitrogen are not drawn. O sp2 sp3 sp sp2 sp3 C sp2 sp3 sp sp3 sp2
a) b) c) d) e)
O H3C C C N H
14. (4 pts) What is the energy difference between the highest and the lowest-energy conformations of 2,3dimethylbutane? a) b) c) d) e) 26 kJ/mol 33 kJ/mol 7.6 kJ/mol 18.4 kJ/mol 4.4 kJ/mol
15-18. (2 pts each) A schematic representation of -type molecular orbitals for 1,3,5-hexatriene (built out of six p atomic orbitals on carbon atoms) is shown below. The orbitals are shown in a random order.
1,3,5-hexatriene
a)
b)
c)
d)
e)
f)
15. 16. 17. 18.
Which orbital corresponds to the HOMO of 1,3,5-hexatriene? Which of the orbitals have 2 nodal planes (in the bonding sense)? Which of the orbitals has the highest energy? If an extra electron was added to the -system of 1,3,5-hexatriene, which orbital would it occupy?
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Chem 210
19. (4 pts) Which of the following is not a resonance structure of the others?
Practice Exam 1A
O a) b)
O
O c) d)
O
20. (4 pts) How many constitutional isomers are there for the alkyl halide C5H11Cl? a) 6 b) 7 c) 8 d) 9 e) 10
21. (4 pts) Which statement about the following equilibrium is true?
O K t-butoxide
+
H2O
OH pKa = 18
+
K+OH-
pKa = 15.7
a) The equilibrium favors the products. b) t-Butoxide is the dominant anionic species in the equilibrium. c) Water is the weaker acid. d) t-Butoxide is stabilized by resonance.
22.
(4 pts) Which of the following is the product of the reaction between AlCl3 and CH3OCH3?
a) Cl
Cl - + CH3 Al O CH3 Cl d) Cl
b) Cl
Cl + - CH3 Al O CH3 Cl e) Cl
c) Cl
Cl + + CH3 Al O CH3 Cl
Cl - + Al CH3OCH3 Cl
Cl + Al CH3OCH3 Cl
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Chem 210
Practice Exam 1A
23-28. (2 pts each) The molecular orbitals for formaldehyde (O=CH2) are shown on the right. They are arranged in the order of decreasing energy, from the highest at the top (a) to the lowest at the bottom (g).
23. Which orbital corresponds to the highest-energy nonbonding electrons on oxygen? 24. Which orbital holds bonding electrons?
25. Which orbital would be the most important in the reaction of formaldehyde with hydride (H)? 26. Which picture represents the lowest-energy type orbital? 27. Which orbital can be designated as ? 28. Which orbital represents the C-O bond?
29-30. (4 pts each) Each of the following molecules or ions contains at least one C-N bond. The following two questions are based on the Lewis structures of these species. Remember to consider resonance structures, where appropriate. Please note that lone pairs on nitrogen atoms are not shown explicitly. c) (H3C)4N+ d) (H2N)3C+ e) (H2C)2N+
a) H2CNCH3
b) H3CCN
29. Which molecule or ion has the longest C-N bond? 30. Which molecule or ion has the shortest C-N bond?
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Chem 210
Practice Exam 1A
Table 4.1 Energy Costs for Interactions in Alkane Conformers Energy Cost Energy Cost Interaction kJ/mole kcal/mole H-H eclipsed 4.0 1.0 H-CH3 eclipsed CH3-CH3 eclipsed CH3-CH3 gauche 6.0 11.0 3.8 1.4 2.6 0.9
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Chem 210
Practice Exam 1A
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Chem 210
Answer Key: 1A 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. d. b. d. b. c. a. d. c. a. e. c. b. a. d. f. f. b. e. d. c. a. a. b. c. a. g. a. g. c. b.
Practice Exam 1A
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Chem 210
Practice Exam 1B
Organic Chemistry: Chem 210
Practice Exam 1B
There are 28 questions on this exam. Check that you have done all of the problems and filled in the first 28 bubbles on the scantron. Most questions are worth 4 points; there are several two-point questions clearly labeled in the text. The maximum score on this exam is 100 points. Instructions Answer sheet 1) On the scantron, you need to clearly fill: your name and your student number, section number: (it is 001) test form (white = test form A; yellow = test form B). 2) Use a #2 pencil Exam policy 1) No electronic devices of any kind, such as calculators, cell phones, or even more advanced digital watches, are allowed. Possession of such devices during the exam, whether in use or not, is grounds for awarding a zero on the exam. 2) Molecular models are allowed (no instruction pages are permitted, however). 3) There are some blank pages at the end of the test that can be used as scratch paper. 4) Relevant tables, including the periodic table, are attached at the end of this exam. 5) Numerical values given in one question apply only to that question, and should not be used in other questions, unless there is a specific instruction to do so. If necessary, the values from the provided tables should be used, even if they differ from values that you may remember from different sources. 6) The exam results are based strictly on scantron's marks. No extraneous information is used to adjust the scores. Mark your choices with extra care. 7) Feel free to take this copy of the exam with you. The answer key will be posted on the web after the exam (under "News"). Hints 1) As you read the question, underline or circle key words to highlight them for yourself. 2) Questions have only one correct answer, unless indicated otherwise. No partial credit will be given. 3) There is no penalty for guessing.
