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Chem 210 Training Set I

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Organic Chemistry: Chem 210

Training Set I

The questions are arranged approximately in the order of the coverage of material in thelecture. 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 sametime?

2. Which of the following compounds has the shortest C-H bond?

3. Which of the following statements about sp2

orbitals isnot true?

a) they are always present as a set of three sp2orbitals

b) they are usually used to form bonds (sometimes lone pairs)c) they are 33%sd) the ideal angle between two sp2orbitals is 120

f)e)d)

c)b)a)

bond

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4. In a hypothetical molecule, a four-bonded carbon atom uses one sp and two sp5 hybridsto make three of its bonds. What is the remaining hybrid that is used to make thefourth bond?

a) pure s e) sp3

b) pure p f) sp

4

c) sp g) sp5d) sp2 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 diagrambelow. At the lowest energy points, the lone electron pair on nitrogen resides in ansp2.1hybrid, but during the flip the hybridization of nitrogen changes continuouslyto finally reach the hybridization of the starting point again. What is the orbitalthat the lone pair resides in at the highest energy point during that transformation?

a) s b) p c) sp d) sp2

e) sp3 f) sp4 g) sp5 h)

7. Lets assume that the H-P-H bond angle in PH3 is 90o (it really is 92o). What is thenature of the orbital holding the lone electron pair?

a) sp3 hybrid d) pure sb) sp2 hybrid e) pure pc) sp hybrid f) intermediate between sp3 and sp2

N

H

H

HN

H

H

H

N

HH

H

(all atoms are in one plane)Energy

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8. Which of the following molecules have at least one sp hybridized carbon atom?

OCO CH3CN CO2

OH

I II III IV V 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 F: CO

B: CH3OCH3 G: OCHCH3

C: CH2CCH2 H: CH3CN

D: CH3CCH I: CH3NH2E: CH2CHCH3

a) A andC e) B, D, andHb) F andH f) B, C, F, andGc) B and I g) C, D, F, andHd) A, G, and I h) A, C, E, andG

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. In 1-bromo-3-chloro-1,2-propadiene (BrHC=C=CHCl) each terminal carbon isbonded to halogen and hydrogen atoms. Excluding the central carbon, what is therelative 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 60

o

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.

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12.Assuming that oxygen and sulfurdo not form double bonds in SO42, determine the

formal charges for this anion (S is the central atom).

Atom: O (all equivalent) Sa) +1 -4

b) +1 -2c) +1/2 -2d) +1/4 -2e) -1 +4f) -1 +2g) -1/2 +2h) -1/4 +2

13. What is the formal charge on nitrogen in the following species?

a) 1 b) 0 c) +1 d) +2

14. Which of the following compounds has the highest dipole moment?

15. 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.

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16. Examineonly theshortest CC bond in each of the following molecules. Arrangethe molecules in order of increasing bond lengths of their shortest CC bond.(Hint: Think about how hybridization affects bond lengths.)

a) A

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19. Some resonance structures of diazomethane are shown below. Which statement aboutthese structures iscorrect?

a)The central nitrogen atom is sp hybridized inA andB, but sp2 hybridized inCandD.

b) StructureD contributes more to the description of the molecule than structureC.

c) All structures satisfy the octet rule for all atoms.d) The equilibrium betweenA andB 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.

20. Consider the allyl anion. Which orbital diagram represents the LUMO?

21-22. Consider the system of 1,3,5-hexatriene (see below). The molecular orbitalsare shown schematically in a random order.

21. Which orbital corresponds to the HOMO of this hexatriene?22. Which orbital corresponds to the LUMO of this hexatriene?

N NH2C NH2C N NH2C N NH2C N

A B C D

a) b) c)

d) e) f)

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23-26. The seven lowest molecular orbitals of formaldehyde (H2CO) are arrangedaccording to their energy, from the lowest at the bottom (g) to the highest at thetop (a).

23. Which orbital will interact the strongest with the empty s orbital of H+in theprotonation reaction?

24. Which orbital will interact the strongest in the reaction of formaldehyde with thehydride (H)?

25. Which is theC-O orbital?

26. Which orbital can be characterized as mainlyC-O?

27. Consider the energy diagram (MO theory) of N2. Which orbital is the highest filledorbital in the energy diagram?

a) the orbital built mainly ofsatomic orbitals

b) the two orbitals (same energy)

c) the orbital built mainly ofpatomic orbitals

d) the* orbital

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28. Below are the four lowest energy molecular orbitals of the pentadienyl cation (the

highest energy is omitted). Which corresponds to the LUMO?

29. Which of the following best describes the lowest energy molecular orbital of an

imine?