Page 1 of 10
Chem 210
1. (4 pts) What is the name of the following hydrocarbon according to the IUPAC rules?
Practice Exam 1B
a) b) c) d) e)
2,6-diethyl-2-propyl-3-methylheptane 4-ethyl-4,5,8-trimethyldecane 7-ethyl-3,6,7-trimethyldecane 3,4,7-trimethyl-3-propylnonane 7-ethyl-3,4-dimethyl-3-propyloctane
2. (4 pts) Which of the following isomeric structures has the lowest energy?
a) H H CH3 d) H H 3C H CH3 H H H
b) H H H H H H H CH3 CH3 H H
c) H H H H H H H CH3 H H CH3
H CH3 e) H
H
HCH3 H CH3
3. (4 pts) The compound with the formula of C4H10O contains the alcohol functional group (OH), but has no multiple bonds or rings. How many constitutional isomers are possible for this alcohol? a) 2 b) 3 c) 4 d) 5 e) 6 f) 7
4. (4 pts) Which of the following is not a valid resonance structure of the others?
a)
b)
c)
d)
e)
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Chem 210
Practice Exam 1B
5. (4 pts) Which of the following molecules do not have a dipole moment?
H Br
H Br A
Br H Br C B C H CH2Br2 C
Br C C H D
H Br Br E
Br
a) A, D, E
b) D, E
c) A, B, C
d) D, C
e) B, D, E
6. (4 pts) The barrier to rotation about the C-C bond in bromoethane is 15 kJ/mol (3.6 kcal/mol). What energy value can you assign to an H-Br eclipsing interaction? a) b) c) d) e) 0.8 kcal/mol 1.0 kcal/mol 1.2 kcal/mol 1.4 kcal/mol 1.6 kcal/mol
7-10. A schematic representation of -type molecular orbitals for 3-methylene-1,4-pentadiene (built out of six p atomic orbitals on carbon atoms) is shown below. The orbitals are arranged in random order and the information on the relative size of orbital lobes is omitted.
3-methylene-1,4-pentadiene
a)
b)
c)
d)
e)
f)
7. 8. 9. 10.
(4 pts) Which orbital corresponds to the HOMO of 3-methylene-1,4-pentadiene? (4 pts) Which of the orbitals has 2 nodal planes (in the bonding sense)? (2 pts) Which of the orbitals has the highest energy? (2 pts) Of all occupied orbitals, which one does not contribute any electron density to the third (central) carbon?
Page 3 of 10
Chem 210
Practice Exam 1B
11. (4 pts) Which of the underlined atoms (in the molecules shown below) are of sp2-hybridization type? A: Cl2CCCl2 a) A and D B: CH3OCH3 C: CH3NH2 D: CH3COCH3 E: HCCH
b) B and E
c) A and C
d) C and E
e) B, C, and D
12.
(4 pts) Consider the two chair conformations of cis-1-chloro-2-methylcyclohexane. Which of the following statements about these two isomers is true? a) Both conformations have equally unfavorable 1,3-diaxial interactions, resulting in no net energy difference between them. b) The conformation with the methyl group in an equatorial position more is stable by 4.8 kJ/mol. c) The conformation with the methyl group in an axial position is more stable by 5.2 kJ/mol. d) The conformation with both substituents in equatorial positions is more stable by 10.4 kJ/mol. e) The energy difference between both conformations cannot be established without knowing the value of Cl-CH3 gauche interaction.
13-15. Consider the following reaction of boron trifluoride (BF3) and acetone:
O BF3 + product CH3 CH3
13. (4 pts) What is the critical HOMO of this reaction? a) nonbonding orbital on boron b) orbital of acetone c) orbital of acetone d) nonbonding electron pair orbital on oxygen 14. (4 pts) What is the critical LUMO of the reaction? a) p orbital of BF3 b) orbital of BF3 c) * orbital of acetone d) nonbonding electron pair orbital on oxygen 15. (4 pts) Which of the following is the correct product of this reaction? (Lone electron pairs are not shown explicitly).
a) O BF3
b) O BF3
c) O BF3
d) O BF3
e) OH BF3
Page 4 of 10
Chem 210
16-17. Consider the following species: a) H2CO b) H3CCOO c) CO d) CO32
Practice Exam 1B
16. (2 pts) Which one has the longest CO bond? 17. (2 pts) Which one has the shortest CO bond?
18. (4 pts) Which statement about the following equilibrium is true?
O OH + I pKa = +4 ClO4 II III
O O + HClO4 IV pKa = -10
a) IV is a acting as a base b) II is acting as an acid c) I and III are resonance structures d) the equilibrium favors the reactants e) I is 14 times less acidic than IV 19. (4 pts) If stands for a wavefunction and r is the distance from the nucleus, which of the following physical observables does the picture below represent?
2
(0,0)
r
a) electron density of a 1s orbital b) electron density of a 2s orbital c) electron density of a 3s orbital d) probability of finding a proton in the nucleus e) probability of finding an electron in the valence shell of hydrogen
Page 5 of 10
Chem 210
Practice Exam 1B
20. (4 pts) A carbon atom in H2CXY forms four bonds. It uses an sp2 hybrid to bond to X, and an sp5 hybrid to bond to Y. The two bonds to hydrogens are made using identical hybrids. What are the hybrids that are used to make the C-H bonds? a) sp b) sp2 c) sp3 d) sp4 e) sp5
21. (4 pts) Compare the hybridization of the central carbon atoms in carbon dioxide (CO2) and allene (H2CCCH2). Which statement is correct? a) The hybridization types of these two carbons cannot be compared because of large electronegativity difference between carbon and oxygen in CO2 that does not exist in allene. b) The hybridization of carbon in CO2 cannot be determined, because the lone electron pairs on oxygen do not allow for angle measurements that are necessary to determine the hybrids involved. c) In CO2 the carbon is sp2 hybridized, but in allene the central carbon is sp hybridized. d) In CO2 the carbon is sp hybridized but in allene the central carbon is sp2 hybridized. e) The central carbons in both compounds are sp hybridized. 22-23. Assume that the three structures below fully describe the carbonate ion (CO3-2). Note that in all of these structures lone-pair electrons are not shown explicitly.
O O -1 C O -1 -1 O
-1 O C O O
-1 O C O -1
22. (2 pts) What is the formal charge on the top oxygen atom? a) -1/3 b) -2/3 c) -1/2 d) -3/2 e) -1 f) -2 g) -3
23. (2 pts) What is the bond order of the top oxygen atom? a) 1/3 b) 2/3 c) 4/3 d) 1 e) 2 f) 3 g) 4
24. (4 pts) Which of the following statements about molecular strain is correct? a) b) c) d) e) Cis and trans stereoisomers of substituted cycloalkanes always have equal amount of strain. Large angle strain is present in all cycloalkanes. Gauche strain in butane is steric in origin. All staggered conformations of small alkanes have torsional strain. Strained alkanes yield less heat when burned in oxygen than their strain-free isomers.
Page 6 of 10
Chem 210
Practice Exam 1B
25. (4 pts) Taking into account the standard conventions used to draw three-dimensional structures, which of the following is the correct 3D representation of 2,3-pentadiene?
H C C C H3C CH3 H H C C C CH3 H H C C C H3C CH3
H H3C
a)
H H3C H CH3
b)
H H C C C H3C CH3
c)
C C C
d)
e)
26. (4 pts) A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for an alkane? 2o H 2o C a) b) c) d) e) 3 4 3 3 8 3 4 5 6 4
27. (4 pts) Which of the following statements about resonance structures is false? a) b) c) d) e) Individual resonance structures are imaginary, not real. Resonance forms differ only in the placement of their or nonbonding electrons. Different resonance structures of a substance don't have to be equivalent. The superposition of resonance forms is always more stable than any individual resonance form. In valid resonance structures, all atoms from the second row of the periodic table must have an octet of electrons.
28. (4 pts) Which of the following statements about valence bond (VB) and molecular orbital (MO) bonding theories is true? a) In VB theory only bonding orbitals are formed when bonds are made. b) In MO theory resonance is necessary to account for delocalized electrons. c) Electronegativity differences between bonded atoms are important in VB theory, but are totally irrelevant in MO theory. d) MO and VB theories almost always yield contradictory information about structure of molecules. e) In MO theory all electrons need to be considered to understand the structure of molecules, but the reactivity is controlled to a large degree only by the appropriate HOMOs and LUMOs in the reactants.