30. Which of the following molecules contain two conjugated bonds?

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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 sp

3

-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 reactionbetween BH3and H2O?

a) an empty porbital on boron and a non-bonding lone pair on oxygenb) an empty sp3orbital 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.

34. For the following two acid base reactions, which statement is true?

a) I is favored to the right, II is favored to the leftb) I is favored to the left, II is favored to the rightc) 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

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35. Consider the following reaction involving two acids shown below: formic acid andHF. Which of the following statements about this reaction are true?

A) Formic acid is the strongest Brnsted acid in the reaction.B) HF is the strongest Brnsted acid in the reaction.C)

KF is the strongest Brnsted base in the reaction.D) KO2CH is the strongest Brnsted base in the reaction.

E)The equilibrium favors the reactants.F)The equilibrium favors the products.G) Formic acid has a weaker conjugate base.H) HF has a weaker conjugate base.

a) only A and D f) B, D, and Hb) only B and F g) A, C, and Hc) only C and H h) B, D, E, and Hd) only D and G i) A, D, F, and He) A, D, and F 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

37. There are nine constitutional isomers with a formula C7H16. How many of theseisomers havenosecondary hydrogens?

a) none b) one c) two d) three e) four f) five

pKa=3.2pKa=3.8

HF+H

O

O-K

+H

O

OH+K+F

-

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38. According to the IUPAC rules, what is the best name for the following compound?

a) 3-ethyl-4,7-dimethylnonaneb) 3,6-diethylisononanec) 3,6-diethylisodecaned) 3,6-diethyl-7-ethylnonanee) 1,1-diethyl-2,5-dimethylheptanef) 3,6-diethyl-7-(methylethyl)octaneg) 3-ethyl-4,7-dimethylninenane

39. What is the correct IUPAC name for the following molecule?

a) 4-ethyl-2,5-dimethylnonaneb) 5,7-dimethyl-6-ethylnonanec) 3-isobutyl-4-methyloctaned) 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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41-46. Fill in the missing words in the passage below with words from the provided listto best preserve the meaning of the paragraph. Some words may be used morethan 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 itsbonds 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 repulsivevan der Waals interactions, and (3) (46) strain attributable to angle distortions.

a) torsional f) hystericb) rotational g) eclipsedc) conformational h) staggeredd) angle i) repulsivee) steric j) aligned

47. Which equilibriumis 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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50. What energy diagram would you expect for rotation about the central bond of 2-methylbutane?

51. Which of the following molecules have dipole moments?

A: gauche conformation of 1,2-dibromoethaneB: anti conformation of 1,2-dibromoethaneC: trans-1,4-dibromocyclohexaneD: cis-1,4-dibromocyclohexaneE: tetrabromomethaneF: dibromomethane

a) A andB b) A andD c) B andE d) C andFe) A, D, andF f) A, C andE g) B, C andE h) B, D andE

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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 ofcyclopropane?

a)The hybrids making the C-C bonds have increased s character as compared tothose of ethane.b)The hybrids making the C-C bonds have increased p character as compared tothose of ethane.

c)The bend C-C bonds have increased electron density above and below the planeof carbon ring (like bonds).

d) The hybrids making the C-C bonds have more s character than the hybridsmaking 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-dimethylcyclohexaneb) cis-1,4-dimethylcyclohexane

54. What is the energy difference between the two structures (the negative value meansthat 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 morestable than the other by 15.2 kJ/mol. What is the energy cost of a 1,3-diaxialinteraction between a chlorine and a methyl group?.

a) 2.1 kJ /molb) 3.7 kJ/molc) 5.4 kJ /mold) 7.9 kJ/mol

e) 10.0 kJ /molf) 13.5 kJ/molg) 15.2 kJ /mol

56. Which of the following chair cyclohexane structures has the lowest energy?

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57. Which is the most stable form of 1-ethyl-4-methylcyclohexane?

Note: Me =-CH3, Et =-CH2CH3

H

Me

H

EtH

H

H

HH

Et

H

HMe

H

H

H

H

H

H

EtH

H

H

MeH

H

H

HH

Me

H

Et

H

H

H

HH

H

Me

EtH

H

H

HH

Et

Me

H

a) b)

c) d)

e) f)

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Answer key: Set I

1. D2. C3.

A4. G

5. B6. B7. D8. B9. H10.D11.C12.F13.C14.

B15.C

16.C17.C18.D19.F20.A21.E22.A23.B24.A25.C26.G27.C28.A29.B30.C31.C32.D33.A34.A35.H36.D37.B38.A39.A40.D41.H42.G43.A

44.H45.E46.

D47.B

48.C49.C50.A51.E52.B53.A54.D55.E56.D57.

C

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