End of the exam
Page 7 of 10
Chem 210
Practice Exam 1B
Table 4.1 Energy Costs for Interactions in Alkane Conformers Energy Cost Energy Cost Interaction kJ/mole kcal/mole H-H eclipsed 4.0 1.0 H-CH3 eclipsed CH3-CH3 eclipsed CH3-CH3 gauche 6.0 11.0 3.8 1.4 2.6 0.9
Page 8 of 10
Chem 210
Practice Exam 1B
Page 9 of 10
Chem 210 Answer key: 1 B 1. C 2. D 3. C 4. D 5. B 6. E 7. A 8. A 9. D 10. B 11. A 12. B 13. D 14. A 15. A 16. D 17. C 18. D 19. B 20. C 21. E 22. B 23. C 24. C 25. B 26. E 27. E 28. E
Practice Exam 1B
Page 10 of 10
Chem 210
Practice Exam 1C
Organic Chemistry: Chem 210
Practice Exam 1C
There are 33 questions on this exam. Check that you have done all of the problems and filled in the first 33 bubbles on the scantron. Most questions are worth 4 points; there are several two-point questions clearly labeled in the text. The maximum score on this exam is 100 points. Instructions Answer sheet 1) On the scantron, you need to clearly fill: your name and your student number, section number: (it is 001) test form (white = test form A; yellow = test form B). 2) Use a #2 pencil Exam policy 1) No electronic devices of any kind, such as calculators, cell phones, or even more advanced digital watches, are allowed. Possession of such devices during the exam, whether in use or not, is grounds for awarding a zero on the exam. 2) Molecular models are allowed (no instruction pages are permitted, however). 3) There are some blank pages at the end of the test that can be used as scratch paper. 4) Relevant tables, including the periodic table, are attached at the end of this exam. 5) Numerical values given in one question apply only to that question, and should not be used in other questions, unless there is a specific instruction to do so. If necessary, the values from the provided tables should be used, even if they differ from values that you may remember from different sources. 6) The exam results are based strictly on scantron's marks. No extraneous information is used to adjust the scores. Mark your choices with extra care. 7) Feel free to take this copy of the exam with you. The answer key will be posted on the web after the exam (under "News"). Hints 1) As you read the question, underline or circle key words to highlight them for yourself. 2) Questions have only one correct answer, unless indicated otherwise. No partial credit will be given. 3) There is no penalty for guessing.
1
Chem 210
Practice Exam 1C
1. (4 pts) Cycloalkanes with the formula of C6H12 have 12 different constitutional isomers. What is the maximum number of tertiary carbons possible in any of them? a) 1 b) 2 c) 3 d) 4 e) 5
2. (4 pts) Dipole moments usually are measured on collections of molecules, and represent average values for all conformations present. Which of the following collections of molecules will show a dipole moment (lone pairs are not shown explicitly)? A: ClH2CCH2Cl a) A and C e) A, B, and D B: NH3 b) A and E f) A, C, and E C: BF3 D: H3COCH3 c) B and C g) B, C, and E E: (CCl3)2 d) B and E
3-5. Consider all equivalent resonance structures for the cyclopentadienyl cation shown below.
1
2
3. (2 pts) What is the formal charge on carbon 1? a) 0 b) +1/5 c) +1/3 d) +1/2 e) +1
4. (2 pts) What is the bond order between carbons 1 and 2? a) 1 b) 7/5 c) 3/2 d) 5/3 e) 2
5. (4 pts) This cyclic carbocation has two sets of degenerate molecular orbitals. What is the correct ordering of energies of the MO's for this system?
A
B
C
D
E
a) b) c) d) e)
E=A<D<C=B E<D=B<A=C C=B<A=D<E C=D<B<A=E E<A=D<B=C
2
Chem 210
Practice Exam 1C
6. (4 pts) Taking into account hybridization and resonance effects, rank the following C-C bonds in order of decreasing length.
A
B
C
D
a) b) c) d) e)
A>C>D>B D>B>C>A C>A>D>B A>C>B>D B>D>C>A
7. (4 pts) Which one of the following isomeric structures has the lowest energy?
a)
Br OH Br
b)
HO Br
c)
HO
d)
Br OH
e)
Br
OH
8. (4 pts) Which statement about the following equilibrium is true?
O O O + O H2O O pKa = 16 I II pKa = 13 III IV O O O + HO
a) b) c) d) e)
The equilibrium favors III and IV. I and III are resonance structures. III is 3 times more acidic than II. II is the conjugate acid of I. IV is the strongest base in the system.
9. (2 pts) The torsional strain of a pair of eclipsing C-C bonds in cyclopentane is the same as the torsional strain of a pair of eclipsing C-H bonds. How much strain (in kJ/mol) is relieved on ring puckering from the flat cyclopentane to the envelope conformation, if the angle strain for both conformations is negligible? a) 7 b) 11 c) 21 d) 33 e) 45 f) 52
3
Chem 210
Practice Exam 1C
10. (4 pts) In the methyl anion (H3C-) the H-C-H bond angles are ca. 107. On the other hand, in the allyl anion (H2C=CH-CH2-) the H-C-H bond angles on the terminal carbons are ca. 120. Which statement accounts for this difference? a) The structures are not really different. In both species the lone pair is in an sp3 hybridized orbital, but with a different s character b) In the allyl anion the structure is stabilized by resonance, and in the methyl anion the charge is stabilized by increasing s-character in the orbital with the lone pair. c) A lone pair of electrons is always more stable in a p-type orbital than in an sp3 type orbitals. d) The HOMO of the allyl systems has orbital lobes on all carbons, but the HOMO of the methyl anion is localized mainly on hydrogens. 11. (4 pts) What is the name of the following hydrocarbon according to the IUPAC rules?
a) b) c) d) e) f) g)
3,3-dimethyl-5-propylhexane 6,6-dimethyl-4-propyloctane 3,3-diethyl-5-propyloctane 5,5-dimethyl-3-butyloctane 6,6-diethyl-4-propyllheptane 3,3-dimethyl-5-propyloctane 5,5-diethyl-3-butylheptane
12-13. Imagine a molecule of a flat (all carbons in one plane) cyclobutane with C-C bonds made of pure p orbitals. Let's call it A. 12. (2 pts) What is the H-C-H angle in A? a) 60o b) 90o c) 109.5o d) 120o e) 180o
13. (2 pts) Compare A to another flat cyclobutane, B, where the C-C bonds are made of hybrids with a large p character (and are bent like in cyclopropane), and to the real cyclobutane, C. Arrange these molecules according to the diminishing torsional strain. a) A > B > C d) B > C > A b) A > C > B e) C > A > B c) B > A > C f) C > B > A
4
Chem 210
Practice Exam 1C
14. (4 pts) Which of the following molecules would have the following potential energy diagram for rotation about its central bond? Note: The 0o angle conformation was selected arbitrarily.
21 20 Energy kJ/mol 15 10 5 3.8 o 180 120o 7.6 o 60
21 18
3.8 0 o o 60 o 120
3.8 o 180
torsion angle
a) butane
b) 2-methylbutane
c) 2,3-dimethylbutane d) 2,2,3-trimethylbutane
15. (4 pts) Which statement about strain is incorrect? a) Strain energy is caused by imperfect bond lengths or angles. b) Perfectly staggered bonds do not contribute to the torsional strain. c) Cycloalkanes are puckered (except for cyclopropane) to minimize their torsional strain. d) Strained compounds release less energy during combustion than their strain-free isomers. e) Steric strain is defined as the repulsion that occurs when atoms are forced closer together than their van der Waals contact distance. 16. (4 pts) What is the energy difference (in kcal/mol) between the two compounds shown below? The negative sign indicates that the compound on the right is more stable.
Cl
Cl
a) 2.20
b) 1.55
c) 0.7
d) 1.55
e) 2.20
17. (4 pts) A carbon atom with a formal charge of +1 is bonded to three different substituents. The carbon uses one sp hybrid and one sp3 hybrid to make two of the bonds. What orbital is used to make the third bond? a) sp b) sp1.5 c) sp2 d) sp3 e) sp4 f) sp5
5
Chem 210
Practice Exam 1C
18-20. Consider the following reaction of benzoic acid and titanium tetrachloride. The lone pairs are not shown explicitly here.
O OH + TiCl4 product
18. (2 pts) Which of the following statements about this reaction is false? a) TiCl4 has low energy empty orbital available. b) Benzoic acid is the Brnsted base in this reaction. c) This is a Lewis acid-base reaction. d) Benzoic acid is the Lewis base in this reaction. 19. (2 pts) What is the critical HOMO of the reaction? a) nonbonding electron pair orbital on carbonyl oxygen (C=O) b) nonbonding electron pair on hydroxyl oxygen (OH) c) nonbonding electron pair on chlorine (not shown) d) lone pair in a d orbital on Ti (not shown explicitly here) e) * orbital of benzene moiety 20. (2 pts) Which of the following is the main product of this reaction? (Lone electron pairs are not shown explicitly).
a) TiCl4 O OH b) TiCl4 OH c) O O H TiCl4 d) O O H TiCl4
O
21. (4 pts) Which one of the following structures is not a resonance form of the others?
a) b) c) d) e)
6
Chem 210
Practice Exam 1C
22. (4 pts) One chair conformation of cis-1,3-dimethylcyclohexane is 23.0 kJ/mol less stable than the other chair conformation. What is the strain energy difference (in kJ/mol) of the two chair conformations of 1,1,3-trimethylcyclohexane? (a) 8.3 b) 12.0 c) 15.4 d) 17.8 e) 20.0
23-28. The MOs of methylene, :CH2, in which all electrons are paired, are shown on the left. They are arranged according to their increasing energy from the bottom to the top, with (f) having the lowest energy and (a) having the highest. Answer the following questions by selecting the appropriate orbital. 23. (2 pts) Which orbital is the HOMO? 24. (2 pts) Which orbital is the LUMO? 25. (2 pts) Which orbital will be the most important in reaction of methylene with the hydride ion? 26. (2 pts) Which orbital corresponds to the lone electron pair? 27. (2 pts) If methylene were to be protonated, which orbital would be involved in the reaction? 28. (2 pts) All orbitals shown are combinations of atomic orbitals, except one. Which one?
29. (4 pts) Methylene, :CH2, with all its electrons paired (see questions 23- 28) reacts with H+ to give a new product. Which of the following reactions will yield the same product? a) removal of a proton from methane b) removal of a hydrogen atom from methane c) removal of a hydride from methane d) removal of two hydrogen atoms from methane e) removal of a hydride and a proton from methane
7
Chem 210
30-31. Consider the following compounds (Lone pairs are not explicitly shown).
a) O OH OH N HO CF3 b) c) d)
Practice Exam 1C
e)
N H
30. (2 pts) Which of the compounds is the strongest Brnsted acid? 31. (2 pts) Which of the compounds is the strongest Lewis base? 32. (4 pts) Which of the underlined atoms (in the molecules shown below) are of the sp3-hybridization type? Lone electrons pairs are not shown explicitly. A: (CH3)2O a) A and B B: (CH3)3N b) C and D c) A and C C: (CH3O)3B d) B, C, and D D: (CH2=CH)3C e) A, C, and D
33. (4 pts) The H-N-H bond angle in ammonia (:NH3) is 107o. Compared to ammonia, what would be the expectation for H-P-H bond angle in phosphine (:PH3)? a) The bond angles will be essentially the same because phosphorus is in the same column of the periodic table as nitrogen. b) The bond angle would be larger because phosphorus needs more p-character in its hybrids to form its longer bonds to hydrogen atoms. c) The bond angle would be larger because phosphorous is bigger and needs more space for its bonds. d) The bond angle would be smaller because phosphorous forms stronger bonds with hydrogen atoms. e) The bond angle would be smaller because phosphorus is less electronegative and needs more scharacter in its lone-pair hybrid.
------------------------------------------------------------------------------------------------------------------------------
End of the exam
8
Chem 210
Practice Exam 1C
Total Strain Energies in Cycloalkanes ring size kJ/mol kcal/mol 115 27.6 3 110 26.4 4 27 6.5 5 0 0 6 26 6.3 7 40 8.6 8
Energy Costs for Interactions in Alkane Conformers Interaction kJ/mole kcal/mole H-H eclipsed 4.0 1.0 H-CH3 eclipsed 6.0 1.4 CH3-CH3 eclipsed 11.0 2.6 CH3-CH3 gauche 3.8 0.9
9
Chem 210
Practice Exam 1C
Practice Exam 1C Answer key 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. C E B B E A A E D B F E A B D A D 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. B A B C C D C C D D C C A E A E
10
Chemistry 210 Exam I September 25, 2007 Form A There are 33 questions on this exam. Check that you have done all of the problems and filled in the first 33 bubbles on the scantron. Most questions are worth 4 points; there are several two-point questions clearly labeled in the text. The maximum score on this exam is 100 points. Instructions Answer sheet 1) On the scantron, you need to clearly fill: your name and your student number, section number: (12:20 class is section 001, 3:25 class is section 002) test form (white = test form A; yellow = test form B). 2) Use a #2 pencil Exam policy 1) No electronic devices of any kind, such as calculators, cell phones, or even more advanced digital watches, are allowed. Possession of such devices during the exam, whether in use or not, is grounds for awarding a zero on the exam. 2) Molecular models are allowed (no instruction pages are permitted, however). 3) There are some blank pages at the end of the test that can be used as scratch paper. 4) Relevant tables, including the periodic table, are attached at the end of this exam. 5) Numerical values given in one question apply only to that question, and should not be used in other questions, unless there is a specific instruction to do so. If necessary, the values from the provided tables should be used, even if they differ from values that you may remember from different sources. 6) The exam results are based strictly on scantron's marks. No extraneous information is used to adjust the scores. Mark your choices with extra care. 7) Feel free to take this copy of the exam with you. The answer key will be posted on the web after the exam (under "News"). Hints 1) As you read the question, underline or circle key words to highlight them for yourself. 2) Questions have only one correct answer, unless indicated otherwise. No partial credit will be given. 3) There is no penalty for guessing.
1
1. (4 pts) How many constitutional isomers of C4H9Br are there? a) 2 b) 3 c) 4 d) 5 e) 6
2. (4 pts) If the CH bond eclipsing in cyclopropane and ethane are the same, approximately, how much of angle strain (in kJ/mol) can be attributed to each of the 105o C-C-C bond angles in cyclopropane? a) 5 b) 10 c) 20 d) 30 e) 40
3. (4 pts) Taking into account hybridization and resonance effects, rank the following C-C bonds in order of increasing length.
A
B C
D
a) b) c) d) e)
A=D=C<B B<D<A<C C<D<B<A B<C<D <A D=A<B<C
4. (4 pts) What is the name the following compound according to the IUPAC rules?
a) b) c) d) e)
2,3-dimethyl-4-propylhexane 4-ethyl-2,3-dimethylheptane 4-ethyl-5,6-dimethylheptane 3-propyl-4,6-dimethylhexane 2-isopropyl-3-ethylhexane
2
5-6. Imagine a hypothetical molecule of a flat (all carbons in one plane) cyclobutane with C-C bonds made of sp5 hybrids. Let's call it A. 5. (2 pts) What is the H-C-H angle in A? a) 60o b) 90o c) 109.5o d) 120o e) 180o
6. (2 pts) Compare A to the real cyclobutane, B. Which statement is correct? a) A has smaller torsional strain than B, but very large steric strain. b) A has larger torsional strain than B, and no steric strain. c) Angle strain in A is about half of the angle strain in B. d) A and B have almost identical total strain. 7. (4 pts) Which statement about the following equilibrium is false?
O CH 3 O I + HO II CH 3 OH III + O IV O
a) b) c) d) e)
The equilibrium is shifted almost completely toward III and IV. I is the strongest base in the system. III is a weaker Brnsted acid than II. IV is the conjugate base of III. The conjugate base of the strongest acid in the system is resonance stabilized.
8. (4 pts) Which of the following structures is not a resonance form of the others? Lone pairs are not explicitly shown.
a) b) c) d) e)
N H
N H
N H
N H
N H
3
9. (4 pts) What term is used for the extra energy present in a molecule in which some of its atoms (that are not directly bonded to each other) are forced closer together than their van der Waals contact distance? a) eclipsing strain b) torsional strain c) steric strain d) angle strain e) compound strain 10. (4 pts) In which of the following species does the central atom (underlined) have sp2 hybridization? A: SO3 a) A and B B: +CH3 b) B and C c) C and D C: H2CCCH2 d) A and C D: H3CCN
e) B and D
11. (4 pts) If ClBCl bond angle in BFCl2 is 109.5o, what hybrid is used by boron to make the BF bond? a) sp b) sp1.5 c) sp2 d) sp3 e) sp4 f) sp5
12. (4 pts) What is the energy difference (in kcal/mol) between the two compounds shown below? The negative sign indicates that the compound on the right is more stable.
CH 3 (H3 C) 3C CH 3 C(CH 3 )3 CH3 CH 3
a) 2.7
b) 1.8
c) 0.0
d) 1.8
e) 2.7
13. (4 pts) The oxygen in water is sp3-hybridized, yet the H-O-H bond angle is only 104.5o. Which factor is behind that angle deviation from 109.5o? a) increasing s character in the lone pair orbitals lowers their energy b) increasing p-character in the OH bond hybrids minimizes the repulsion between bonding electrons c) increasing s-character in the O-H bond hybrids makes the bonds stronger d) increasing p character in the lone pair orbitals makes them bigger and as the result they push the OH bonds closer together e) decreasing both s and p character in the OH hybrids makes them smaller allowing more space for lone pairs
4
14 - 19 The seven lowest energy MOs of the vinyl cation are shown on the left, arranged vertically according to their increasing energies, with (a) having the lowest energy. Answer the following questions by selecting the appropriate orbital.
H H H vinyl cation
14. (2 pts) Which orbital is the HOMO? 15. (2 pts) Which orbital is the LUMO? 16. (2 pts) Which orbital will be the most important in a reaction of the vinyl cation with a hydride ion? 17. (2 pts) Which orbital can be classified as *? 18. (2 pts) Which orbital is built from s orbitals of all atoms in the molecule? 19. (2 pts) Which MO is built mainly of just one atomic orbital?
20. (2 pts) How many orbitals are missing (i.e. are not shown) in the picture of MO's of the vinyl cation? a) 2 b) 3 c) 4 d) 5 e) 6
21. (4 pts) What is the energy difference between the two lowest-energy conformations of 2,3dimethylbutane? a) b) c) d) the energy of one gauche interaction the energy of two gauche interactions the energy of two H <> CH3 eclipsing interactions the energy of three H <> CH3 eclipsing interactions e) the energy of two CH3 <> CH3 eclipsing interactions
5
22. (4 pts) Below is the energy diagram for the rotation about the central bond of 2-methylbutane. Which Newman projection correctly depicts the second highest energy conformation? Note: Me is the abbreviation for methyl, CH3.
25 20 Energy (kj/mol) 15 10 5 0 -180
-120
-60
0 Torsion angle
60
120
180
a)
Me H Me H H Me H H
b)
Me H Me Me H H
c)
Me Me Me H H H
d)
Me H Me Me H H
e)
Me Me H Me
23. (4 pts) Dipole moments usually are measured on collections of molecules, and represent average values for all conformations present. Which of the following collections of molecules will not show a dipole moment? a) CH2F2 d) F2C=C=CF2 b) FCH2CH2F e) FH2CCHF2 c) cis-1,4-difluorocyclohexane f) cis-1,2-difluorocyclohexane
24-25. Consider the following compounds (lone pairs are not explicitly shown).
a) O NH2 NH b) c) CH 3CN N d) e) [N(CH 3) 4] +
24. (2 pts) Which of the compounds is the strongest Brnsted acid? 25. (2 pts) Which of the compounds is the strongest Lewis base?
6
26-27. Consider the following Lewis acid-base reaction (lone pairs are not explicitly shown).
O 1 O 2 + AlCl3 product
O
3
26. (4 pts) What is the critical LUMO of the reaction? a) the nonbonding electron pair on oxygen-1 b) the nonbonding electron pair on oxygen-2 c) the nonbonding electron pair on oxygen-3 d) the p orbital on aluminum e) the * orbital of the carbonyl group with oxygen-3 27. (4 pts) What is the main product of this reaction?
a) AlCl3 O b) AlCl3 O O c) O O AlCl3 d) O O e) O O
O
O
O
O
O
AlCl3
O
AlCl3
28. (2 pts) If the origin of the plot below has [0,0] coordinates, which of the following can the function represent?
a) b) c) d) e)
2s wavefunction 2s electron density 3s radial electron density (i.e. probability fraction per shell) 3p wavefunction 2p electron density
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29. (4 pts) Glucosamine (R = H), a naturally occurring substance found in the exoskeleta of marine invertebrates, is believed by some to be helpful in relieving arthritic symptoms in humans. What are, respectively, the relative positions of the amino group to the CH2OH substituent and OR group?
OH HO HO O OR NH2
glucosamine
a) cis, cis
b) trans, trans
c) cis, trans
d) trans, cis
30 31. Consider a 6-carbon system built of an allyl anion and an allyl cation by connecting their central carbons. The -type MOs of that system are shown below in a random order. Note that some orbitals may be degenerate.
A B C
D
E
F
30. (2 pts) Which orbital is the LUMO? a) A b) B c) C d) D e) E f) F
31. (2 pts) How many unpaired electrons are there in the molecule according to this MO scheme? a) 0 b) 1 c) 2 d) 3 e) 4
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32-33. Consider all equivalent resonance structures for the cyclic molecule below.
32. (2 pts) What is the formal charge on carbon 1? a) 1 b) 2/3 c) 1/3 d) 1/3 e) 2/3
33. (2 pts) What is the bond order between carbons 1 and 2? a) 2/3 b) 1 c) 4/3 d) 2 e) 5/3
End of the exam
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Total Strain Energies in Cycloalkanes ring size kJ/mol kcal/mol 115 27.6 3 110 26.4 4 27 6.5 5 0 0 6 26 6.3 7 40 8.6 8
Energy Costs for Interactions in Alkane Conformers Interaction kJ/mole kcal/mole H-H eclipsed 4.0 1.0 H-CH3 eclipsed 6.0 1.4 CH3-CH3 eclipsed 11.0 2.6 CH3-CH3 gauche 3.8 0.9
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Form A:
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. C D B B D B D D C A A E A E F F G A F C A A D A B D B D B F C C C
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